an experimental renal acidification...

An Experimental Renal Acidification Defect

in Patients with Hereditary Fructose Intolerance

II. ITS DISTINCTION FROMCLASSIC RENALTUBULAR

ACIDOSIS; ITS RESEMBLANCETO THE RENAL

ACIDIFICATION DEFECTASSOCIATEDWITH THE

FANCONI SYNDROMEOF CHILDREN WITH CYSTINOSIS

R. CURTIS MoRms, JR., with the technical assistance of Ibs UEKI

From the Departments of Medicine and Pediatrics, University of California School ofMedicine, San Francisco, California 94122

A B S T R A C T In adult patients with hereditaryfructose intolerance (HFI) fructose induces arenal acidification defect characterized by (a) a20-30% reduction in tubular reabsorption of bi-carbonate (T HCO:-) at plasma bicarbonate con-centrations ranging from 21-31 mEq/liter, (b) amaximal tubular reabsorption of bicarbonate (TmHCO3-) of approximately 1.9 mEq/100 ml ofglomerular filtrate, (c) disappearance of bicarbo-naturia at plasma bicarbonate concentrations lessthan 15 mEq/liter, and (d) during moderately se-vere degrees of acidosis, a sustained capacity tomaintain urinary pH at normal minima and toexcrete acid at normal rates. In physiologic dis-tinction from this defect, the renal acidification de-fect of patients with classic renal tubular acido-sis is characterized by (a) just less than completetubular reabsorption of bicarbonate at plasma bi-carbonate concentrations of 26 mEq/liter or less,(b) a normal Tm HCO3- of approximately 2.8mEq/100 ml of glomerular filtrate, and (c) dur-ing acidosis of an even severe degree, a quantita-

This work was presented in large part at the generalsession of the national meeting of the American Societyfor Clinical Investigation, Atlantic City, April, 1966.

Address requests for reprints to Dr. R. Curtis Morris,Jr., Department of Medicine, University of CaliforniaSchool of Medicine, San Francisco, Calif. 94122.

Received for publication 6 December 1967 and in re-

vised form 22 March 1968.

tively trivial bicarbonaturia, as well as (d) a uri-nary pH of greater than 6.

That the fructose-induced renal acidification de-fect involves a reduced H+ secretory capacity ofthe proximal nephron is supported by the magni-tude of the reduction in T HCO3- (20-30%o) andthe simultaneous occurrence and the persistencethroughout administration of fructose of impairedtubular reabsorption of phosphate, alpha aminonitrogen and uric acid.

A reduced H+ secretory capacity of the proximalnephron also appears operative in two unrelatedchildren with hyperchloremic acidosis, Fanconi'ssyndrome, and cystinosis. In both, T HCO3- wasreduced 20-30%o at plasma bicarbonate concentra-tions ranging from 20-30 mEq/liter. The bicarbo-naturia disappeared at plasma bicarbonate con-centrations ranging from 15-18 mEq/liter, andduring moderate degrees of acidosis, urinary pHdecreased to less than 6, and the excretion rate ofacid was normal.

INTRODUCTION

So-called renal tubular acidosis (RTA) is a clini-cal disorder of renal acidification expressed bio-chemically as a characteristic syndrome that in-cludes minimal or no azotemia, hyperchloremia,metabolic acidosis, yet alkaline or minimally acid

1648 The Journal of Clinical Investigation Volume 47 1968

urine (1-5). This syndrome, when combined withreduced rates of excretion of titratable acid andammonium, is generally considered diagnostic ofRTA (1-5). This combination can be reproducedexperimentally in adults with hereditary fructoseintolerance (HFI) by administration of fructoseafter ammonium chloride loading (6).

To further compare the physiologic character-istics of the experimental renal acidification defectwith those of clinical renal acidification defects,the renal tubular reabsorption of bicarbonate(T HCO3-) and urinary acid excretion were mea-sured over a wide range of plasma bicarbonateconcentration in four patients with hereditaryfructose intolerance during administration of fruc-tose, in four patients with classic RTA,1 and intwo with the Fanconi syndrome, cystinosis, andhyperchloremic acidosis. The renal acidificationdefect induced by fructose, unlike that of patientswith classic RTA (2-5), was characterized bya significant reduction in the maximal tubular re-absorption of bicarbonate (Tm HCOJ-) and, dur-ing moderately severe degrees of acidosis, by aurinary pH of less than 5 and normal rates of acidexcretion. An acidification defect of similar physio-logic character was demonstrated in the patientswith cystinosis and Fanconi's syndrome. In fur-ther comparison, an impairment in the renal tubu-lar reabsorption of alpha amino nitrogen, phos-phorus, and uric acid accompanied the fructose-induced acidification defect of patients with HFI.

