atrial natriuretic factor and liver disease

SDecial Article

Atrial Natriuretic Factor and Liver Disease

LEONARD WARNER,3 KARL SKORECKI,3 LAURENCE M. BLENDIS3 AND MURRAY EPSTEIN’, ’Nephrology Section, Veterans Affairs Medical Center; =Department of Medicine, University of Miami School of Medicine,

Miami, Florida 33125; and 3Department of Medicine, University of Toronto, Toronto, Ontario, Canada M5S 1A8

The clinical course of patients with decompensated Laennec’s cirrhosis is frequently complicated by progressive impairment of renal sodium handling, leading to formation of ascites and peripheral edema (1, 2). Despite intense study, the basis of the impairment of renal sodium and water excretion remains incompletely defined.

With the characterization of atrial natriuretic factor (ANF) and the demonstration that it participates in the regulation of volume homeostasis in animals and human beings (3-7), theoretical considerations suggest a potential role for ANF in the pathogenesis of the impaired sodium homeostasis of cirrhosis. On one hand, patients with cirrhosis manifest clinical findings indicative of reduction in effective circulating blood volume (1, 2, 8, 9). Furthermore, direct measurements indicate that central blood volume is often reduced in cirrhotic patients (1, 10). Because atrial distention is considered a major stimulus for ANF release, reduction in effective circulating blood volume and reduced filling pressure could lead to diminished ANF release. On the other hand, according to the “overflow” hypothesis (1, 8, 11) the central hypervolemia that results from a primary disturbance in renal sodium handling might tend to favor an increase in ANF release. The possible role of ANF in this disorder has not, however, been established.

The past decade has witnessed numerous studies investigating the role of ANF in regulating renal sodium and water handling in patients with liver disease. Initially, most of these studies focused on determina- tions of circulating ANF levels. More recently, several investigators have used experimental maneuvers that acutely alter volume distribution, such as head-out water immersion (HWI) and peritoneovenous (PV) shunting, as a means of assessing the relationship between ANF and concomitant kidney function.

The aims of this review are several. The first is to briefly review the relationship of plasma ANF levels with

Received August 6, 1992; accepted November 24, 1992. M. Epstein was supported by Department of Veterans Affairs grant 2456.

K. Skorecki’s laboratory is supported by the Medical Kesearch Council and the Kidney Foundation of Canada.

Address request reprints to: Murray Epstein, M.D., Nephrology Section ( I I lCl ) , Veterans Affairs Medical Center, 1201 N.W. 16th Street, Miami, FL 33125.

HEPATOLOGY 1993;17:500-513. 0270-9139193 $1.00 + . I0 31/1/44524

the stage of liver disease. The second is to consider the effects of dietary sodium challenges and volume- expanding maneuvers such as HWI and PV shunting on ANF responsiveness. Finally, we will review the ex- tensive experience with infusion of exogenous ANF on kidney function and the implications of these studies in delineating a pathophysiological role for abnormal responses to ANF in the salt and water retention of cirrhosis.

OVERVIEW OF ANF ACTION In 1981, deBold et al. (12) reported that a crude pro-

tein extract derived from atrial myocytes could induce a rapid and reversible marked rise in renal sodium and water excretion. In a few years, the factor responsible for this effect was identified as the circulating 28-amino- acid polypeptide ANF. The precursor has been identified as a 126-amino-acid propeptide stored primarily in atrial myocytes (13). ANF release is enhanced by atrial distention and, possibly, by vasoconstrictor hormones (7). ANF receptors have been identified in vascular smooth muscle, kidney, adrenal gland and brain (14). More recently, three classes of ANF receptors have been charac- terized (14). These include two biologically active receptors (A and B) and a C-type or clearance receptor, which may serve as a reservoir for sequestration and release of ANF (15). The second messenger-signaling system in the cellular response to ANF has been identified as the particulate or membrane-bound guanylate cyclase, which induces generation of cyclic GMP (cGMP). A prompt rise in serum and urinary cGMP can be measured after stimulation with exogenous or endogenous ANF (16). In fact, the type A biologically active ANF receptor has guanylate cyclase catalytic activity as part of its cytoplasmic domain; this function is directly activated on occupancy of the receptor (14). Many physiological and cellular responses to ANF occur in different organ systems. In the kidney, increased sodium excretion is attributed to inhibition of sodium reabsorption in the inner medullary collecting duct (17). Other renal effects include an increase in glomerular filtration rate (1 8); an inhibition of vasopressin action in the medullary collecting duct, leading to increased solute-free water clearance (19); and the blockade of renin secretion (20). In the adrenal glands, aldosterone secretion is inhibited (20). In vascular tissues, ANF is both a direct vasorelaxant (21) and an antagonist of the

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vasoconstrictive effects of angiotensin 11 (22). ANF inhibits release of vasopressin in the brain (23).

ALTERATIONS OF ANF IN LIVER DISEASE In late stages of cirrhosis, patients manifest clinical

findings indicative of reduction in effective circulating volume (1,9). Direct measurements indicate that central blood volume is often reduced (10). Because atrial distention is a major stimulus for ANF release, a reduction in central filling pressure with a decreased effective circulating volume might lead to reduced ANF secretion. Despite these considerations, the available data indicate that ANF levels are not suppressed but rather are normal or elevated (24). In most studies, ANF concentrations in cirrhotic patients without ascites, as a group, do not differ significantly from those observed in normal control subjects (25-30). In patients with ascites, plasma ANF values are equal to (26, 30, 31) or exceed those of (27-29, 32-34) normal subjects. In functional kidney failure of cirrhosis (hepatorenal syndrome), plasma ANF levels have been reported to be elevated compared with those of patients with liver disease and normal kidney function or compared with those of normal subjects (32). In one study that found that cirrhotic patients with ascites had reduced ANF levels, more than half of these patients were receiving diuretic therapy (25). In general, the variability of plasma ANF levels may be attributable to several factors, including technique of plasma extraction, RIA, posture, dietary sodium restriction intake, concomitant diuretic treatment and, possibly, variations in hepatic and renal clearance of ANF.

