investigation vol. of - jci

Journal of Clinical InvestigationVol. 42, No. 9, 1963

INFLUENCE OF SODIUMINTAKE ON EXCHANGEABLESODIUMIN NORMALHUMANSUBJECTS*

By PAUL I. JAGGERt GERALDJ. HINE, JOHNA. CARDARELLI, AND BELTONA.BURROWSWITH THE TECHNICAL ASSISTANCE OF VALENTINE BIKERMAN

(From the Evans Memorial Department of Clinical Research, Massachusetts Memorial Hos-pitals, the Radioisotope and Medical Services, Boston Veterans Administration Hos-

pital, and the Department of Medicine, Boston University School of Medicine,Boston University Medical Center, Boston, Mass.)

(Submitted for publication November 19, 1962; accepted May 31, 1963)

Several methods for determining exchangeablesodium in human subjects by using the gamma-ray emitter Na22 (half-life 2.6 years) in conjunc-tion with measurements of body radioactivityhave previously been described (1-3). Withthese methods, it is possible to make repeatedexchangeable sodium determinations over a periodof several weeks or months after giving a singletracer dose of the radioisotope. Sodium metab-olism can thus be studied in a way that has certainadvantages over conventional metabolic balancetechniques in that sodium losses from all sourcesare accounted for and the results obtained reflectspecifically changes in the active sodium pool.

For the determination of exchangeable sodium,a 24- to 48-hour period is usually allowed forexchange of the tracer sodium with body sodiumin adult subjects, but some observations havesuggested that there is significant continuing ex-change after this interval (1, 4-7). The fractionof body sodium not exchanged in 24 to 48 hoursis estimated at 30% of total body sodium and isthought to consist mostly of sodium in bone (4,5, 8-11). If these observations are correct, theywould seriously limit the interpretation of long-term exchangeable sodium determinations after asingle Na22 injection, for this slow exchangewould result in a continually increasing calculatedexchangeable sodium, and it would be difficult atany given time to determine how much of a meas-ured increase in exchangeable sodium was due tothe continuing internal exchange and how much

* Submitted in honor of Chester S. Keefer, M.D., andthe Golden Anniversary of the Evans Memorial De-partment of Clinical Research, Boston, Mass. Publishedin part in abstract form: Clin. Res. 1959, 7, 283.

t Work done during tenure of a U. S. Public HealthService Postdoctoral Fellowship. Present address: U. S.Naval Hospital, San Diego, Calif.

to an actual increase in body sodium. Further-more, if our interest is in the active sodium pool,that is, that part of body sodium available forrelatively rapid metabolic accommodations, itwould be erroneous to represent this pool by anexchangeable sodium value which included sodiumthat had taken several days to equilibrate withthe tracer.

In the present report, Na22 and a body radio-activity counter have been employed for repeatedexchangeable sodium determinations in five nor-mal individuals, each given widely varied amountsof sodium over extended periods. The influenceof changes in sodium intake on and the contribu-tion of slowly exchanging body sodium to suchexchangeable sodium values were evaluated.

METHODS

Experimental procedure. Five healthy men varying inage from 23 to 40 received iv tracer doses of Na22 (10 to15 /Ac). Three of the subjects (B.E., L.M., and P.J.)were taking fixed low-sodium diets at the time of the in-jection, and the other two (B.B. and J.C.) were takingtheir normal diets with a variable sodium content andlater were placed on fixed low-sodium diets. In threeof the subjects (P.J., B.B., and J.C.), the fixed low-sodium diet consisted of a liquid synthetic preparationthat provided 2 mEq of sodium per day or less.' Thelow-sodium diet for the other two subjects (B.E. andL.M.) was a hospital low-sodium diet supplying an aver-age of 20 mEq of sodium per day. The period of fixedlow-sodium intake was followed in each subject by a pe-riod of fixed relatively high-sodium intake. During theperiod of high intake, two of the subjects (P.J. and B.B.)took their daily sodium load of 200 mEqorally in the first

1 The major component of the diet for J.C. and B.B.was kindly supplied as Product 3060-1-A by Mead John-son & Co., Evansville, Ind. The synthetic diet for P.J.was generously furnished by Dr. Jacob Lemann, Jr.; itsmajor constituents were soybean flour, corn oil, andglucose.

