STUDIES IN PHOSPHORUSMETABOLISMIN MAN. III. THE DISTRIBUTION,EXCHANGEANDEXCRETIONOF PHOSPHORUSIN MANUSING

RADIOACTIVE PHOSPHORUS(P32) AS A TRACER1

By STANLEYM. LEVENSON,2.X,4 MARGARETA. ADAMS,3 HYMANROSEN,2 ANDF. H. LASKEYTAYLOR8

(From the U. S. Army Medical Nutrition Laboratory 9937 TSU-SGO, 1849 West PershingRoad, Chicago 9, III.)

(Submitted for publication August 25, 1952; accepted March 4, 1953)

INTRODUCTION

In the course of some metabolic studies in man,it became important to know in some detail thefate of injected radiophosphorus. Although p32has been used extensively as a tracer in animalmetabolic studies, its use in man has been largelylimited to the therapy of various hematologic andmalignant disorders, and to the determination ofcirculating red cell volume. Accordingly, we havefollowed the distribution of radiophosphorus invarious plasma, red cell, and urine fractions, afterthe intravenous administration of 100 to 200 mi-crocuries of p82, as inorganic phosphate, into nor-mal men. The effects of glucose and insulin onphosphorus partition were also observed. Thesedata form the basis for a discussion of the kineticsof phosphorus transfer and distribution.

METHIODS

Healthy young adult males served as subjects. Theyweighed, on the average, about 75 Kg. and were on nor-mal food intakes prior to study. Observations were madeafter a 12 hour fast. Radioactive phosphorus (100 to200 pc P), in the form of Na.HP0S5 (carrier-free)dissolved in about 3 ml. of pyrogen-free water, was in-jected intravenously in a few seconds into an antecubitalvein. Blood samples were thereafter collected seriallyfrom the opposite arm. The blood was collected in bot-tles containing dried potassium-ammonium oxalate ad-

1 This work was aided in part, by gifts from Smith,Kline and French Laboratories, Philadelphia, and theAmerican Cancer Society (Massachusetts Division), toHarvard University.

2 Army Medical Service Graduate School, Walter ReedArmy Medical Center, Washington 12, D. C.

s Thorndike Memorial Laboratory, 2nd and 4th MedicalServices (Harvard), Boston City Hospital, Boston, Mas-sachusetts.

' Surgical Service of the Medical College of Virginia,Richmond, Virginia.

* P' was supplied by Monsanto Chemical Co., ClintonLaboratories, Oak Ridge, Tennessee.

justed to produce no change in red cell size. The bloodsamples, cooled in ice, were centrifuged for 10 minutes at2,000 rpm as soon as possible, and the plasma removed.The red cells were washed once with ice cold isotonic so-dium chloride solution, centrifuged again, and the super-natant washings removed as completely as possible anddiscarded. The washed red blood cells were rapidlyfrozen at - 20° C. to mnimize hydrolysis of phosphoruscompounds and to produce hemolysis. Plasma and he-molyzed red blood corpuscles were each analyzed chemi-cally for total phosphorus, total acid soluble phosphorus,and inorganic phosphorus by the methods of Fiske andSubbarow (1). The radioactivity of these phosphatefractions was determined according to methods previouslydescribed (2). Values for the trichloroacetic acid solubleorganic and the trichloroacetic acid insoluble fractionswere calculated. In some subjects, the effects of intra-venous glucose (25 gm.), or crystalline insulin (0.1 unitper Kg. body weight) were observed. Urine samples.were also collected serially and total P" and phosphorumeasured.

RESULTS

1. Disappearance of P82 from plasma

The course of the disappearance of pB2 fromthe plasma, following the single rapid intravenousinjection of labeled phosphate, is shown in Figure1. The experimental points have been plottedsemi-logarithmically and represent observationsmade on seven normal young adult males.