These discrepancies in plasma ANF levels may also be explained in part by the different stages of cirrhosis. ANF levels and their effects need not be the same in the early, preascitic phase of the disease as they are when ascites is far advanced. The demonstration in most studies that ANF levels are normal or elevated may be interpreted as supporting the overflow theory of ascites formation in cirrhosis. According to this formulation, slight degrees of hypervolemia and atrial enlargement might lead to elevation of ANF levels or resetting of the threshold for ANF release. As patients retain more sodium, ANF levels may continue to rise. Warner et al. (35) have shown that in preascitic hepatic cirrhosis or fibrosis, ANF levels rise in parallel with increases in intrasinusoidal pressure (Fig. 1) to the extent that ANF reflects a mild degree of volume expansion. This finding is also consistent with the overflow hypothesis in early stages of the disease. As ascites develops, fluid is translocated out of the intravascular compartment; this may lead to diminished atrial stretch. Thus ANF levels may fall toward the normal range (33). How- ever, atrial enlargement and elevated atrial pressures are not necessarily features of cirrhosis with ascites. Furthermore, factors besides mechanical atrial stretch may enhance ANF release. These include a-adrenergic stimulation and stimulation by other vaso- constrictors (36-40). It is also conceivable that the markedly enhanced lymph flow in patients with decom-

502 WARNER ET AL. HEPATOLOGY March 1993

impairment (serum creatinine > 300 kmol/L and urine output < 100 m1/24 hr).

Additional studies were conducted to assess the responsiveness of ANF to alterations of volume and right atrial pressure induced by hemodialysis or by infusion of 5% human albumin solution (44). The changes in circulating ANF levels induced by hemodialysis correlated with changes in volume and right atrial pressure. In six patients with no or mild kidney failure, infusion of 900 ml of 5% human albumin solution induced significant increases in changes in circulating h-ANF that correlated with volume and right atrial pressure changes (p < 0.001 and p < 0.05, respectively). The augmentation of ANF was not accompanied by concomitant natriuresis or diuresis, a finding com- patible with the hypothesis that resistance to h-ANF may take place in this group. Collectively the above-cited findings indicate that there is no deficiency of h-ANF in fulminant liver failure and that known mechanisms of h-ANF release are not impaired.

METABOLISM AND ARTERIOVENOUS EXTRACTION OF ANF

Plasma disappearance of ANF after intravenous infusion is rapid, indicating a substantial plasma turnover rate of this peptide (45-47). Other peptides with fast turnover rates, such as insulin, neurotensin and vasoactive intestinal polypeptide, undergo substantial hepatic degradation (47-491, raising the possibility that splanchnic removal of ANF is impaired in liver disease. Consequently, several investigators have assessed this possibility.

Gin& et al. (34) investigated ANF metabolism in patients with cirrhosis and ascites to assess the relative contribution of impaired degradation of ANF to the elevated plasma levels. Eleven patients with cirrhosis and ascites without kidney failure and 11 control patients were studied. The plasma concentrations of immunoreactive ANF in coronary sinus, right atrium, pulmonary artery, hepatic vein and peripheral vein were significantly elevated in cirrhotic patients compared with controls. No significant differences were seen in splanchnic and peripheral extraction between control subjects and cirrhotic patients. The cardiac release of ANF, calculated as the product of blood flow and immunoreactive ANF concentration in the coronary sinus, was eight times greater in the cirrhotic patients than in control subjects. These studies provide definitive evidence indicating that elevated plasma ANF levels in cirrhotic patients are attributable to increased cardiac reIease and not to impaired splanchnic or peripheral extraction of the peptide.

Hollister and co-workers (50) have suggested that reduced liver function as found in patients with cirrhosis is associated with reduced plasma ANF clearance. Their suggestion was based on the demonstration of the presence of a substantial hepatic-intestinal clearance of ANF in subjects without reduced liver function.

Recently, Henriksen et al. (51) investigated the hepatic-intestinal removal rate of endogenous circu-

lating ANF in cirrhosis. Thirteen patients with cirrhosis (six of Child-Turcotte class A, five of class B and two of class C) and eight control subjects were studied. The Fick principle was applied during hepatic vein catheterization. These investigators demonstrated that the arteriohepatic venous extraction ratio of ANF, hepatic- intestinal clearance and the removal rate in cirrhotic patients were very similar to those of controls. Hen- riksen et al. (51) interpreted their data to indicate that hepatic-intestinal disposal of ANF in cirrhosis does not differ from that of controls; indeed, the values were similar. They concluded that patients with cirrhosis and decreased hepatic function do not have decreased disposal of circulating endogenous ANF.

The possibility that splanchnic disposal of ANF is influenced by food intake has recently been addressed. Vierhapper et al. (52) found a basal splanchnic uptake of ANF of 8.5 ymol/min in healthy human subjects. Recently Henriksen, Bendtsen and Gerbes (53) assessed the effect of food ingestion on splanchnic disposal of human a-atrial natriuretic peptide (ANF), in six subjects referred for hepatic vein catheterization but without hepatic disease. Hepatic-intestinal removal of ANF was determined before and after a test meal. They observed enhanced splanchnic removal of ANF after food intake and proposed that this is due to increased hepatic- intestinal clearance of the peptide consequent to increased splanchnic blood flow rather than to altered fractional extraction of ANF.

CHARACTERIZATION OF ANF Previous studies aimed at characterizing immunore-

active ANF on HPLC have suggested the presence of an immunoreactive component of higher molecular weight in some patients (54). Epstein et al. (28) carried out chromatographic analyses on reverse-phase HPLC in extracts of plasma from several cirrhotic patients, both under control conditions and after marked stimulation of ANF secretion by water immersion. The pattern obtained was identical to that demonstrated in normal human subjects (Fig. 2) (55). Therefore elevated levels of ANF in cirrhosis cannot be attributed to release of an altered molecular species recognized by conventional RIA.

ANF RESPONSIVENESS TO VOLUME-EXPANSION MANEWERS (HWI)

Although attempts have been made to assess renal and hormonal responsiveness with saline solution infusion, several limitations of study design confound interpretation of the results. As we have detailed previously (56, 57), rapid volume expansion with solu- tions such as saline have several drawbacks related to their lack of specificity. For example, saline solution infusion increases the volume of all fluid compartments and induces concomitant alterations in plasma compo- sition.