1459

P. I. JAGGER, G. J. HINE, J. A. CARDARELLI, AND B. A. BURROWS

TABLE I

Body weight;, sodium intake, output, and serum concentration; serum and body Na22 values;and exchangeable sodium determinations in Subject P.J.

Bodyretention,

Urine Na percentageDay Wt Na intake output Serum Na Serum Na22 of dose Nae

kg mEq mEg mEqIL percentage of % mEqdose/L

1 80.5 1 18.3 143 4.355 99.42 3,2602 80.1 1 5.8 137 99.613 79.6 1 4.3 97.634 79.6 1 2.6 136 4.224 97.61 3,1405 79.5 1 1.3 98.336 79.4 1 1.6 137 4.302 96.43 3,0707 79.3 1 0.9 96.168 79.1 1 0.6 136 96.579 78.9 1 3.2 138 4.470 95.92 2,960

10 78.1 1 3.9 139 95.3111 77.7 1 4.1 94.5312 77.5 1 2.7 138 4.341 96.21 3,06013 77.1 1 2.2 138 96.5814 77.1 1 1.6 138 94.3815 77.2 1 0.4 140 4.194 94.55 3,16016 77.3 1 0.5 138 93.9517 77.2 1 0.4 137 4.258 93.24 3,00018 77.1 1 0.4 92.5319 77.0 1 0.4 138 4.266 92.75 3,000

20 77.0 200 1.3 145 4.140 90.82 3,18021 78.3 200 5.7 141 3.803 91.36 3,39022 78.8 200 36.6 150 3.921 91.09 3,48023 79.6 200 82.8 145 3.378 89.04 3,82024 79.5 200 206 91.5125 79.2 200 213 145 3.111 77.14 3,60026 79.0 200 254 149 2.905 73.95 3,790

27 adlib 37628 ad jib 49729 79.0 adlib 353 146 2.265 53.77 3,47030 ad jib 314 48.5131 ad lib 347 45.5432 77.6 ad lib 267 40.6933 77.7 ad jib 243 144 1.652 38.42 3,35034 77.6 ad lib 269 34.6535 77.3 ad jib 285 31.8936 77.0 ad fib 349 143 1.309 28.37 3,10037 76.9 adlib 266 26.0638 77.3 ad lib 212 24.1439 77.6 ad jib 184 144 1.008 23.13 3,30040 77.2 ad jib 312 20.7241 77.3 ad jib 236 139 0.883 19.48 3,07042 76.9 adlib 19243 77.5 ad jib 176 16.5544 76.8 ad jib 244 134 0.693 15.08 2,92045 77.1 adlib 18546 76.5 adlib 137 14.1647 77.4 adlib 177 128 0.590 13.05 2,83048 77.9 adlib 205 12.5749 78.0 adlib 189 11.9150 77.6 ad jib 171 142 0.477 10.88 3,24051 77.0 ad jib 17352 77.4 adlib 19753 77.5 adlib 372 8.90654 77.7 ad jib 196 145 0.385 8.375 3,15055 76.9 ad lib 186 7.95756 77.2 ad fib 43457 77.1 ad jib 222 144 0.294 7.013 3,430

1460

INFLUENCE OF SODIUM INTAKE ON EXCHANGEABLESODIUM 1461

mEq aauu I - -\3400

32003000-

2800-

82 B.ODY WEIGHTKg 80 -

78-

5004 UR/NE SODIUM400-

mEq 300 -

200-

100

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

DAYS

FIG. 1. EXCHANGEABLESODIUM VALUES, BODY WEIGHT, AND URINE SODIUM PLOTTED AGAINST TIME. Salt loadingin this subj ect was accomplished by the ingestion of NaCl tablets. At his own request, the subject was on a low-calorie diet during the first two dietary periods. Note that his weight at the end of the study (day 59) was 3.5kg less, but his exchangeable sodium was slightly higher. This suggests the loss of sodium-free tissues, presum-ably mainly fat.

8 to 10 hours of each 24-hour period, and the other three(B.E.., L.M., and J.C.) received their sodium load of 181to 359 mEq per day intravenously in the form of 5%o sa-line solution in the first 2 hours of each 24-hour period.After the period of high-sodium intake, each subject re-sumed his normal diet and was followed for an additionalperiod on a variable sodium intake.