The data suggest a very rapid transcapillary mi-gration of P82. This is evident from the followingcalculation: if the mixing phase of p82 in theplasma is over, and the uptake of pB2 by the bloodcells is minimal in the first few minutes (3, 4,vide infra) the dilution of P'2 at the end of thistime would represent the plasma volume werethe P32 wholly contained in the vascular tree.It can be seen from Figure 1 that after the firstfew minutes only some 1 per cent of the total in-jected p52 is found in each 100 ml. plasma. Thisleads to a calculated plasma volume of some 10

497

S. M. LEVENSON, M. A. ADAMS, H. ROSEN, AND F. H. L. TAYLOR

MINUTES - - " on - _FIG. 1.

*- Semi-logarithmic plot of the course of disappearance of P' from plasma. X X

These data are derived from the experimental curve, and illustrate its double exponential nature.

litres. This figure is much too high for plasmavolume, but only slightly lower than an expectedvalue (12 litres) for total extracellular fluid (5),and suggests a very rapid transcapillary migrationof radiophosphorus. A true value for extracellu-lar space is not to be expected from this calcula-tion, since it would necessitate homogeneous dis-tribution of the radiophosphorus in the interstialfluid and limitation of the radiophosphorus to thisspace. Pappenheimer, Renkin, and Borrero (6)have- shown that there is an arterio-venous differ-ence in the concentrations of various injected ionsand that a homogeneous plasma ion pool can onlybe approximated. Because of the access of phos-phate to intracellular pools, it is unlikely to be uni-formly mixed in the extravascular fluid after itsescape from the capillaries.

An apparent rapid transcapillary movement ofp32 was also observed by Kleiber, Smith, and Ral-ston in cows (7). Walker and Wilde (8) simi-larly found that 90 per cent of injected K'2 movesout of the plasma of rabbits within one minute fol-lowing its intra-arterial injection.

Following the initial very rapid decrease in

plasma radioactivity in the first few minutes afterthe injection of labeled phosphate, its disappearanceslows. A smooth curve drawn through the ex-perimental points plotted semilogarithmically (Fig-ure 1) fits the exponential type equationP = aie-blt + a2e-b2t . . . + a,e-bnt + Peq. (1)

In the equation, P = per cent injected P82found in each 100 ml. plasma at time t after injec-tion, P,. represents the "steady state" level ex-pressed in the same units, and a1, a2, an and bl, b2,bn are constants.

By the seventh hour after the injection of thelabeled phosphate the rate of its disappearance fromthe plasma has become very slow. As a first ap-proximation, it is assumed that a steady state hasbeen reached. In Figure 1 the straight line drawnfrom this point and extrapolated back to zero timerepresents P.. When this "equilibrium" line issubtracted graphically from the experimentally ob-tained curve, another curve is obtained. Theprocess of extrapolation and subtraction is repeatedwith-this curve, and a last, steep straight line is theresult.

498

STUDIES IN PHOSPHORUSMETABOLISM

The three straight lines can be formally con-

sidered to represent three processes, in which a isthe y intercept of each line, and b is the slope.The equation now becomes

P = 0.85 e-008t + 0.27 e-01t + 0.03. (2)

One other formal mathematical manipulationcan be applied to the derived equation. This isthe assignment of "transfer coefficients" to thetwo processes represented on the graph by thetwo steep straight lines, and in the equation by thefirst two terms. This is a formal step, and doesnot imply that the processes are separate or real,or that they are taking place in homogeneous"pools."

According to Solomon (9), the transfer co-

efficients of the processes represented by the semi-logarithmic plots can be evaluated with a degreeof accuracy depending on the validity of the follow-ing assumptions:

1) The various body compartments maintaina constant size during the period of study;

2) The transfer coefficients across membranesare the same in both directions;

3) The total injected p32 iS maintained withinthe body compartments under study, i.e., no ex-

cretion. It is clear that this assumption is tosome extent unwarranted, since it was found thatin the first seven hours after injection, 5 to 10 per

cent of the injected P32 was excreted (Figure 5).Neglecting this error, the slopes (b, and b2) bearthe following relationships to the transfer coeffi-cients (k1 and k2):

bi = (k1 + k2) + 4k12- k1k2 + k22,

b2 = (k1 + k2) - Vk12- k1k2 + k?.Solving these equations simultaneously,

ki = 38 X 103 per min., t = 18 min.

k2= 7.9 X 10-3 per min., tj = 87 min.

transfer. The double exponential nature of thisphase of the disappearance suggests that phos-phate enters one group of cells rapidly and an-other group relatively slowly. More precise char-acterization of these transfers depends on specificcellular analyses.