Studies from Epstein’s laboratory over the past 24 yr have succeeded in circumventing many of these meth- odological problems by applying a newly developed

HEPATOLOGY Vol. 17, No. 3, 1993 WARNER ET AL. 503

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Plasma ANF 50-

(fmol/ml) 25 -

0 - ‘Pre-study ’ 1 ’ 2 ‘ 3 ’ Recovery’

Hours FIG. 2. Reverse-phase HPLC of C,, Sep-Pak extracts of plasma

obtained from a cirrhotic patient before (control; 0) and during (immersion; 0) stimulation of ANF secretion by HWI. Samples were injected onto a 0.39 x 30 cm FBondapak C,, column and eluted at 1.0 mumin with a linear gradient of 10% to 60% acetonitrile in 0.1% trifluoroacetic acid over 50 min, as previously described (13); the gradient was begun coincident with sample injection at 0 min. The major peak of immunoreactive ANF has a retention time (25 min) identical to that of synthetic ANF (99-126). No immunoreactivity was detected in either sample at a retention time of 38 min, corresponding to results with ANF precursor, ANF 1-126 (14). (Reprinted by permission from “Relationship between plasma ANF responsiveness and renal sodium handling in cirrhotic humans” by Epstein M, Loutzenhiser R, Norsk P, Atlas S, in Am JNephrol, vol9, pp. 133-143. Copyright Q 1989 by S. Karger AG.).

investigative tool-the model of HWI-to the as- sessment of renal sodium and water homeostasis in normal humans and in patients with diverse edematous states (56,571. Earlier studies from our laboratory have demonstrated that this maneuver produces prompt, sustained diuresis and natriuresis and markedly sup- presses levels of PFtA, plasma aldosterone (PA) and arginine vasopressin (56, 57).

Epstein et al. (58, 59) and Warner et al. (60) have demonstrated that HWI is a useful model for investigating the responsiveness of the ANF system to acute reversible central blood augmentation in normal subjects. HWI elicits a marked rise in ANF levels and a natriuresis (55, 59) and increases in plasma cGMP (60) and urinary cGMP excretion (55,59-61). Urinary cGMP excretion is a marker of biochemical responsiveness to ANF signaling (62, 63).

Recently Epstein et al. (28) used HWI to determine whether responsiveness of ANF to volume expansion is impaired in cirrhosis. Eight of the nine patients studied had marked ascites. After equilibration on a 10 mEq/day sodium diet, during 3 hr of HWI, five of the nine cirrhotic patients manifested exaggerated peak ANF responses, whereas the other four manifested increases similar to those of normal subjects (Fig. 3). The concomitant natriuretic response varied widely, ranging from absent to markedly exaggerated. In contrast to normal subjects, the natriuretic responses of cirrhotic patients were dissociated from the concomitant increases in ANF. These observations indicate that no impairment occurs in the release of ANF in cirrhosis.

FIG. 3. Effect of HWI on plasma ANF levels in 11 cirrhotic patients. The shaded area represents means c S.E.M. for 13 normal subjects (9) undergoing an identical immersion study. Cirrhotic subjects manifested a wide variety of responses, with patients 1 to 4 and 9 manifesting markedly exaggerated augmentation of ANF. (Reprinted by permission from “Relationship between plasma ANF responsiveness and renal sodium handling in cirrhotic humans” by Epstein M, Loutzenhiser R, Norsk P, Atlas S, in Am J Nephrol, vol 9, pp. 133-143. Copyright 0 1989 by S. Karger AG.).

A similar conclusion was reached in a study by Skorecki et al. (33) in which 12 sodium-retaining cirrhotic subjects were studied during HWI. The patients were maintained for 7 days on a 20 mEq/day sodium intake and then studied on both control and immersion days. In six subjects, immersion resulted in marked natriuresis sufficient to induce negative sodium balance by hr 3. These subjects were called “responders.” In these six patients, baseline preimmersion levels of PRA and serum aldosterone were all below 3 ng/l/sec and 4 nmol/L, respectively. In the other six patients, the natriuretic responses to HWI were markedly blunted and insufficient to induce negative sodium balance. ‘l’hese patients were termed “nonresponders.” In contrast to responders, baseline preimmersion levels of PRA and aldosterone were all greater than 3.5 ng/L/sec and 5 nmol/L, respectively, in nonresponders. These levels were significantly elevated compared with levels in responders and normal subjects consuming the same amount of sodium. Furthermore, during HWI none of the nonresponders had aldosterone levels suppressed below 1.4 nmoVL, a level previously proposed to be permissive of natriuretic response (64). In both groups of cirrhotic subjects, baseline levels of plasma ANF and urinary cGMP excretion were significantly and compa- rably elevated compared with the normal range for control subjects ingesting the same amount of sodium. Despite the marked difference in natriuretic response to immersion between responders and nonresponders, significant and comparable further elevations of plasma ANF and urinary cGMP excretion were seen during immersion compared with the control day values in both groups. These results indicate that even in nonresponders, no defect in ANF release or biochemical responsiveness is present, at least at the level of cGMP signaling.


The pattern of urinary cGMP responses suggests that the relative renal resistance to the natriuretic action of ANF in nonresponders is mediated at a level parallel to or beyond that of coupling of ANF to its guanylate cyclase-linked receptor. However, it is possible that urinary cGMP excretion predominantly reflects cGMP of glomerular origin and therefore may not reflect guanylate cyclase activation at tubule segments relevant to the natriuretic action of ANF. Resistance to the natriuretic action of ANF appears to be associated with activation of antinatriuretic factors, including the renin- angiotensin-aldosterone system, which is clearly activated in nonresponders. Epstein et al. (28) also observed significant elevation of basal PRA in nonresponders compared with responders.

Saline-induced Volume Expansion. As a more conventional volume-expanding maneuver, Tesar et al. (65) infused 2 L saline solutionl70 kg body wt over 2 hr into six normal subjects, six compensated cirrhotic patients and six decompensated cirrhotic patients. In the nonascitic cirrhotic patients and in the normal controls, infusion of saline solution was associated with a significant rise in ANF levels with concomitant, significant natriuresis and diuresis. The cirrhotic patients with ascites responded to infusion of saline solution with increases in ANF levels comparable to those in nonascitic cirrhotic patients and controls. In contrast, the ascitic cirrhotic patients did not have significant natriuresis or diuresis. In this group of ascitic cirrhotic patients, PRA and plasma aldosterone level were incompletely suppressed by saline solution infusion. Tesar et al. (65) also concluded that the renal resistance to the natriuretic action of ANF could be mediated in part by the incomplete suppression of the renin-angiotensin- aldosterone system.

Peritoneovenous Shunting. Another maneuver that has been utilized to assess ANF responsiveness and its relationship with renal excretory function is peritoneovenous shunting (PVS). The physiological consequences of PVS have been well documented (66). Shortly after shunt insertion, a marked translocation of volume into the intravascular compartment occurs, as demonstrated by an average 15% fall in hematocrit without a change in RBC mass. Increases in cardiac output, renal plasma flow and creatinine clearance take place, accompanied by a diuresis and a natriuresis. Although declines even- tually occur in elevated levels of PRA, serum aldosterone levels, plasma catecholamine levels and vasopressin concentrations during the postoperative period, few if any changes occur in these parameters during the first 2 to 4 hr after PVS, when there are maximal changes in systemic hemodynamics and urinary sodium and water excretion. Because an intravascular volume load is known to be a potent and rapid stimulus for ANF release, it was of interest to investigate the response of this hormone to PVS.