A study day ran from one morning to the next andended with the patient fasting. Urine was collected eachday for determination of 24-hour urine sodium excretion.One subject (P.J.) also collected all stools during theperiods of fixed low- and fixed high-sodium intakes, andpooled stool specimens were analyzed for sodium con-tent after nitric acid digestion. Venous blood sampleswere drawn, and body radioactivity measurements weremade at the end of the study day.

Body radioactivity measurements. Body radioactivitymeasurements were made with the subject seated in acontour chair beneath the open end of a shielded sodiumiodide crystal 4 inches in diameter by 4 inches in height.The center of the crystal was placed 50 cm from the mid-line of the body. The details and efficiency of this count-ing arrangement have been described elsewhere (3).Separate measurements of the radioactivity of the kneeand the posterior aspect of the head were made withsmaller sodium iodide crystals. At the knee, the collima-

tion for the crystal was such that the NaO radioactivitymeasured came from a limited area composed mainly ofthe bony structures (5). At the posterior aspect of thehead (lower occipital area), there was less collimation,and radioactivity from a wider area containing cerebro-spinal fluid and brain tissue as well as bone was measured.

Sample measurements. The Na' radioactivities of se-rum and urine samples were determined in a standardsodium iodide well-counter. In one study where Na' wasmeasured in the presence of Na22 (Subject B.E.), a dif-ferential pulse height analyzer was used. Chemical so-dium concentration of samples was measured by flamephotometry.

Calculations. Exchangeable sodium was derived fromthe retention of Na22 determined from body radio-activity measurements, serum Na22 concentrations, andserum concentration of stable sodium. The formula em-ployed was: Exchangeable sodium= body Na22 retention(per cent) X seritmn sodium concentration (mnilliequiva-lents per liter)Iserwm Na22 concentration (per cent perliter), where Na22 retention in the body and Na22 concen-tration in the serum are expressed as percentages of theinitial dose.

The body counter was calibrated for each patient byequating the measurement at 24 hours with Na' reten-tion at that time as determined from urinary excretion.


3600mEq 3400

3200

3000

2800

90

88Kg 86

84

400

300mEq

100

J.C.

%~~~~~~~~~~.~~ ~ ~

0

EXCHANGEABLEBODYSODIUM

O.* O,

,/ * %e

6DY WEIGHT

URINE SODIUM

o Z 4 6 8 10 12 14 16 18 20 24 28 32 36

DAYS

FIG. 2. EXCHANGEABLESODIUM, BODYWEIGHT, AND URINE SODIUM PLOTTED

AGAINST TIME. Sodium supplementation during the period of high-sodiumintake was administered intravenously. Exchangeable sodium during thisperiod gradually increased to a peak value and then fell off to intermediatelevels. The subject was also on a low-calorie reducing diet and lost a totalof 5.3 kg over the course of the study without a decrease in the final ex-

changeable sodium values as compared to the initial ones.

Long-term Na2 retention. In three subjects (P.J., B.B.,and J.C.), measurements of body radioactivity due to Na22were continued at intervals up to 21 years after the ini-tial injections with a more sensitive counting arrangement,consisting of an 8- X 4-inch NaI (Tl) crystal mountedin a steel room and connected to a multichannel pulseheight analyzer.2 The purpose of these measurementswas to determine the residual amount of the tracer doseof Na22 and the long-term turnover of this residual.

RESULTS

Exchangeable sodium. The three subjects whowere taking a fixed low-sodium diet at the startof their studies (B.E., L.M., and P.J.) showedrelatively constant values for exchangeable sodiumthroughout the period of fixed low-sodium intake(Figure 1, Table I). The two subjects whostarted their studies on regular diets (B.B. andJ.C.) showed varying exchangeable sodium valuesas long as they remained on their regular diets,

2 Radioactivity Center, Massachusetts Institute of Tech-nology, Cambridge, Mass.

but when they changed to the fixed low-sodiumdiets, their exchangeable sodium values decreasedfor the first few days, and then also remainedrelatively constant (Figure 2, Table II).

When each subject changed from a fixed low-sodium intake to a fixed high-sodium intake, therewas a definite increase in his exchangeable sodiumvalues. In two of the subjects (B.B. and J.C.,Figure 2), there was a gradual increase to a peakvalue and then a fall-off to intermediate levels.In the other three subjects, there was no suchclearly defined pattern. Despite this variation inexchangeable sodium values on the high-sodiumintakes, there was a significant group increasein exchangeable sodium with increased sodiumintake (Table III).