2. Appearance of p32 in the red cells

The in zivo uptake of p32 by red cells is showngraphically in Figure 2. This curve can be rep-resented by

Q = al( - e-blt) + a2(1 - e-b2t). (5)When Q = per cent injected P32 per 100 ml. redcells,

Q = 0.07(1 -e-e t) + 0.13(1 -e- 02t). (6)The first term is significant during only the

first 10 to 20 minutes. The concentration of P32in the red cell increases sharply during this timeand then rather abruptly levels off. The rapidleveling off is probably due to the simultaneous es-

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What these processes represent physiologicallyis not certain. They may represent intracellular

at

.40 _

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.0600I

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FIG. 2. SEMI-LOGARITHMIC PLOT OF P2 UPTAKF BY RED

CELLS IN VIVO

The curve is a composite of data on four subjects.

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499

,_. -1b

S. M. LEVENSON, M. A. ADAMS, H. ROSEN, AND F. H. L. TAYLOR

movement of P32 out of the plasma was observed(Figure 4). In the case of glucose, this movementwas approximately three times normal, while in-sulin gave rise to a rate four times normal. In allcases, the eventual equilibrium level of p82 foundin the plasma was about 0.03 per cent P32 injectedper 100 ml. plasma.

No information regarding the effect of glucoseor insulin on the rate of entry of p82 into the redcell was obtained in our in vivo studies since thenet uptake was practically over at the time of glu-cose and insulin administration.

4. Urinary excretion of injected p32

The cumulative values of per cent injected p82excreted in the urine are shown in Figure 5 andTable I. These curves were found empirically tobe represented by the equation

R = Roa(l - e-bt),where R = per cent injected p32 found in urine af-ter t minutes, and a and b are constants. Sincethe first urine collections were made one hour af-ter p32 injection, the single term of this equationapplies only to later data. A rearrangement ofequation 7 gives

MINUTES Li-on

FIG. 3. GRAPHIcAL DERIVATION OF THE TRANSFRCOEFFICIENT OF Pn FROMPLASMATO REDCELLS, AT 370 C.IN VITRO

cape of p32 from the plasma into other body com-

partments. Experiments, done by ourselves andothers, on the uptake of p32 by red cells in sitro(8), indicate that the uptake from plasma by redcells is relatively slow. During the first two hoursthe measured disappearance of p32 from the plasmain vitro occurs exponentially. From our in vitrodata (Figure 3), we have calculated a transfercoefficient of 0.4 per cent per min. at 370 C.

3. Effect of insulin and glucose on plasma radio-phosphate disappearance

The effect of insulin and glucose on the move-

ment of p82 was studied in a few subjects.Crystalline insulin (0.1 unit per Kg. I.V.), or

glucose (25 gm. I.V.) was administered one hourafter injection of p82. In each case, an accelerated

In I - ) = - bt. (8)

A plot of ln 1-RR against t should yield a

straight line, the slope of which is - b (Figure 6),representing the rate of passage of P82 into urine.Failure of this function to pass through the value1.0 at zero time probably reflects the effect of an-

other term in equation 7 which we have neglected,as mentioned. The average rate of urinary p32excretion obtained in this way is 7.8 x 10-3 per

min. (Table II), a figure identical to that of rateconstant k2 derived from the plasma radiophos-phate determinations. The effects of a single in-travenous injectiort of glucose or insulin were notreflected in the urine phosphorus.

5. Body phosphorus pool

We have calculated the size of a body "phos-phorus pool," and its turnover rate, using themethods which Sprinson and Rittenberg developedfor calculation of the body "nitrogen pool" (10).