Campbell et al. (29) studied six cirrhotic patients with massive refractory ascites under strict metabolic conditions while the patients were receiving a 20 mEq/day sodium diet. Baseline ANF levels were significantly elevated in these subjects compared with levels in normal subjects and cirrhotic subjects without ascites on

a 20 mEq/day sodium diet. After shunt insertion, immediate natriuresis and diuresis were observed in five of the six cirrhotic patients with refractory ascites. In these five, right atrial pressure and ANF increased, immediately followed by an increase in urinary cGMP excretion. In the sixth patient, the increase in right atrial pressure was delayed by initial mechanical failure of the shunt; the increase in ANF and the diuresis and natriuresis were correspondingly delayed. The changes in ANF after PVS were positively correlated with changes in right atrial pressure, urinary cGMP excretion, urine excretion of sodium and urine volume. These results suggested that a rise in ANF level contributes to the immediate urinary response to PVS. These results confirm that even cirrhotic patients with severe sodium retention and refractory ascites do not have defects in ANF release or biochemical responsiveness at the level of cGMP signaling. These studies suggest that in these patients a relative resistance to the natriuretic action of ANF occurs that can be overcome by the consequences of PVS.

Klepetko et al. (67) reported similar responses to PVS and measured these parameters at subsequent intervals. Ten patients with cirrhosis and ascites underwent PVS, which resulted in acute increases in plasma ANF levels. Mean preoperative plasma ANF level was 82 2 13.5 ng/L and rose significantly to 308 k 60 ng/L immediately after surgery. ANF levels gradually fell but were still significantly elevated 7 days after PVS (149 ? 28 ng/L). However, 3 mo after shunt insertion, plasma ANF levels had fallen to preoperative values (75 2 9 ng/L).

It should be noted that alternative explanations have been proposed for the augmented levels of ANF after PVS. Salerno et al. (68) have shown that immediately after large-volume paracentesis before colloid re- placement, a significant increase in plasma ANF levels occurs, with a concomitant decrease in PRA and serum aldosterone. The authors concluded that the rapid relief of high intraabdominal pressure increases central venous blood return and cardiac output. It appears, therefore, that the rise in ANF that occurs immediately after PVS is mediated initially by the relief of tense ascites and by intravascular fluid translocation. The potential role of ascitic fluid itself in stimulating ANF release, in addition to relief of elevated intraabdominal pressure and central volume expansion, has not been evaluated. This would require an isovolemic infusion of ascitic fluid.

Renal Responsiveness to Infusion of Exogenous Peptide. The volume-expanding maneuvers described in the previous section induce complex shifts in several parameters of the regulation of sodium excretion. To assess responsiveness to ANF per se, investigators have infused exogenous ANF into cirrhotic patients. The available reports indicate that many patients manifest blunted natriuretic response to infusion of the peptide.

Legault et al. (Unpublished observations, 1989) compared the natriuretic response to HWI with the response to infusion of a-human ANF in 12 cirrhotic subjects. ANF was administered as an initial bolus of 35 ng/kg/min for 5 min; this was followed by a constant infusion of 15 ng/kg/min for 115 min. No significant

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change in mean arterial blood pressure was found with either maneuver in any of the subjects. Five patients displayed natriuretic responses to HWI sufficient to achieve negative sodium balance, analogous to the responders in the previously cited study by Skorecki et al. (33). Each of these five patients also had a natriuretic response to ANF infusion. In contrast, the other seven patients consistently failed to exhibit natriuretic responses to either maneuver. In these nonresponders, levels of PRA and serum aldosterone were significantly elevated, and basal urinary sodium excretion and plasma sodium concentration were significantly depressed in comparison to levels in responders. The congruency among patients in the response to ANF infusion on the one hand and HWI on the other indicates that resistance to the natriuretic effect of HWI does indeed represent resistance to the natriuretic action of ANF. It should also be noted that ANF levels achieved with infusion were comparable to peak levels achieved immediately after PVS. Failure of nonresponders to demonstrate natriuresis in response to this ANF infusion protocol clearly suggests that factors in addition to ANF release per se mediate the natriuretic response to PVS.

Salerno et al. (69) also studied the effects of ANF infusion in cirrhotic patients. After a bolus injection of ANF (1 bg/kg), seven patients with elevated baseline PRA and aldosterone levels demonstrated attenuated or absent natriuretic responses. Seven patients without sodium retention or with moderate sodium retention and normal baseline PRA and aldosterone levels had marked natriuresis, as did normal controls. This supraphysiological dose of ANF caused a transient drop in blood pressure in both cirrhotic subjects and normal controls.

Fried et al. (70) evaluated the effects of anaritide, a 25-amino-acid synthetic analog of ANF, in 28 cirrhotic patients with ascites and edema. Patients received infusions ranging from 0.015 to 0.300 Fg/kg/min. The investigators saw significant natriuresis at all doses of anaritide, with the exception of the highest dose. With infusion rates of 0.60 pg/kg/min or greater, a progressive significant decline in mean arterial pressure was noted. The authors concluded that the therapeutic potential of anaritide could be limited by its hypotensive effect at high doses. Petrillo et al. (71) showed that the hypotension caused by high-dose anaritide was associated with a rise in plasma catecholamine levels and plasma renin activity. Thus it appears that the attenuated renal response to ANF infused at high doses in turn may be mediated by activation of the sympathetic nervous system and the renin-angiotensin systems and perhaps by other factors, such as a decrease in glomerular filtration rate, that abrogate the natriuretic activity of ANF.

Investigators have attempted to improve the therapeutic potential of intravenous ANF infusion in patients with cirrhosis by the simultaneous infusion of norepinephrine to prevent the hypotensive effects of ANF. However, two groups of investigators have observed that the blunted natriuretic response to ANF in patients with advanced cirrhosis could not be reversed by prevention or correction of hypotension with intravenous norepi-

nephrine (72, 73). In both studies, norepinephrine infusion had no significant effect on PRA or aldosterone. Thus the systemic hypotensive state per se, which is characteristic of advanced cirrhosis, does not appear to be the critical determinant of sodium retention.