Values for exchangeable sodium during the finalperiods when the subjects resumed their normaldiets again showed considerable variability; aver-

age values for this period were intermediate tothe low- and high-sodium intake averages in three

VARIALN LOW No IHIGH No VARIABLE No

1462

0

0% 0..*0%0

INFLUENCE OF SODIUM INTAKE ON EXCHANGEABLESODIUM

TABLE II

Body weight; sodium intake, output, and serum concentration; serum and body Na22 values;and exchangeable sodium determinations in Subject J.C.

Day Wt

kg

1 90.22 90.03 90.84 90.45 90.86 90.57 90.0

8 88.59 88.0

10 87.511 87.012 86.913 86.814 86.515 86.316 86.017 85.418 85.6

19 85.820 86.021 86.522 85.823 85.524 84.625 84.826 84.4

27 84.328 84.029 84.830 84.631323334 85.835 84.836 84.437 84.338 84.9

Na intake

mEq

ad libad libad libad libad libad libad lib

245228218238229240257236

ad libad libad libad libad libad libad libad libad libad libad libad lib

Urine Naoutput

mEq

185136

64.9265114143193

79.524.814.44.044.522.321.952.311.473.172.59

14.4157204252235302186240

13291.2

11355

113181174202248177

95.182.5

Serum Na

mEqIL

139136

136

142138

Serum Na22

percentage ofdose/L4.2914.141

3.659

3.4103.232

139 3.286

139 3.323

138 3.140

142 3.166

144 3.267141 3.190

145 3.116140 2.786144 2.674141 2.429141 2.279141 2.169142 2.029141 1.904

140 1.763

139 1.682

141 1.277

138 1.179

142 1.051

TABLE III

Changes in serum sodium concentration and exchangeable sodium with increased sodium intake

Low-sodium period High-sodium period

Na Serum Na Na Serum Na A SerumSubject intake i SD Nae i SD intake i SD Nae i SD Na* A Naet

mEq mEq/L mEq mEq mEqIL mEq mEq/L mEqL.M. 20 141.9 4 1.62 3,247 4- 29.2 181 145.0 i 2.77 3,481 i 34.0 +3.1 +234

[9]t [9] [7] [7]B.E. 20 140.9 i 1.25 2,635 i 32.6 358 140.5 i 1.69 2,830 i 30.0 -0.4 +195

[8] [8] [8] [8]B.B. 1 140.1 i: 1.58 3,452 i 140 200 138.8 i 1.64 3,616 + 107 -1.3 +164

[7] [6]§ [5] [5]J.C. 2 140.5 ± 2.28 2,876 + 27.9 236 141.8 i 1.73 3,089 i 25.2 +1.3 +213

[6] [6] [8] [8]P.J. 1 138.1 i 1.73 3,081 ± 57.7 200 145.8 i 1.21 3,543 ± 1.13 +7.7 +462

[14] [8] [6] [6]

* Group change is insignificant (p greater than 0.2), according to paired I test (12).t Group difference significant (p less than 0.01).$ Numbers in brackets refer to number of determinations.§ Exchangeable sodium value for first day omitted from calculated mean.

1463

Bodyretention,

percentageof dose

93.6889.8087.8483.1579.3376.2671.22

69.7267.5867.0967.3766.9465.4365.5165.8464.9565.2464.96

65.3362.5158.4754.8250.5145.9643.7240.73

38.3137.0135.8335.13

28.3126.2724.5524.1523.56

Nae

mEq

3,0302,9503,090

3,1803,040

2,860

2,820

2,880

2,950

2,8802,870

3,0403,1403,1503,1803,1302,9903,0603,020

2,940

2,900

3,130

2,870

3,180


mEq3000

2800

2400

3600

3400

3200

4000

3800

3600

3400

3200

3000

4000

3800

3600

3400

3200

3200

3000

2800

.-. Estimated from Sodium Bolancex-x Cc/cu/a/ed from Body No22

B.E.

Low Na, x x

-Asx| High Na

LM.