METHODSA total of 34 studies were carried out. All were startedin the early morning with the subjects fasting. All fe-male subjects were comfortably supine during the courseof each study which involved long intravenous infusions;urine was collected at 10- to 20-min intervals via an in-dwelling catheter that emptied under a layer of mineraloil. In the male subjects, voided urine was collected un-der mineral oil. At appropriate intervals blood sampleswere collected for determination of pH and CO2 tension(Pco2). Arterial blood was drawn from a brachial arteryvia a Cournand needle, or arterialized blood was drawnfrom a superficial vein on the back of the hand, that hadbeen heated with an electric heating muffler to 450C orgreater for more than 30 min.

In most studies the concentration of bicarbonate in theplasma was manipulated by continuous intravenous infu-

1 Classic RTA is used here to mean unremitting RTAunassociated with impaired tubular reabsorption of aminoacids or glucose, and characteristically associated withnephrocalcinosis.

sion of a 3.75% solution of sodium bicarbonate adminis-tered with a constant infusion pump. In many of thesestudies the plasma bicarbonate concentration was in-creased before the infusion of bicarbonate by oral ad-ministration of sodium bicarbonate. Inulin clearance wasmeasured throughout the infusion periods.

Fructose studies

Subjects. The subjects were four patients with HFI(D.M., E.A., A.H., who had also been studied in anearlier investigation (6), and L.R., the 36 yr old sisterof A.H.) and seven control subjects (D.M.'s son, K.M.,a 13 yr old fructose-tolerant boy presumed to be heterozy-gous for the defect of HFI, M.P., a 45 yr old womanwith classic RTA, and five normal subj ects ranging inage from 27 to 46 yr).

Procedure. 16 of the 18 studies in this series includedthe intravenous administration of two fructose solutions:a priming dose of 4-13 g was given as a 25% solutionover an interval of 6-9 min, and a sustaining infusionof 4.5-12 g/hr was given as a 9 or 10% solution for atleast 1 hr. Throughout the administration of fructose tothe patients with HFI and for at least 3 hr thereafter, a10%o solution of glucose was administered at a constantrate calculated to deliver 0.06-0.12 g/kg per hr.

In four studies on three patients with HFI (studies1-4) and in single studies on each of the control sub-jects, fructose was administered after the plasma bicar-bonate concentration had been increased to levels greaterthan that of Tm HCOs-. In a single study on one patient(study 5), fructose was administered at a normal plasmabicarbonate concentration. In study 1 and in studies onthe control subjects, sodium bicarbonate was infused at asingle constant rate of 0.764 ml/min. In studies 2-5, therate of infusion of sodium bicarbonate was increasedwhen fructose was begun in order to prevent or minimizea decrease in the plasma bicarbonate level such as oc-curred in study 1. In one study on a patient (E.A.), glu-cose (without fructose) was administered intravenouslyat a rate calculated to induce sustained hyperglucosemia.The control subjects received 1.3-1.7 times as much fruc-tose as the average amount administered to the patients.

In three studies on E.A. (studies 6-8), fructose wasbegun during moderately severe metabolic acidosis in-duced by ammonium chloride administered orally (0.18-0.2 g/kg). In two of these studies (studies 6 and 7), theplasma bicarbonate concentration was progressively in-creased by intravenous infusion of sodium bicarbonate.In two other studies in which a mild degree of acidosiswas induced by ammonium chloride (0.1 g/kg),- eitheronly a prime of fructose was given (study 9, D.M.), orthe amount of fructose administered was half that ad-ministered previously during a comparable degree ofacidosis (study 10, E.A.) (6).

Nonfructose studies

Subjects. The subjects were patient M.P., three otherpatients with classic RTA, one of whom was a child,

Renal Acidification Defect in HFI: Distinction from Classic RTA 1649

and two children with Fanconi's syndrome and cystinosis the higher range, the plasma bicarbonate concentration(Table I). was increased approximately 1 mEq/liter per hr. At the

Procedure. Renal reabsorption of bicarbonate was de- time of each study, the patients with RTA had beentermined in single studies on each of the adult patients maintained in a nonacidotic, normokalemic state for atwith RTA. In each study the plasma bicarbonate con- least 1 yr. Blood volume was neither reduced nor in-centration was increased from normal values to values creased in the adult patients as measured with 'I-labeledgreater than those of the maximal tubular reabsorption albumin. In other reported studies in which renal reab-of bicarbonate (Tm HCOs-). In three separate studies sorption of bicarbonate was measured in patients withon the child with RTA, bicarbonate reabsorption was RTA (2, 3, 5), TmHCO3- could have been increased bymeasured at three different ranges of plasma bicarbonate: potassium depletion (12). At the time the measurements11-18, 23-26, and 28-32 mEq/liter. In the two studies at were made the serum potassium level was either re-

TABLE IClinical and Physiologic Data in Patients with Renal Acidification Defects