Beutler et al. (74) compared the responses of patients with biopsy-proven cirrhosis but without ascites (proved by sonography) with those of normal subjects when given comparable infusions ofANF. The normal subjects were studied while consuming a 20, 100 or 200 mmol sodium/day diet; the cirrhotic patients were consuming 40 or 120 mmol sodium/day. Mean basal ANF level in the cirrhotic patients was 34.4 k 4 pmol/L, compared with 11.3 k 3 pmol/L in the controls. Blood pressure, glomerular filtration rate (inulin clearance), renal plasma flow (para-aminohippurate clearance), solute-free water clearance, and maximal urine flow were all normal in the cirrhotic patients. In response to a single injection of 100 pg ANF, the normal subjects manifested much greater proportional increases in sodium excretion at all sodium intake levels compared with the cirrhotic patients. In addition, no rise in glomerular filtration rate occurred in the patients, and the maximal urine flow was significantly less. These observations suggest that the alterations in ANF levels and ANF response occur in the early, preascitic phase of cirrhosis as well as in established disease. An intriguing feature of this study - but one that may confound the interpretation of the results- is that in these patients with compensated cirrhosis, ANF administration caused large decreases in blood pressure (mean decrease = 22/13 mm Hg) that were not seen in normal subjects. The concomitant drop in blood pressure could be explained by impending underfill or possibly by an abnormality in the regulation of systemic vascular tone, as recently proposed according to the “peripheral arterial vasodilatation” hypothesis (75). This hypothesis proposes that vasodilatation is the earliest hemodynamic abnormality that is the ante- cedent of renal sodium retention in early cirrhosis.

Laffi et al. (76) found that in cirrhotic patients with refractory ascites and striking activation of the renin- angiotensin-aldosterone system, a bolus infusion of a-human ANF (1 p,g/kg intravenously every 3 hr for four doses) was ineffective in causing an appreciable natriuresis or diuresis. In contrast, when the a-human ANF was infused as a smaller initial bolus (50 ngkg) followed by an infusion of 0.1 pg/kg/min for 45 minutes there was a heterogeneous natriuretic response (77). In responders, the extent of the natriuretic response paral- leled the increase of effective renal plasma flow and glomerular filtration rate. In nonresponders, the absent natriuretic response was associated with reduction of these parameters. As noted by other investigators (711, PRA was increased in nonresponders but not in responders to ANF infusion.

CIRCADIAN PATTERN OF CIRCULATING ANF In normal subjects, urine volume flow and sodium

excretion are subject to diurnal variation, with reduction of both during the night (78, 79). This pattern is reversed or lost in patients with cirrhosis (80, 81). In studies of circulating aldosterone concentration in cir-


rhosis during which patients remained supine for 24 hr, Bernardi et al. (82, 83) demonstrated loss of the circadian pattern of PRA and aldosterone and norepinephrine concentrations in patients with cirrhosis and ascites. Recently Colantonio et al. (84) reported the absence of variation of plasma ANP levels in cirrhotic patients under similar conditions. Unfortunately, these investigators failed to report data on concomitant renal excretory function.

Recently, Panos et al. (85) investigated whether the circadian pattern of circulating ANF is altered in cirrhosis and whether ANF alterations contribute to the reversed nocturnal natriuretic and diuretic patterns observed in cirrhotic patients. They studied 21 patients with cirrhosis and moderate-to-marked ascites and compared them with 10 age-matched controls. In eight of the cirrhotic patients, PRA, plasma aldosterone and urinary sodium excretion were measured every 4 h r for 24 hr. Subjects were ambulatory between 8 AM and 11 PM and supine from 11 PM to 8 AM. In control subjects, urinary sodium excretion was highest between 4 PM and midnight and lowest between midnight and 8 AM. In patients with cirrhosis, urinary sodium excretion was 0.6 ? 0.1 bmol/min between 8 AM and midnight and 1.9 ? 0.7 bmol/min (p < 0.08) between midnight and 8 AM. Concomitantly, mean plasma ANF concentration did not change during the day, but large, sustained increases in PRA and plasma aldosterone were noted. A correlation was observed between ANF and urinary sodium excretion between midnight and 8 AM (r = 0.65; p < 0.02) and 4 PM and midnight (r = 0.54, p < 0.05) but not between 8 AM and 4 PM. PRA declined from 12.5 5 2.5 pmol/hr/ml at midnight to 7.4 f 0.9 pmol/hr/ml at 8 AM (p < 0.051, and PA decreased from 1,032 f 101 pmol/L to 798 k 56 pmoVL (p < 0.05). Panos et al. (85) interpreted their findings to suggest that ANF contributed to the nocturnal natriuresis of cirrhosis. They postulated that suppression of the activity of the renin-aldosterone system during recum- bency may allow the natriuretic effect of ANF to become manifest.

The Response of ANF to Dietary Sodium Challenges. To delineate the role of ANF under more physiological conditions, Warner et al. (35) studied the effect of ANF levels and urinary sodium excretion of varying dietary sodium intake in 11 patients with chronic liver disease (six without and five with histories of clinical sodium retention [ascites or edema]).

The patients were prescribed a 20 mmol/day constant sodium diet for 1 wk, followed by a constant 100 mmoVday sodium diet under strict metabolic conditions. After 5 days of equilibration on each diet, blood and urine samples were collected for assay of plasma ANF levels and urinary sodium excretion. Normal controls (n = 61, as expected, remained in sodium balance on both the 20 mmol/day (urinary sodium excretion = 19 ? 2 mmol/day) and the 100 mmol diet (urinary sodium excretion = 99 k 7 mmoVday). ANF levels rose on the higher sodium diet (from 10 -t 4 pg/ml to 19 2 4 pg/ml). Patients without histories of clinical sodium

retention (n := 6) were able to achieve near sodium balance in 5 days on a 20 mmoVday sodium diet (urinary sodium excretion = 17 % 3 mmol/day) and 100 mmoVday sodium diet (urinary sodium volume = 80 ? 5 mmol/day). ANF levels rose on the 100 mmol/day diet (21 ? 7 pg/ml to 30 5 7 pg/ml) but were not significantly elevated compared with levels in normal controls. In contrast, patients with histories of clinical sodium retention were in significant positive balance on both the 20 mmoVday sodium diet (urinary sodium excretion = 9.5 -+ 3.3 mmol/day) and the 100 mmol/day sodium diet (urinary sodium excretion = 37 k 13 mmol/day). This occurred despite significantly elevated plasma ANF levels on a 20 mmoVday diet (29 ? 4 pg/ml) and a significant increase in plasma ANF levels on a 100 mmoVday sodium diet (62 ? 9 pglml). These results are also consistent with renal resistance to natriuretic actions of ANF in this group of patients.