<* X High Na

Pd.

|LLowNa N

l~~~~~X,*e / High NaK

F <ax x XNx. 1I

>- ~~~Low Na

l xK~x'x High Na

_~~ ~ ~~~~~~~~~~I _iJ. C.

)t ~~~~~~~~II Id I I2 4 6 8 10 12 14 16 18 20 22 24 26 28

DAYSFIG. 3. EXCHANGEABLESODIUM VALUES CALCULATEDFROMSERUMSPE-

CIFIC ACTIVITY AND BODY-COUNTINGDATA COMPAREDWITH THOSE ESTI-MATEDFROMSODIUM BALANCE. The latter values were obtained from thevalue calculated for exchangeable sodium at the end of the first day oflow-sodium intake, corrected for each subsequent day for the sodium bal-ance determined from intake and urinary excretion.

subjects (L.M., P.J., and J.C.) and greater thanthe fixed high-sodium intake averages in theother two subjects (B.E. and( B.B.).

Body weights. There were no significantchanges in body weights for the group as a wholein going from low-sodium periods to high-sodium

periods, perhaps because caloric intake was notregulated to energy requirements. Two subjects,in fact, were placed on low-calorie diets l)ecausethey wished to lose weight. However, bodyweights usually decreased when the subjects wereplaced on fixed low-sodium diets and usually in-

1464

L

I


creased when they were changed to fixed high-sodium diets. It is worth noting that the twosubjects on low-calorie diets lost totals of 3.5 kg(P.J.) and 5.3 kg (J.C.) without a significantdifference in exchangeable sodium at the begin-ning and end of the studies.

Serum sodium concentration. The increase inserum sodium concentration for the group in go-ing from a low-sodium intake to a high-sodiumintake was not significant (Table III). Thethree subjects (P.J., L.M., and J.C.) who showedan increase in serum sodium concentration alsoshowed the greatest increase in average exchange-able sodium between the low-sodium and high-sodium periods. The calculated values for ex-changeable sodium are dependent on the serumsodium values, but the validity of these observa-

LOWNa HIGH Na

O iXV

.V-~~~~

144~~~~~XK~~~~~~~~

K~~~~~~~~

ZZ~~~~~~~~~~

tions is supported by the fact that these subjectshad been on the low-sodium diets for the longestintervals (11 or more days) before changing toa high-sodium intake.

Sodium balance. Daily 24-hour urine sodiumexcretions fell gradually to low levels, remainedrelatively constant when the subj ects were placedon fixed low-sodium diets, and then gradually in-creased again after the institution of the fixedhigh-sodium intakes. A negative sodium balance,as determined from the urinary excretion, tendedto coincide with a fall in measured exchangeablesodium, while a positive balance coincided withan increase.

A curve of estimated exchangeable sodiumvalues can be constructed for the periods of fixedsodium intake by beginning with the value for

0 4 8 12 16 20 24 28 32 36 40 44 48 52DAYS

FIG. 4. REPRESENTATIVESET OF RADIOACTIVITY MEASUREMENTSOBTAINED BY THE BODYCOUNTERAND BY COUNTERSCLOSETO THE HEADAND THE KNEE. The curve of serum radioactivity is also included. The count rates are plottedon a logarithmic scale against time; the actual count rates were multiplied or divided by some arbitrary factors tobring all curves close together for better comparison. This example shows the prolonged equilibration time of thehead, but not that of the knee as seen in other cases; it was chosen because of the long follow-up (51 days). Afterthe initial period of equilibration, the curves are roughly parallel throughout. There is no evidence for continuedslow penetration of Nab into the bone of knee or head.

1465


14001° JA.C.* J.A.C.

----x G.,J.H.1500

12001-

0look1ooo0

9001

0k0.