Urinary excretionRenal acidificationt

GlucosePatient,* age (yr) UpHmin a-Amino a-Amino N. (glucose

and sex Clinical diagnosis (CO2) UTAVmaX UNHI4Vma. N§ creatinine N§ oxidase) GFR

,uEq/min mg/24 hr g/g ml/min per1.73 m2

Adult patientsNormal values <5.31 >25.0 >39.0 50-150C. V. Classic renal 6.48 12.9 28.1 89.6 Negative 48.851 tubular acidosis, (13)F nephrocalcinosisB. M. C. Classic renal 6.80 13.0 14.0 76.0 Negative 45.032 tubular acidosis, (17)F nephrocalcinosis

M. P. Classic renal 6.33 12.8 21.8 122.0 Negative 120.043 tubular acidosis, (19)F nephrocalcinosis

ChildrenNormal values (a) <5.60 >13.9 >45.7 0.30-0.62

(b) <4.90 >47.0 >76.0C. M. G. Classic renal 6.72 13.7 17.3 0.318 Negative 46.46-8 tubular acidosis, (10)F nephrocalcinosis

T. B. Cystinosis, Fanconi 5.90 36.5 68.7 3.070 1+-2+ 73.11.5-3 syndrome, hyper- (18.5)F chloremic acidosisE. S. Cystinosis, Fanconi 5.29 70.3 75.0 1.300 1+-4+ 36.42-6 syndrome, hyper- (13.3)M chloremic acidosis

UpHmin, minimal urinary pH (numerals in parentheses indicate lowest measured total serum CO2 content in mmoles/liter); UTAHV. and UNH4VinS, maximal rate of excretion of titratable acid and of ammonium, respectively (values inchildren were corrected to a standard body surface area of 1.73 M2); GFR, glomerular filtration rate measured as inulinclearance (average of at least three successive 20-min urine collections).* Data on three of the patients have been reported previously: B. M. C. (7), C. V., and C. M. G. (8).t Renal acidification response during existent acidosis or after administration of a single dose of 0.1 g of NH4Cl/kgorally; procedure of Wrong and Davies (4). Normal values for adults were established in a previous study (7); normalvalues for children were derived by measurements (a) on the last day of a 3-5 day period of ammonium chloride-inducedacidosis in children aged 1-16 yr (9), (b) after a single dose of ammonium chloride (0.029 g/kg) in children aged 7-12yr (10).§ Ninhydrin method (11).

1650 R. C. Morris, Jr.

duced or not stated, or the possibility that chronic aci-dosis may have caused persisting potassium depletioncould not be evaluated from the data reported.

The boy and girl with Fanconi's syndrome were stud-ied on three and four occasions, respectively. As in thechild with RTA, bicarbonate reabsorption was measuredat different ranges of plasma bicarbonate in separatestudies. In most of these studies the plasma bicarbonateconcentration was increased at a rate no greater than 1mEq/liter per hr.

Laboratory methods. Laboratory determinations werecarried out as described previously (6). Plasma bicarbo-nate was calculated from the arterial pH and Pco2 by theHenderson-Hasselbalch equation; pK was taken as 6.1and a as 0.0301. The renal reabsorption and excretion ofbicarbonate were plotted according to the method ofPitts, Ayer, and Schiess (13). The Donnan equilibriumand the transit time between glomerulus and urinary

FRUCTOSE

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RESULTS

Fructose studies

Renal acidification. When fructose was with-held from the patients with HFI, the bicarbonatetitration curves obtained were like those of normalsubjects, and the values of Tm HCO3- were 2.8(E.A.), 2.7 (L.R.), and 2.5 (A.H.) mEq/100 mlof glomerular filtrate (Fig. 1), the last value beingperhaps slightly below normal. Within 40 minafter beginning fructose, tubular reabsorption ofbicarbonate decreased 20-30%7o in each patient.This magnitude of decrease occurred throughout

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-60 -30 0 30 60 90TIME (MINUTES)

FIGURE 1 Effect of administering varying amounts of fructose on blood fructose levelsand renal tubular reabsorption of bicarbonate (T HCOs-) in four studies on threepatients with hereditary fructose intolerance. f = study 2, 0 = study 3, 0 = study 4,O = study 5. Alkalosis was produced by bicarbonate infusion (before the data shown)in all control and experimental subjects except in study 5. Each pair of vertical bars(top left) indicates the amount of fructose administered as a priming dose (P) andas a sustaining infusion (S) in each study. Open and closed symbols indicate beforeand during fructose infusion, respectively. The smaller, bracketed closed circles indi-cate the mean and standard deviation of values in seven control subjects.