As mentioned previously, the six patients without sodium retention were further studied on the 100 mmoVday sodium diet along with seven similar patients. All underwent measurement of estimated portal pressures (corrected sinusoidal pressure), which were positively correlated with ANF levels (n = 13; p < 0.01, r = 0.75) (Fig. 1). These results are consistent with the proposal that in these patients intrasinusoidal hypertension may play a role in the initial development of sodium retention and that sodium retention is maintained on various sodium diets at the expense of gradual elevation in ANF levels.

RELATIONSHIP BETWEEN CIRCULATING ANF AND RENAL EXCRETORY FUNCTION; CONSIDERATION OF RESISTANCE TO

NATRIURETIC ACTION OF ANF The studies previously mentioned, on the whole,

indicate that no deficiency in plasma ANF levels occurs in cirrhosis and ascites. Nor is there any impairment in release of ANF after volume-expansion maneuvers. There is no apparent impairment in generation of cGMP, as evidenced by the rise in urinary cGMP levels after HWI, even in nonresponders (33), indicating that the relative resistance to the natriuretic action of ANF is likely mediated at a level beyond ANF release or coupling to its receptor.

Nevertheless, a clear dissociation is evident between circulating ANF levels and renal responsiveness to the natriuretic actions of this hormone. What mediates resistance to ANF? Possibilities include (a) impaired delivery of salt and water to distal nephron sites responsive to ANF mediated by antinatriuretic factors and reduced glomerular filtration rate, (b) presence of forces favoring sodium retention that override the natriuretic effects of ANF and its site of action in the medullary collecting duct, (c) down-regulation of a population of ANF receptors at a distal nephron site not reflected in urinary cGMP, (d) a biochemical defect beyond the level of cGMP production such as cGMP-dependent kinase and (e) decreased delivery of permissive cofactors that allow appropriate ANF ac-


tion to be effected at the luminal distal tubule (see below).

Review of available evidence indicates that the presence of opposing antinatriuretic factors is likely important in mediation of renal resistance to the natriuretic actions of ANF. Experiments using HWI have shown that nonresponders have significantly greater activation of the renin-angiotensin system than do responders (28, 33). None of the nonresponders demonstrated nadirs of aldosterone levels lower than 1.4 nmol/L after HWI (33), a value that has previously been shown by others (64) to be permissive of a natriuretic response.

Gerbes et al. (86) have shown that ANF, PRA and aldosterone alone did not correlate with sodium excretion but that the ratio of ANF to plasma aldosterone was closely correlated with basal and stimulated natriuresis in cirrhotic patients. These results do not imply that activation of the renin-angiotensin-aldosterone system is the only important antinatriuretic factor but rather that this system is a suitable marker for enhanced antinatriuretic activity caused by several efferent antinatriuretic factors. The finding that some sodium- retaining cirrhotic subjects show hyperresponsiveness to HWI, both in terms of enhanced ANF release and natriuresis in comparison to normal subjects (41), strongly suggests that no intrinsic biochemical resistance to ANF exists in cirrhosis. Once antinatriuretic factors have been adequately suppressed, the full effect of increased levels of ANF can be expressed by the kidney and reveals itself in an exaggerated natriuretic response.

Renal resistance to the natriuretic actions of ANF may be mediated in part by increased renal sympathetic nerve activity. Studies by Koepke et al. (87-89) implicate a role for heightened renal nerve activity in experimental animal models of ANF resistance, including cirrhosis. Nicholls et al. (90) showed that HWI caused significant suppression of norepinephrine in patients in whom natriuresis developed. Lumbar sympathetic block was shown to increase sodium excretion in six of eight sodium-retaining cirrhotic patients (91).

A recent study by Morali et al. (92) is of interest in this regard. They studied the potential role of the sympathetic nervous system as a determinant of the responsiveness of ANF by utilizing microneurography to measure muscle sympathetic nerve activity (SNA). Twenty-six patients with biopsy-proven cirrhosis and seven age- and sex-matched normal volunteers were studied after a week of a 20 mmol/day sodium diet without diuretic administration. Muscle SNA was re- corded from the peroneal nerve and correlated with responsiveness to 2-hr ANF infusion. Lithium clearance was used as a marker of sodium reabsorption proximal to the intramedullary collecting duct, the main site of ANF action. Muscle SNA was greatly increased in the 12 ascitic patients who did not respond to ANF infusion compared with that in the normal subjects (64 2 4 bursts/min vs. 27 k 7 bursts/min; p < 0.001) and mod- erately increased in 5 ascitic responders (47 +- 6 bursts/min; p < 0.05) but not significantly increased in

9 nonascitic patients with cirrhosis (34 2 5 burstdmin). SNA correlated positively with plasma norepinephrine levels (r = 0.69; p < 0.005) and correlated inversely with peak sodium excretion during the ANF infusion (r = 0.63; p < 0.0001). Consistent with previous studies, PRA and aldosterone levels were markedly elevated in ascitic nonresponders and normal in ascitic responders and nonascitic patients. Lithium clearance was reduced in ascitic patients compared with that in nonascitic patients, did not change after ANF infusion and was correlated inversely with SNA (r = 0.61; p < 0.01). These results support the concept that the sympathetic nervous system is a factor in renal sodium handling in cirrhosis and that refractoriness to ANF might be explained in part by increased neurally mediated sodium reabsorption proximal to the intramedullary collecting duct, the main site of ANF action.

The mechanism whereby these candidate antinatriuretic forces impair the response to ANF warrants consideration. One obvious potential mechanism is the action of angiotensin I1 and catecholamines enhancing fractional proximal reabsorption of filtered fluid and hence diminishing delivery to ANF-responsive sites in the distal nephron. This in turn may be the result of direct effects on sodium handling by the proximal tubule and disturbances in peritubule capillary Starling forces following adjustments in glomerular hemodynamics (93). Both micropuncture and clearance studies in human beings and animals confirm that abnormal salt and water processing in cirrhosis occurs at several tubule sites, even in the face of normal glomerular filtration rate (93). The necessary and sufficient site for ANF-induced natriuresis is the medullary collecting tubule, although actions at more proximal segments may be contributory (93-98). Indeed indirect evidence from HWI studies indicates that, in some circumstances, resistance to ANF action at the distal nephron contributes to the blunted natriuretic response (33). In these studies, nonresponders and responders demonstrated significant kaliuresis during HWI, but only the responders had significantly increased solute-free water clearance. Thus the delivery of sodium to distal nephron segments was at least adequate for a kaliuretic response, even in the nonresponders.