401-J0.zI

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(7)

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30

0

att\ tlseO f lt0 190

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FIG. 4. EFFEc~r OF INTRVENOUSGLUCOSEAN'D INSULINPTFROMPLASMA

The body "phosphorus pool" with which we are phosphiconcerned is the rapidly exchangeable phosphorus. calculatSprinson and Rittenberg showed theoretically, cows to

E E + S phospha=E +S and b = p , (9) phorus

phoruswhere a and b are the constants of equations 7 mi. orand 8 above, E = mg. nitrogen excreted per min., figuresS = rate of turnover of body pool nitrogen in rmg, urine (per min., and P = size of the nitrogen body pool rate ofin mg. sentedl

Wehave assumed these same relationships for rapidlyphosphorus. The results of our calculations are over aFlisted in Table II. The average size of the rapidly conditicexchangeable phosphorous body pool (P) is about1,200 mug. in our subjects. These subjects, as men- 6. Dist:tioned, were young adult males in good health, phos~weighing on the average about 75 Kg., and wereon normal food intakes prior to study. The ob- Tabl4servations were made after a 12 hour fast. The distributotal phosphorus in such an individual is about 150 fractiongin. (11). Thus, under the conditions of this cose, astudy, the rapidly exchangeable phosphorus ControJamounts to about 0.2 per cent of the total body in two

MINUTES

ON THE RATE OF DISAPPEARANCEOF

orus. Kleiber, Smith, and Ralston (7),ted the exchangeable body phosphorus forbe about 0.5 to 0.9 per cent of the total body

orus. The average rate (S) at which phos-leaves the rapidly exchangeable body phos-pool in our subjects is about 8.5 mg. perabout 0.7 per cent of the pool per min., a

similar to the rate at which p82 enters the0.8 per cent injected p82 per min.) and thedisappearance of p82 from plasma repre-

by k2 (0.8 per cent per min.). Thus, theexchangeable body phosphorous pool turnspproximately ten times per day under theons of this study.

Iribution of injected p32 in various plasmapkorus fractions

es III, IV, and V present detailed data onition of p82 in various plasma phosphorousas of control subjects, those receiving glu-ind those receiving insulin, respectively.I data are available as serial observationssubjects over a four hour period, and in six

501

S. M. LEVENSON, M. A. ADAMS, H. ROSEN, AND F. H. L. TAYLOR

TABLE I

Urinary excretion of Pa foUowing the intravenousadministration of P3 labeled phosphate

a

a.

I -0

z

0 60 120 180 240 300 360 420MINUTES -

FIG. 5. CUMULATVEURINARY PS' EXCRETION OF SnxNoRMALMALES

subjects at one hour after injection. Variations insome of the chemically measured fractions in thep!asma and red cells, somewhat greater than ex-pected from possible technical error, were observedin some of the control subjects. During the pe-riod of time under consideration, most of the in-jected p'2 remained in the inorganic fraction.The percentage Of p82 in the organic phosphorusfractions varied from 0 to 12 per cent, with a meanof 6 per cent. This average percentage is just out-side the error of the methods of measurement.Essentially all of the p82 which had gone into theorganic fractions was in the acid insoluble phase.These in vivo data demonstrate substantially thesame findings as the results obtained previously inin vitro studies at 37° C. (12).

Regarding the organic fractions, there does notappear to be any progressive shift of p82 to thevery small acid soluble organic component. Thisraises the question of whether there actually ex-ists, in human plasma, an acid soluble organicfraction, since one would expect that during a pe-riod of four hours, there would be a measurable

Timeafter

injectionhrs. Subject

1 12356789

101113141516

2 2389

101113141516

3 2356789

101113141516

4 23

125 5

6789

137 5

624 4

67

12161718

Total Pexcreted

(cumulative)mg.