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FIGURE 2 Relationship between plasma concentration, renal tubular reabsorption, and urinary excre-tion of bicarbonate before and during administration of fructose in two patients with hereditaryfructose intolerance. A = study 1, 0 = study 2, 0 = study 3, 0 = study 5 at plasma bicarbonateconcentrations greater than 20 mEq/liter and study 6 at lesser concentrations, V = study 7. Open andclosed symbols indicate before and during infusion of fructose, respectively. Fully closed symbols indi-cate measurements made at blood fructose concentrations greater than 15 mg/100 ml, and speckledsymbols indicate concentrations less than 15 mg/100 ml. The "titration curves" were drawn by eye.Half-closed symbols denote the initial period after beginning fructose. Open rectangles indicate valuesbefore and hatched rectangles indicate values during infusion of glucose without fructose.

the range of plasma bicarbonate studied (21-31mEq/liter) and persisted for as long as bloodfructose remained at levels greater than 15 mg/100 ml. At such levels Tm HCO:- was approxi-mately 1.9 mEq/100 ml of glomerular filtrate(Fig. 2, Table II). In general, the magnitude ofreduction in T HCO3- varied directly with theblood fructose concentration. TmHCO- was notreduced by blood glucose concentrations in excessof 250 mg/100 ml sustained for 1 hr. With theexception of study 1 (Fig. 2), in which the plasmabicarbonate concentration decreased from 32.6 to24.1 mEq/liter during administration of fructose,plasma bicarbonate levels changed little during thedecrease in bicarbonate reabsorption.

When fructose was administered to patient E.A.during moderately severe acidosis, urinary pH re-mained less than 5 (studies 6 and 7) or increasedtransiently to 5.5 (study 8) (Table III, Figs. 3

and 4); the excretion rate of acid remained nor-mal. In study 6 the blood fructose concentrationranged from about 45-20 mg/100 ml; this rangewas comparable to that maintained in studies onE.A. and L.R. in which fructose was administeredat high or normal plasma bicarbonate concentra-tions (studies 2, 4, and 5). When plasma bicar-bonate concentration was increased from 13 to 16.6mEq/liter in study 6, urinary pHI was successively4.5, 4.75, 5.2, 5.3, and 6.3, and urine flow variedlittle. Bicarbonaturia occurred at a plasma bicar-bonate concentration of 16.6 mEq/liter and, alongwith urinary pH, increased briskly as the plasmabicarbonate concentration was increased further.As urinary pH increased to values progressivelygreater than 5.3, the rate of excretion of ammo-nium and titratable acid became progressivelysubnormal for the state of acidosis. In study 7blood fructose was maintained at lower levels than


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FIGURE 3 Effect of continuous intra-venous infusion of fructose on urinary pHand bircarbonate in two studies on a pa-tient with hereditary fructose intoleranceand ammonium chloride-induced acidosis.O = study 6, V = study 7. Open andclosed symbols, same as in Fig. 2. Duringthe studies the degree of acidosis wasprogressively -lessened by continuous in-travenous administration of sodium bicar-bonate. In study 7, a 10% solution offructose was infused at 0.764 ml/min for205 min with a prime of 16 ml of a 25%solution of fructose over the first 7 min.Details of study 6 are given in Table III.GFR, glomerular filtration rate. UV, uri-nary excretion.

in study 6, but the response to bicarbonate loadingwas qualitatively similar; the reduction in THCO3- was less but comparable to that occurringin study 2 after the blood fructose concentrationhad decreased to values of less than 15 mg/100 ml(Fig. 2).

When fructose was administered to the patientswith HFI during metabolic acidosis, urinary pHincreased regardless of the plasma bicarbonateconcentration or the amount of fructose adminis-tered. For a given range of blood fructose, themagnitude of increase in urinary pH varied di-rectly with the plasma bicarbonate level (Fig. 4).For a given range of plasma bicarbonate concen-trations, the smaller the blood fructose concentra-tion, the smaller was the increase in urinary pH(Fig. 4).

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mained at normal concentrations throughout thefructose infusions, and the glomerular filtrationrate decreased slightly but transiently.

In both the patients with HFI and the subjectswithout HFJ, urinary excretion of sodium in-creased during administration of fructose andsodium bicarbonate (Fig. 5). Predictably, the rateof sodium excretion was greater in the studies onpatients with HFI in which the rate of infusionof sodium bicarbonate was increased when fructosewas administered (studies 2-5) than in the studieson subjects without HFI and in study 1 on apatient with HFI (E.A.). Since the rate of urinarysodium excretion in study 1 approximated the ratein the five normal subjects during fructose admin-istration, the reduction of Tm HCO3- in the pa-tients with HFI cannot be related to osmoticdiuresis.


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findings indicate that fructose induced a greaterdegree of impairment of tubular reabsorption ofphosphorus in the patients with HFI than in thecontrol subjects.