Another action of ANF in the collecting tubules is to antagonize the hydroosmotic action of vasopressin (19), and it is tempting to speculate that resistance to both the diuretic and the natriuretic actions of ANF occurs in the collecting tubule. It has been demonstrated that at peak natriuretic effect urinary potassium excretion is significantly greater in responders than in nonresponders during both ANF infusion and HWI (Legault L, Unpub- lished observations, 1989). This occurs despite lower aldosterone concentrations in responders, suggesting that distal delivery is limiting in nonresponders. The lack of kaliuresis after infusion of physiological doses of ANF in responders suggests that the dominant effect of ANF is to reverse the antinatriuretic factors at the collecting tubule and that improved distal sodium delivery during HWI is mediated more by other factors


associated with volume expansion in addition to the rise of ANF per se. Furthermore, the ability of an ANF infusion to cause natriuresis only in responders, without a concomitant change in lithium clearance, confirms a distal site of ANF resistance in nonresponders (92).

Recently Morali et al. (99) studied 10 patients with massive, resistant ascites, off diuretics and on a 20 mmollday sodium diet for 7 days. ANF responsiveness was confirmed by the failure of a 2-hr infusion of ANF to induce natriuresis. The next day each patient received a 40-gm infusion of mannitol and, subsequently, a combined infusion of mannitol and ANF. Six of the 10 patients responded to mannitol alone with increased natriuresis (0.27 2 0.05 mmolhr to 1.65 & 0.53 mmolhr). The combination of ANF and mannitol induced further significant increases in sodium excretion (3.28 & 0.68 mmolhr). Lithium clearance increased in the responders after mannitol infusion but did not increase further with ANF and mannitol. Inulin and para-aminohippurate clearances were equally low in mannitol responders and nonresponders and did not change with ANF and mannitol infusions. The mannitol responders had significantly lower PRA and aldosterone and norepinephrine levels than did the mannitol nonresponders. Thus ANF unresponsiveness in some cirrhotic patients was shown to be due in part to increased sodium reabsorption proximal to the distal site of ANF action as well as to resistance to the action of ANF in the collecting tubule. The increased proximal reabsorption could be mediated by disturbances in physical forces governing fluid reabsorption in the proximal tubule or by direct actions of angiotensin I1 or catecholamines.

A potential biochemical mechanism for resistance of the collecting tubule to ANF in cirrhosis remains to be clarified. The well-preserved cGMP responses might suggest that resistance to ANF is mediated in a pathway distal to or parallel with cGMP signaling. However, as noted previously, urinary cGMP appearance may be a predominant reflection of cGMP of glomerular origin such that inhibition of guanylate cyclase at relevant downstream tubule sites might not be readily discerned with measurements of urinary cGMP excretion. Rela- tively high concentrations of calcium are required to inhibit ANF-stimulated guanylate cyclase in cultured rat renal glomerular mesangial cells (100). In these cells, activation of protein kinase C with phorbol esters also causes marked inhibition of guanylate cyclase activation. These results are consistent with the studies of Jaiswal, Jaiswal and Sharma (1011, who proposed that the guanylate cyclase coupled to the ANP receptor is partially under the regulation of protein kinase C-mediated phosphorylation. No vasoconstrictor hormones have been found that inhibit cGMP production in intact glomerular mesangial cells, in agreement with unaltered urinary cGMP responses in cirrhosis. In contrast, vasoconstrictor hormones have been shown to inhibit the cGMP response of vascular smooth muscle cells in culture (102, 103). There is a less likely possibility that resistance to ANF is mediated at the level of receptors not coupled to guanylate cyclase or at the level of cGMP kinase.

ENDOPEPTIDASE AND OTHER URINARY PEPTIDE COFACTORS

A novel approach to enhancing ANF activity is to inhibit metabolism of the peptide. In uitro studies have demonstrated that ANF is a substrate for neutral endopeptidase (membrane metalloendopeptidase) (14). The major cleavage site is the Cys105-Phe106 bond, which yields an inactive metabolite. The enzyme is widely distributed in the body but is most abundant in the renal cortex, in particular the proximal tubule (104). Several inhibitors of neutral endopeptidase are known; one such compound is tbiorphan (DL-3mercapto-2benzylpropan- oyl-glycine). Given alone to rats, it is natriuretic and diuretic (105). Coadministered with ANF, it has been reported to enhance plasma ANF levels and to potentiate the renal response to the peptide (105).

Recently Wilkins et al. (106) investigated the effect of thiorphan in rats in which an aortavenocaval (A-V) shunt was established distal to the renal arteries to create a model of high-output heart failure with chron- ically elevated plasma ANF and urinary cGMP levels. They demonstrated that this inhibitor induced natriuresis in A-V fistula rats exceeding that seen in control animals given these compounds and matching the peak natriuresis produced in sham-operated animals with high doses of ANF. These investigators speculated that the greater potency of thiorphan in heart failure compared with infusions of the peptide may be attributable to the ability of this compound to protect filtered ANF from degradation in the kidney, thereby enabling the peptide to reach normally inaccessible renal tubule sites. Consistent with this hypothesis was the appearance of ANF in the urine of animals treated with thiorphan. In contrast, very little ANF was detected in the urine of sham-operated or A-V fistula rats under basal conditions, or even when plasma ANF levels were greatly elevated by infusion of the peptide.

Levy et al. (Personal communication, 1992) have studied the effect of inhibition of neutral endopeptidase 24-11 on dogs with partial inferior vena cava ligation that were unresponsive to a supraphysiological dose of intravenous ANF. Inhibition of neutral endopeptidase 24-11 converted a nonresponding dog with partial inferior vena cava ligation into a responder. This exciting result was found by these investigators after many previous strategies to convert nonresponder to responder dogs with partial inferior vena cava ligation had been unsuccessful. Administration of ANF in the face of a-adrenergic blockade, saralasin (an angiotensin I1 antagonist), dypridamole to block adenosine uptake and elevation of blood pressure could not alter the initial response to ANF (107). Recently, Levy’s group also observed (Personal communication, 1992) that bradykinin allowed nonresponding dogs with partial inferior vena cava ligation to respond to ANF, urodilatin (see below) and 8-bromo cGMP. Conversely, pretreatment of dogs with partial inferior vena cava ligation with aprotinin or a specific bradykinin antagonist significantly attenuated the natriuretic action of ANF. These results indicate that in nonresponding dogs with partial inferior vena cava ligation there appeared to be a deficiency in kinins