800p

rook

- 6/4/60- 5/29/6/- Background

600k

500k

400kNao

300k

200k

100

0 10 20 30 40 50 60 70 80 90 00 I110 I

CHANNELNUMBER

FIG. 5. GAMMA-RAYSPECTRAOBTAINED IN IRON ROOMAT 15 AND 27 MONTHSAFTER ADMINISTRATION OF TRACER

DOSE OF NA= TO SUBJECT J.C. Measurements of body radioactivity of Subject G.J.H., who had not received Nab,were obtained to serve as body background radioactivity in areas of Na' photopeaks. The 1.28 Mev Na22 peak

activity was approximately one-half that of the 1.46 Mev K' peak activity on 6/4/60. Also shown are the 0.511Mev Na22 photopeak and an adventitious 0.622 Mev Cs1" photopeak. With the K4 photopeak as a standard, thedecrease in Na22 photopeak activities during the following year was approximately one-third.

calculated exchangeable sodium at the end of thefirst day of fixed intake and then correcting on

each subsequent day for the sodium balance as

determined from intake and urinary excretion.In Figure 3, the curve determined by the abovemethod and the curve calculated from serum

specific activity and body radioactivity are shownfor each of the subjects. The curves show simi-lar trends in each case; however, there is more

variation in the day-to-day values calculated fromserum specific activity and body radioactivity, andthese values are usually lower than the 1)alallcevalues for the same day.

Head and knee radioactivity mneasurements.Figure 4 shows a representative set of curves forradioactivity as recorded by the body counter and

by the counters at the head and at the knee, to-gether with the curve for serum radioactivity.As might be anticipated, times for equilibration ofthe Na22 recorded by the counters at the headin this subject and in both the head and knee inothers are prolonged as compared with the timefor equilibrium recorded by the body counter andin the serum samples. After equilibration, how-ever, all the curves are parallel throughout therest of the study periods.

Na24 study. On day 71 of the study in sub-ject B.E., an exchangeable sodium value was

determined from the Na'22 data in the usual man-

ner. A tracer dose of Na24 was then adminis-tered intravenously, and urine was collected forthe next 24 hours while the patient took approxi-

Nat"

Cs'37 0.6621 860 710 650 500

Nog2 1.28 J 320 240 250 170

330 370

Decoy of No' in 1 Year = 76%

1466

10.51 f 15 1035 1050 730

1. 46 290 330

.1


TABLE IV

Peak ratios of Na2l relative to KO0*

Na22 Date of measurementgamma

Subject rays 61/4/60 5/29/61 6/1/62

Mev %J.C. 0.51 100 61 42

1.28 100 60 40

P.J. 0.51 100 55 501.28 100 63 39

B. B. 0.51 100 58 331.28 100 41 31

* Na22 to K40 ratios are given for both Na22 annihilation radiation(0.51 Mev) and for the Na22 gammaray (1.28 Mev). The average ofall data yields an effective half-life of about 1.3 years. Correcting forphysical half-life (2.6 years) yields a biological half-life of approximately2.6 years. At 15 to 19 months after the tracer doses of Na22, heights ofthe Na22 1.28 Mev photopeak and the K40 1.46 Mev photopeak in thegamma-ray spectra were approximately the same in all subjects. Ifwe assume a body burden for K40 of 0.12 juc and a gamma-ray abund-ance for Na22 ten times that for K40, this indicated a retention of ap-proximately 0.1 %of the initial dose of Na22. The ratio of the Na22photopeaks to the K40 photopeak on 6/4/60 was taken as 100%0 forcomparison with the Na22: K40 photopeak ratios obtained at yearlyintervals thereafter.

mately the same diet as during the previous day.At the end of the 24 hours, an exchangeable so-dium value was determined from the serum spe-cific activity of Na24 and body retention of thatisotope, as indicated by urine data. The ex-changeable sodium value determined from Na2271 days after its administration was 2,920 mEq,and the value determined the next day from Na24,just 24 hours after its administration, was 2,910mEq.

Lonig-term Na22 retention. Initial measure-ments in the steel counting-room were made 15to 19 months after injection of the Na22 doses.The gamma-ray spectra that were obtained atthis time showed a retention of Na22 that was lessthan the naturally occurring K40 in all three sub-jects. Subsequent determinations expressed aspercentages of this initial Na22: K40 ratio showeda markedly prolonged biological half-time, aver-aging approximately 2.6 years, as compared to an11- to 15-day half-time for fall-off of serum Na22activity in the same patients on their normal diets(Figure 5, Table IV).

DISCUSSION

Several lines of evidence bear upon the ques-tion of whether or not there is a later significantpenetration of the Na22 tracer into that fraction ofbody sodium that has not equilibrated in the first24 to 48 hours. The first is found in the curvesfor exchangeable sodium in the three subjects who

began their studies on fixed low-sodium diets.There is some variability inl day-to-day values forexchangeable sodium, but for periods of 8 to 21days. there is no definite trend toward an increasein valuies that Nvould occur with continuing pene-tration of Na22 into a slowly exchanging sodiumspace.