In the patients with HFI, the mean rate ofexcretion of alpha amino nitrogen increased strik-ingly during administration of fructose, 173 ug/min as compared with 24 jug/min in the controlgroup (Table IV). In each of the four studies onthe patients, the plasma alpha amino nitrogen levelincreased slightly during administration of fruc-tose; however, with one exception (E.A.) thefiltered load of alpha amino nitrogen actually de-creased because the glomerular filtration rate de-creased slightly. Thus, in three of the four studies,urinary excretion of alpha amino nitrogen in-creased despite a decrease in filtered load. In thecontrol subjects (T.M. and M.B.) in whom uri-nary excretion of alpha amino nitrogen increased(> 100 ,g/min), the filtered load increased mark-edly (> 1000 /ig/min), but the fraction of thefiltered load reabsorbed remained the same. These

--10 ~15 20U 25PLASMAHC03- (mmoles/liter).

FIGURE 4 Magnitude of fructose-induced increase inurinary pH as a function of plasma bicarbonate con-

centration and amount of fructose administered in 12studies on two patients with hereditary fructose intoler-ance. Each bar indicates urinary pH values immediatelybefore the fructose infusion and 30-40 min after its initi-ation. Each value was calculated from the average H'concentration derived from measurements of urinary pHin three successive 15-20 min periods. 0 =nine studieson E.A., = three studies on D.M. Closed symbols indi-cate maintained blood fructose concentrations of greaterthan 25 mg/100 ml. Half-closed circles indicate a range

of blood fructose concentrations of 25-15 mg/100 ml andopen symbols a range of 5-14 mg/100 ml. 6 of the 12studies were done during a previous investigation (6).

Other tubular functions in subjects made alka-lotic (Table IV). In both the HFI patients andcontrol subjects the clearance of phosphorus in-creased, and the fractional reabsorption of phos-phorus decreased during administration of fruc-tose. In each of four studies on the patients withHFI (studies 1-4), the serum phosphorus con-

centration decreased during fructose administrationto less than 0.6 mmoles/liter, a level at which theurine becomes virtually free of phosphorus innormal individuals (14). The serum phosphoruslevel did not decrease in the control group. These

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UKV

(pEq/min)

-90 -60 -30 0 30 60 90

TIME (MINUTES)

FIGURE 5 Effect of administering fructose during sodiumbicarbonate loading on the rate of urinary electrolyte ex-

cretion (U . . . V) in patients with hereditary fructoseintolerance and control subjects. C= study 1, 0 = studies2-5, 0 = control studies.


7.5

7.0

URINE pH

6.S-

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't- % ~ 006 iC-4 U C'a¢N -- 6 o

cU -~~~~~~~~~~~~~~~~cCCaa

.t~~~~~~~~~~~~~~~~~0.0s@ NN- +U) D0 )Hc 0 eiCa-0 m m+ t 0 O e°~~~~~~~~~~~-cccct. 0cct- .4O00 t~- ~

9 U) mmo+ e

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w eX =~~~~~~~~~~~~~~~~~c m CK 0 r cq \0-o0 \0 00o.IU

x0 --tv \c a o 0o C) U*;o0 O cs U - + 0% 000 00W OC otn t 00 °i Q

0~~~~~~~~~~~~~~~~~~~ .0

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b-C)¢ -~~ '~1~)0%O t.o-~t..0 + O 0

° 1-F i-W-- -.*

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r-tot-.t.a.0 0 o-da0 -4 (L)

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06 wU :: ¢ *~~.~~ oa--'o-m\ ND6e"

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od 00~ o6i 00 -U. 0

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q. 6 l Ca >

3~~~~~~~2.4666 6-66-~~~~~~~~~~~~~~~~~~~~~~~ 1.0 a4) \ aCC) a) - ~~~~~~~~~~~~~~~~~C4+ C t - l 0 4-

rn O 0 C co0~a)000\e 0,00 ~ ~ ~ ~ ~ ~ 0 a

02 ~~ ~~~~~ ~~~~~~~~~0 '=~~~~~ ~ Ca Ca~~~~~~~~~~I~~L bio 4)- C) Ca

V-q E. a) . .CdC: co C cd &. L

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c

E

UA

8

z0-

4

z

4b.-

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4

11

ic

9

8

7

6

5

4

3

2

0 1 2 3 4 5 6 7 8 9 10FILTERED LOADOF URATE(mg/min)

11 12

FIGURE 6 Change in calculated net tubular reabsorptionof uric acid as a function of increased filtered load of uricacid in patients with hereditary fructose intolerance(HFI), in control subjects infused with fructose, and innongouty subjects infused with uric acid. 0 = patientswith HFI, C1 = control subjects, A = nongouty patients(data of Yui, Berger, and Gutman [15]). Open andclosed symbols indicate before and during infusion,respectively.

0

ma.

a

20

VE0

5E

0

0

Z.0

02E

LU

4Z0

4Vra

findings indicate that fructose resulted in renalaminoaciduria only in the patients with HFI.