I HEPATIC VENOUS OUTFLOW BLOCK I primary renal

sodium retention

intravascular volume expansion .I

loss of volume into

EARLY CIRRHOSIS

antinatriuretic factors n

I LATE CIRRHOSIS

antinatriuretic

FIG. 4. Working formulation for the role of ANF in the renal sodium retention of cirrhosis. The primary hepatic abnormality necessary and sufficient for renal sodium retention is hepatic venous overflow blockade. In early disease, this signals renal sodium retention, with consequent intravascular volume expansion and a compensatory rise in plasma ANF level. At this stage of disease, the increase in ANF level is sufficient to counterbalance the primary antinatriuretic or renal sodium retaining influences; however, it does so a t the expense of an expanded intravascular volume, with the potential for overflow ascites. With progression of disease, disruption of intrasinusoidal Starling forces and loss of volume from the vascular compartment in the peritoneal compartment occur. This underfilling of the circulation may attenuate further increases in ANF levels and promotes the activation of antinatriuretic factors. Whether the antinatriuretic factors activated by underfilling are the same as or different from those that promote primary renal sodium retention in early disease remains to be determined. At this later stage of disease, elevated levels of ANF may not be sufficient to counterbalance antinatriuretic influences. (Modified with permission from “The role of resistance to atrial natriuretic peptide and the pathogenesis of sodium retention and hepatic cirrhosis” by Warner LC. In: Brenner BM, Laragh J, eds. Progress in atrial peptide research. Vol 3. New York: Raven Press, 1989:185-203. Copyright 0 1989 by Raven Press, Ltd.).

below some critical level. It is possible that the renal activity of ANF may be potentiated by inhibition of endopeptidase 24-1 1 by a mechanism involving the accumulation of bradykinin (108,109).

Urodilatin, mentioned previously, is a homologous natriuretic peptide of renal origin first discovered in 1988 (110). The prohormone produced by the kidney tubules may be identical to the 126-residue prohormone of atrial natriuretic peptide (111). This prohormone is cleaved to urodilatin, a 32-residue peptide (identical to circulating ANF with the addition of four amino acids and a NH,-terminal extension) found in urine but not in plasma. Urodilatin has a natriuretic potency at least equal to that of ANF. It is unclear whether under physiological circumstances urodilatin has primacy over ANF in governing natriuretic responses (111). It may be the physiological ligand for more distal ANF receptors.

Drummer et al. (112) found that urodilatin excretion closely parallels circadian renal sodium excretion in nine healthy human subjects and also correlates well with natriuresis after saline solution infusion. They found no similar correlation with plasma ANF and concluded that urodilatin was the important peptide in the physiological regulation of natriuresis. It may well be that ANF is predominantly active in terms of its vasoactive prop- erties in the kidney and elsewhere in the body, whereas urodilatin may mediate the epithelial modulatory responses.

It has been postulated that brush-border peptidases prevent peptides such as ANF from reaching the distal nephron and causing an inappropriate natriuresis (113). In contrast, urodilatin is resistant to degradation by these enzymes and is produced beyond their site of action (112).

We are unaware of studies assessing urodilatin responsiveness in cirrhotics patients. Additional studies are needed to delineate the role of urodilatin in modu- lating sodium homeostasis in normal subjects and in mediating the sodium retention of cirrhosis.

SUMMARY A working formulation for the role of ANF in the

sodium retention of cirrhosis is summarized in Figure 4. Sodium retention is initiated early in cirrhosis, either as a result of hepatic venous outflow block (114,115) or of primary vasodilation (75). The consequent intravascular volume expansion causes increases in ANF levels. At this stage of disease, the rise in ANF level is sufficient to counterbalance the antinatriuretic influences. However, this occurs at the expense of an expanded intravascular volume with the potential for overflow ascites. With progression of disease, disruption of intrasinusoidal Starling forces and loss of volume from the vascular compartment into the peritoneal compartment occur (1 15). This underfilling of the circulation may attenuate further increases in plasma ANF and promotes the

510 WARNER ET AL.

activation of antinatriuretic factors. At this later stage of disease, elevated levels of ANF are insufficient to counterbalance antinatriuretic influences. Thus the role of ANF in cirrhosis is primarily beneficial in that it successfully attenuates the antinatriuretic forces in the compensated stage.

Raised ANF levels have two potential deleterious effects. First, ANF may exacerbate arterial vasodilatation, leading to further sodium retention. The primacy of vasodilatation has been proposed as an alternate formulation to the overflow and underfill hypotheses (76). Second, Epstein et al. (24) found higher basal ANF levels in cirrhotic patients with edema than in those patients without edema. ANF is known to reduce plasma volume in anephric animals (116) and to increase the ultrafiltration coefficients of isolated capillaries (1 17). Therefore it is conceivable that in the clinical setting in which antinatriuretic factors limit the renal responsiveness to ANF but in which ANF levels are elevated Le., cirrhosis, congestive heart failure, primary kidney disease), ANF itself may contribute to edema formation at the level of the peripheral microcirculation. In general, ANF likely has no primary role in the sodium retention in cirrhosis. In early compensated cirrhosis, ANF may maintain sodium homeostasis despite the presence of mild antinatriuretic factors. In late ascitic cirrhosis renal resistance to ANF develops, rendering it ineffective.

Future studies should address the biochemical mechanisms that mediate ANF resistance. More information will be forthcoming from Levy and coworkers on the interaction of bradykinin with ANF to cause natriuresis. Deficiencies in renal kinins may be important in advanced cirrhosis and in other sodium-retaining states.

Another potential area of research involves the iden- tification of antinatriuretic factors in early preascitic cirrhosis. The finding of an association between corrected sinusoidal pressures and steady-state plasma ANF levels (35) suggests that intrasinusoidal hypertension stimulates sodium retention. In patients, preascitic cirrhosis has been associated with a trend toward elevated muscle sympathetic nerve activity compared with such activity in normal subjects, which did not reach statistical significance (92). It remains to be established in animal models whether intrasinusoidal hypertension leads to sodium retention initially through increased renal sympathetic nerve activity or through other mechanisms. In portal vein-ligated rats, plasma ANF levels decreased by 50% (118). A more appropriate model to investigate the relationship between ANF and cirrhosis might be hepatic vein ligation, which would lead to an increase in intrasinusoidal hypertension. Insights gleaned from studies of ANF action in cirrhosis suggest several potential therapeutic prospects. En- dopeptidase inhibitors, which augment ANF and kinin delivery distally, appear to show promise in preliminary studies in nonresponders. ANF infusion alone has been disappointing in that the patients who would benefit the most from the peptide are resistant to it. ANF may be much more effective when coupled with an endopep-

HEPATOLOGY March 1993

tidase inhibitor or other agents; together these may find a role as temporary supportive measures in the man- agement of patients with hepatorenal syndrome. Fi- nally, urodilatin, which is resistant to breakdown by brush-border enzymes, may have added therapeutic potential when used with bradykinin.

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