The second line of evidence comes from thecomparison of curves of Na22 radioactivity fall-offrecorded at the knee and at the back of the headwith the curve for fall-off recorded by the bodyradioactivity counter and with the curve forserum Na22 radioactivity fall-off. The radio-activity at knee and head increases for 24 to 48hours after body and serum curves have leveledoff, but after this the curves are parallel. Ifthere were significant further exchange of the Na22with the sodium in bone after the first few days,the curves at knee and head should have had aslower rate of fall-off during the rest of the studythan the curves for body and serum. This failureto show further exchange is comparable to thework in dogs reported by Edelman, James, Baden,and Moore (9), who compared bone specificactivities to serum specific activities, and foundthat approximately 45% of the bone sodium wasreadily exchangeable in the first 1 to 3 days andthat the rest showed no tendency to exchangeover the next 30 days.

The third line of evidence comes from inspec-tion of the curves for exchangeable sodium pre-dicted from balance data and the curves calcu-lated from serum specific activity and body radio-activity (Figure 3). Continuing exchange of thetracer should result in calculated exchangeablesodium values increasingly larger than those esti-mated from the balance data; however, this isapparently not the case. Still further evidencecomes from the comparison of the exchangeablesodium value calculated from the Na22 data onday 71 of the study in B.E. with the 24-hourvalue for exchangeable sodium calculated fromthe Na24 data on the next day. If there had beena continuing slow exchange of the Na22 over theprevious 71 days, the exchangeable sodium valuecalculated from the Na22 should have been sig-nificantly greater than that calculated from theNa24, but the values were almost identical. Simi-larly close correlations were obtained betweenexchangeable sodium calculated from Na22 on day

1467


98 and from Na24 on day 99 in one edematoussubject and onl days 100 and 101 in another (13).

Other reports have indicated results contraryto the above, with exchangeable sodium values,or with sodium spaces continuing to increase fora period of a week or more (1, 4-7). In thestudies that did not employ direct measurementsof body radioactivity (4-6), these contradictoryresults can probably be attributed to incompleteassessment of body Na22 losses, either because offailure to measure extrarenal losses or becauseof incomplete collections. This would result invalues for body Na22 retention progressivelyhigher than the actual retention, and use of thesein the calculation of exchangeable sodium wouldresult in values for exchangeable sodium thatare increasingly larger than the actual values.The studies that did employ measurements ofbody radioactivity differ from the present one inthat the subjects were not placed on fixed sodiumintakes. As discussed below, exchangeable so-dium varies according to sodium intake, and there-fore it is difficult to draw definitive conclusionsfrom studies in which sodium intake was notcontrolled.

While continuing penetration of the Na22 tracerinto a very slowly exchanging space does not sig-nificantly affect determinations of exchangeablesodium, measurements of body radioactivity overlong intervals indicate that slow penetration ofminute amounts of Na22 does occur. Measure-ments in the steel room indicate the persistencein the body of greater quantities of Na22 thanwould be expected from extrapolation of the dis-appearance slopes of serum radioactivity whilethe subjects were on normal diets; this persistingNa22 has a disappearance half-time measured inyears, rather than in days. This prolonged re-tention of minute amounts of Na22 by humansubjects also has been demonstrated by others(14-16). This very slowly exchanging sodiumprobably represents the incorporation of sodiuminto the internal structure of bone crystals, ata rate dependent upon the normal continuingprocess of bone resorption and deposition (17,18). That part of bone sodium that exchangeswithin 24 hours, on the other hand, probably isthe sodium that is merely absorbed on bone crystalsurfaces (15, 17, 19, 20), or that displaces calciumions from the surfaces of the crystal lattice.

The 24-hour equilibration time used in thispresent study in normal individuals may not al-ways be adeqtuate for patients with certain diseasestates. Suich patients may well require 48 hours,72 hours, or even slightly longer for equilibration(2, 21, 22). In a group of edematous patients inwhom 48 hours was allowed for equilibration,evidence similar to that in normal subjects forlack of significant continuing exchange after thisinitial equilibration period was obtained (13, 23).