During fructose administration, urinary excre-tion of uric acid increased markedly in the patientswith HFI and minimally in the control subjects(Table IV). The serum uric acid level increasedboth in the patients and the control subjects. Thefiltered load of uric acid increased in each of thepatients and in two of the controls; in the two con-trol subjects, the calculated net tubular reabsorp-tion of uric acid increased commensurately (Fig.6). A similar relationship between net tubularreabsorption and filtered load of uric acid (at fil-tered loads less than 10 mg/min) has been ob-served in nongouty subjects in whom the filteredload was increased by intravenous infusion of uricacid (15). In two of the four studies on the HFIpatients (E.A., A.H.), however, the calculated nettubular reabsorption of uric acid decreased as thefiltered load increased. In the other two studies,the rate of increase in calculated net tubular reab-sorption was distinctly less than that in the controlsubjects over a similar range of filtered loads.

In the son of D.M. and in RTA patient M.P.,renal handling of phosphorus, alpha amino nitro-

20 22 24 26 28PLASMAHCO3- (mmoles/liter)

FIGURE 7 Relationship between plasma concentration, renal tubular reabsorption, andurinary excretion of bicarbonate in a child with classic renal tubular acidosis (0)and in two children with hyperchloremic acidosis associated with cystinosis and theFanconi syndrome (A = T.B., V = E.S.).


u

/O

I I I

0A

is ** -*A __ _

2.41.

2.0F

1.2

0.8k-

I I1- I I I I I I I I I10 12 14 16 18 20 22 24 26 28

PLASMAHCOj (mmoles/liter)

30 32 34

FIGURE 8 Relationship between renal tubular reabsorption and plasma concentration of bi-carbonate in eight adult patients with classic renal tubular acidosis. * = three patients inpresent study, *, A, * = 1, 1, and 3 patients, respectively, studied by other investigators(2, 3, 5).

gen, and uric acid was qualitatively and quantita-tively similar to that in the five normal subjects.

Nonfructose studies

In the child and adult patients with classicRTA, the general configuration of the bicarbonatetitration curve was like that of the curve obtainedin normal subjects (Figs. 7 and 8). Tm HCO3-was approximately 2.8 mEq/100 ml of glomerularfiltrate, a normal value. The tubular reabsorptionof bicarbonate, however, was incomplete at plasmabicarbonate concentrations as low as 11 mEq/liter.

In the two children with cystinosis and Fanconi'ssyndrome, the Tm HCO8- was approximately 2.0mEq/100 ml of glomerular filtrate (Fig. 7). Inthe girl, tubular reabsorption of bicarbonate was

complete at a plasma bicarbonate concentration of18 mEq/liter and in the boy at a concentration of15 mEq/liter.

DISCUSSION

In adult patients with classic RTA the urinarypH is inappropriately high during severe as wellas mild degrees of acidosis, and persisting urinary

excretion of bicarbonate is characteristic (1-5).

The amount of bicarbonate excreted, however, is

a trivial fraction of that filtered until the plasmabicarbonate concentration is experimentally in-creased to levels at which tubular reabsorption ofbicarbonate is normally maximal, 26-28 mEq/liter; Tm HCO,- is not reduced, and the bicar-bonate titration curve is little splayed (3, 5). Thisobservation permits two inferences: (a) the totalH+ secretory capacity is not reduced (3, 5), and(b) the rate at which bicarbonate is reabsorbedby the proximal tubule is not greatly reduced(3, 5). The demonstration in one patient withRTAthat the rate of bicarbonate excretion varieddirectly with urinary flow (increased by waterdiuresis) (5) has been interpreted as evidenceagainst leakage of bicarbonate by the proximalnephron at a small fixed rate sufficient to over-

whelm the acidification process of the distalnephron. The measured constancy of urinary pHand bicarbonate levels over a wide range of urineflow is consistent with the generally inferredoperational mechanism of RTA: an inability ofthe distal nephron to maintain normally steeplumen-peritubular H+ gradients (3, 5, 16).

The results of the present investigation indicate

Renal Acidification Defect in HFI: Distinction from Classic RTA

0

* ==

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'no

2

2

0.41-

.e- - - OF-

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1659

that in patients with HFI, fructose induces a renalacidification defect that is physiologically distinctfrom the defect in patients with classic RTA.(a) At plasma bicarbonate concentrations of lessthan 14 mEq/liter, tubular reabsorption of bicar-bonate in the patients with HFI was complete andurinary pH was less than 5.0. (b) As plasma bi-carbonate was experimentally increased from 13-18.7 mEq/liter, urinary pH increased progres-sively to 7.4; urinary excretion of bicarbonateoccurred at a plasma bicarbonate concentration of16.6 mEq/liter and increased sharply thereafter.(c) At plasma bicarbonate concentrations rangingfrom 21-31 mEq/liter, T HCO2- was reduced20-30%. The Tm HCO3- of 1.9 mEq/100 ml ofglomerular filtrate indicates a subnormal H+secretory capacity (5).

The 20-30% reduction of T HCO3- at plasmabicarbonate concentrations ranging from 21-31mEq/liter almost certainly implicates the acidifica-tion process of the proximal nephron. A reductionin T HCO3- of this magnitude presumably wouldnot occur even if the acidification process of thedistal nephron were completely obliterated, sincethe proximal nephron accounts for 85-90%o of therenal reabsorption of bicarbonate in the monkey(17) and the rat (18) at normal concentrations ofplasma bicarbonate. The continuing ability of thedistal nephron to achieve normally steep lumen-peritubular H+ gradients is indicated by the uri-nary pH values of less than 5 at plasma bicar-bonate concentrations of 14 mEq/liter or less. Itcould be argued that the continuing ability ofthe distal nephron to achieve normally steep lumen-peritubular H+ gradients does not preclude a largereduction in the rate at which the distal nephroncan secrete H+ and some reduction of Tm HCO3-on that basis. But the H+ secretory capacity of thedistal nephron cannot be greatly reduced in thefructose-induced renal acidification defect. If itwere, brisk bicarbonaturia would occur when theproximal nephron rejected an amount of bicar-bonate only slightly greater than that which justperceptibly titrated the H+ secreted by the distalnephron. Accordingly, since urinary pH increasedfrom 4.5 to 4.75 when the plasma bicarbonate con-centration was increased from 13 to 13.9 mEq/liter, one would have expected urinary pH toincrease from 4.75 to greater than 7 over a narrowrange of further increase in the plasma bicarbonate

level. Instead, such a range of increase in urinarypH required an increase in the plasma bicarbonateconcentration from 13.9 to 18.5 mEq/liter.

The proximal nephron is clearly implicated bythe impairments in the tubular reabsorption ofalpha amino nitrogen, phosphate, and uric acidthat occurred within minutes after the initiation offructose infusions in the patients with HFI. Theseimpairments and the impairment in tubular reab-sorption of bicarbonate occurred simultaneouslyand persisted for as long as fructose was admin-istered. Over the time course of each study, themagnitudes of these impairments changed essen-tially in parallel. In general, the magnitude oftubular dysfunction varied with the blood fructoselevel (Figs. 1, 2, and 3). These findings and thefact that the enzymatic defect of HFI occurs inthe renal cortex (19) provide strong support forthe hypothesis that both the impairment in tubu-lar reabsorption of bicarbonate and the proximaltubular dysfunction are caused by a fructose-induced, dose-dependent abnormality of renalmetabolism unique to patients with HFI.

The abnormality of renal metabolism may affectonly the renal cortex and spare the medullaryportion of the distal nephron. The metabolic ab-normality presumably depends on intracellularaccumulation of fructose-l-phosphate (F-1-P)(20), which results from the virtual absence ofaldolase activity against F-1-P in the liver andrenal cortex of patients with HFI. Mammalianrenal cortex, like liver, normally extracts fructosereadily and converts it to glucose (21, 22) appar-ently only via F-1-P and the products of its aldo-lase cleavage (21). But renal medulla, like somaticmuscle (23), extracts fructose sparingly (22),converts none to glucose (22), and normally hasbut a small fraction of the aldolase activity towardF-1-P demonstrated in the cortex.2 Quite con-ceivably then, only the cortex has the metabolicpotential for accumulating F-1-P in pathogeneticamounts.

The physiologic characteristics of the renal acidi-fication defect induced by fructose are consistentwith a rate defect of H+ secretion limited to theproximal nephron. With such a defect, urinary pHduring acidosis will be appropriately low or in-appropriately high depending on the relative reduc-

2 Morris, R. C., Jr. Unpublished observations.


tions of plasma bicarbonate concentrations andT HCO3- (Figs. 2 and 3). Since the reductionof T HCO3- varies directly with the blood fructoselevel, urinary pH will also vary directly with theblood fructose level, as well as with the plasmabicarbonate concentration (Figs. 3 and 4).

Such a formulation explains what might other-wise appear to be inconsistent responses of urinarypH to the administration of fructose during mod-erately severe acidosis: the increase in urinary pHfrom 4.7 to 5.5 in one study and the persistence ofurinary pH at values of less than 5 in two otherstudies (Fig. 4). With blood fructose levels in therange of 25-15 mg/100 ml, Tm HCO,- was 1.9mEq/100 ml of glomerular filtrate, bicarbonatethreshold was 16 mEq/liter, and urinary pH wasgreater than 5 at a plasma bicarbonate concentra-tion of 14.3 mEq/liter and less than 5 at a plasmabicarbonate concentration of 13.9. With the demon-strably achievable Tm of 1.7 at blood fructoselevels greater than 25 mg/100 ml (Fig. 1), bicar-bonate threshold would probably be approximately14 mEq/liter, and urinary pH would predictablybe greater than 5 at a plasma bicarbonate concen-tration of 13 mEq/liter.