The mean values for exchangeable sodium dur-ing the periods of high-sodium intake were sig-nificantly higher for the group than the meanvalues during the low-sodium periods (TableIII), even though final equilibrium following thechange in sodium intake might not have beenobtained. In three of the subjects, the agreementbetween the increase in sodium intake and the in-crease in exchangeable sodium was within 100mEq. In one subject, B.E., who had some diar-rhea during the period of high-sodium intake, theincrease in sodium intake exceeded the mean in-crease in exchangeable sodium by 143 mEq, andin one subject, P.J., who was on a low-sodium in-take before the high-sodium period for the longestinterval, the increase in mean exchangeable so-dium exceeded the increase in daily sodium in-take by 263 mEq. In four of the five subjects,the increase in exchangeable sodium with anincrease in sodium intake was about 200 mEq.More studies with a wide range of sodium intakeswould be needed to substantiate this point, butthis may be evidence that homeostatic mechanismsallow for relatively fixed net changes in exchange-able sodium and that, for most normal subjectsat least, in going from a low- to a high-sodiumintake, these changes are around 200 mEq.

The increase in body sodium with increasedsodium intake is also confirmed by balance data.The curves for exchangeable sodium estimatedfrom balance data and the calculated curves showsimilar trends in each subject (Figure 3). Asnoted above, the day-to-day values calculated fromradioactivity measurements show more variabilityand tend to be lower. The lower values, however,may be closer to the true values, particularly inthe later days of the studies, in that measurementsof body radioactivity account for losses of sodiumby nonrenal routes that are missed by the balancemethod. It is true that stool sodium losses by

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INFLUENCE OFSODIUM INTAKE ON EXCHANGEABLESODIUM1

one subject (P.J.) in whom collections were madetotaled only 15 mEq over a 26-day period, butgreater sodium losses may have occurred in Sub-ject B.E. to account for the divergence betweenthe balance curve and the body radioactivity curvcduring his period of high-sodium intake.

A change in exchangeable sodium with a changein sodium intake could be anticipated from pre-vious studies (24). It is possible that the ap-parent magnitude of these changes may be duein part to incomplete mixing of the daily incre-ments of stable sodium with the exchangeablesodium. This would give a falsely low serumspecific activity which, in the subsequent calcula-tions, would result in a falsely high exchange-able sodium. Equilibration of Na22 and Na24with serum sodium is reasonably complete within12 to 24 hours (10, 21, 25, 26). If we canassume that equilibration of stable sodium fol-lows the same time course, the practice of givingthe sodium supplements during the first part ofeach study day should have minimized this ef-fect of increased sodium intake on the estimationsof exchangeable sodium.

The magnitude and the pattern of the rises inexchangeable sodium when high sodium intakesfollowed sodium deprivation were probably duein part to the increased levels of circulatory aldos-terone resulting from sodium restriction (27-32).One may speculate that the increases in exchange-able sodium to peak values over several days fol-lowed by a fall-off to intermediate values seen inat least two of the studies (J.C. and B.B., Figure4) are indicative of levels of circulatory aldoster-one that result in an "overcompensation" for theprevious sodium deficit; then, as the levels of al-dosterone fall, the excess sodium is excreted. Asimilar cyclical pattern with sodium loading haspreviously been reported (31).

It is not possible from the present data to de-termine the relative contribution of different bodycompartments to the sodium lost in going fromthe regular to the fixed low-sodium diets, nor thedistribution among body compartments of thesodium gained in going from the fixed low- to thefixed high-sodium diets. On the basis of reportedanimal studies, however, it can be predicted thatthe extracellular fluid compartment would showthe greatest changes followed by the exchange-able fraction of bone sodium (32-37).

SUAMMIARY

1. l ong-term studies of sodium metabolism infive normal subjects were carried out by use oftracer (loses of Na22 and measurement of bodyradioactixvity to make repeated exchangeable so-dium determinations.

2. Body sodium exchanging after the first 24to 48 hours was not a significant fraction of themetabolically active sodium pool, nor did it inter-fere with long-term measurements of this pool byisotope dilution methods.

3. Exchangeable sodium was shown to vary di-rectly with changes in sodium intake.

4. A small fraction of the tracer doses of Na22was shown to have a prolonged biological half-lifeconsistent with its incorporation within bonecrystals.

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