A reduction in the H+ secretory capacity of theproximal nephron also appears operative in thetwo children with hyperchloremic acidosis, Fan-coni's syndrome, and cystinosis. The renal acidifi-cation defect of these children, like that inducedby fructose in patients with HFI but unlike thatof the child and adults with RTA, was character-ized by a 25-30%o reduction in Tm HCO3- andnormal rates of acid excretion and disappearanceof bicarbonate during moderate degrees of acidosis.The "swan neck" configuration of the microdis-sected cystinotic nephron (24) provides a struc-tural basis for multiple dysfunctions of the proxi-mal nephron. Swamping of the distal nephron withbicarbonate escaping reabsorption proximally hasbeen suggested as the operational mechanism ofthe renal acidification defect of patients with theFanconi syndrome and of infants with RTA(25-30).

A reduction in renal H+ secretory capacity ex-plains why acidosis in children and adults with theFanconi syndrome (31-34), infants with RTA(28), and some patients with unexplained osteo-malacia can be resistant to correction with alkalitherapy (35, 36). At normal plasma bicarbonate

concentrations, patients with marked reductionsof Tm HC03- excrete massive amounts of bicar-bonate, whereas most patients with classic RTAexcrete relatively trivial amounts. Accordingly, inmost patients with classic RTA, correction ofacidosis is sustained by an amount of alkali onlyslightly greater than that amount of nonvolatileacid endogenously produced: 1-1.5 mEq/kg per day(16). In patients with a marked reduction of TmHCO.-, this amount of alkali results in increasedexcretion of bicarbonate but in only minimalchanges in plasma bicarbonate concentration: sus-tained correction of acidosis requires an additionalmuch larger amount of alkali equal to that amountof bicarbonate excreted at normal plasma bicar-bonate concentrations.

Some workers believe that a reduction in bi-carbonate threshold, with or without an associatedreduction in Tm HCO3-, identifies renal tubularacidosis as "proximal" and accounts for extremebicarbonate wasting (30). But a reduction in bi-carbonate threshold indicates only the occurrenceof bicarbonaturia at a reduced plasma bicarbonateconcentration and gives no indication as to themagnitude of the bicarbonaturia at progressivelyhigher plasma bicarbonate concentrations. If theamount of bicarbonate excreted at plasma bicar-bonate concentrations of 22-24 mEq/liter is onlya trivial fraction of that filtered, it seems doubtfulwhether the finding of a reduced bicarbonatethreshold is sufficient to implicate the acidificationprocess of the proximal nephron. The findingclearly does not explain the phenomenon of acido-sis strikingly resistant to correction with alkalitherapy. The phenomenon is, however, explicablein patients with a reduced bicarbonate thresholdbut no measured reduction in Tm HC03- and arelatively trivial reduction in T HCO3- at a plasmabicarbonate concentration of 22-24 mEq/liter. Ina variety of renal acidification defects, includingthose characterized by a marked reduction in TmHC03-, prolonged acidosis increases T HC03-over a broad range of plasma bicarbonate concen-trations; sustained correction of acidosis decreasesT HC03- (37). In one child reported as havingproximal RTA, such an effect of corrective alkalitherapy may account for the unexplained reductionof Tm HCO,- from a normal value (29) to 2.1(30). In other children described with RTA, suchan effect would also explain why sustained correc-


tion of acidosis required an amount of alkali sev-eral times that which corrected the acidosisinitially (38, 39).

The observation that renal H+ secretory capacitycan be increased experimentally in patients withFanconi's syndrome by intravenous administrationof a 0.15 M solution of Na2HPO4/NaH2PO4mayhave therapeutic implications (40). Separation ofrenal acidification defects into rate defects (reducedH+ secretory capacity) and gradient defects wouldappear to have clinical as well as physiologic im-plications.

ACKNOWLEDGMENTSThe author wishes to express his thanks to Dr. MervinGoldman for identifying and referring patient D. M., toDr. John Kane for referring patients A. H. and L. R.,to Dr. Carolyn Piel for referring patient T. B., to Dr.Bruce Marshall for referring patient E. S., and particu-larly to Drs. Arthur A. Badad and Anthony Sebastian forhelp in conducting several parts of this study.

This work was supported by U. S. Public HealthService Grant HE-10044, by a grant from the MedicalEndowment Fund of the University of California CancerSociety, and by a grant from the Blair Fund, administeredby the Committee on Research of the University of Cali-fornia School of Medicine. Many of the studies werecarried out in the General Clinical Research Center, pro-vided by the Division of Research Facilities and Re-sources, U. S. Public Health Service (FR-79).

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an experimental renal acidification...

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