834

11523865939

16054

1208816

280110

12205

71200

9416510535

29012027

25055

13511511525511519012555

300125

63315200175185215190335280300240

1,480905870610940

1,610715

Per centinjected PS

excreted(cumulative)

2.52.91.63.94.22.62.44.84.76.43.11.81.63.13.22.53.76.66.59.24.02.81.93.6

3.72.95.25.84.64.97.87.1

10.04.44.32.23.74.33.67.57.17.46.16.59.26.58.47.8

12.09.4

11.011.0

6.713.0

4.1

Specificactivity*

0.0300.7250.0140.1700.0490.0440.0620.0300.0870.0540.0380.1100.0060.028

0.2670.0120.0520.0330.0690.0560.0380.0800.0070.030

0.1400.0120.0950.0420.0400.0430.0300.0600.0520.0350.0780.0070.0300.0680.0120.0370.0410.0390.0280.0340.0270.0230.0280.0330.0080.0100.0130.0180.0070.0080.006

* Specific activities were obtained by dividing the percent of injected Pa per 100 ml. plasma, red cells and urine,by the mg. per cent of phosphorus in each fraction. Theuse of the percentage of added Pa rather than the numberof counts as the basis of calculation permitted direct com-parison of the different experiments despite variation inthe actual amount of Pa added.

502

-

STUDIES IN PHOSPHORUSMETABOLISM5

TABLE II

Urinary excretion and turnover of phosphorus

Total Pa Rate of Size of Rate at whichRate of excreted phosphorus body phosphorusp32 in 7 hours excretion pool leaves pool

Subject excretion after injection (E) (P) (S)Per cent Per centbody Pa injecedper min. dose mg. per min. mg. mg. per mix.

1 0.78 10.0 1.1 1,400 10.02 0.78 7.0 0.6 1,100 8.03 0.79 8.0 1.0 1,400 10.24 0.76 7.5 0.6 1,050 7.45 0.77 7.5 0.6 1,050 7.46 0.77 5.6 1.3 3,400* 24.5*

Avg. 0.78 7.5 0.9 1,200 8.6

* Not included in average.

movement of pS2 into this fraction. The possibilityexists, however, that a very slow shift of p32 intothe acid soluble organic phase does take place.

Observations were made regarding the possibleeffects of glucose and insulin on the distribution ofp32. With regard to glucose, we have data on twosubjects, and in these, there was no apparent ef-fect on p32 distribution as compared with normals(Tables IV and V). Of the three subjects re-

ceiving insulin, on whom we have detailed data,two showed no differences in p32 distribution com-pared to normal, while in the third, there was aslight additional shift into the acid insoluble organicphase. A pre-insulin level of 6 per cent of the totalp32 increased to 16 per cent in this fraction twohours after insulin administration. The bloodsugar curve was no different in this subject fromthose in the other two subjects nor was the curve ofinorganic or total p32 concentrations different(Table V).

7. Distribution of injected p32 in the various redcell phosphorus fractions

Data are available in four subjects regardingpartition of phosphorus fractions in red cells(Table VI). In each instance the major portion

of the p82 was initially present in the inorganic frac-tion. At the peak uptake, about 85 per cent ofthe total p32 was present as inorganic phosphorus,-a figure which corresponds closely with in suitrodata previously obtained (12). This would seem-definitely to establish the existence of inorganic

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S. M. LEVENSON, M. A. ADAMS, H. ROSEN, AND F. H. L. TAYLOR

TABLE VII

Distribution of Pa2 between plasma and red blood cellsSpecific activities*Inorganic fractions

Red bloodPlasma cells

In vitro, after 4 hrs.incubation at 37° C. 14 13

In vivo, 5 hrs. afterI.V. injection 0.019 0.018

* See Table I.

phosphorus in human erythrocytes. What P32 iSpresent as organic is in the acid soluble fraction.

It may be seen in Table VII that specific activi-ties in the inorganic fractions of plasma and redcells are identical at equilibrium. This steadystate has occurred both in zivo and in sitro by fourto five hours. The in vitro experiments were car-ried out at 370 C. In qivo, there is a gradual shiftof p32 from the inorganic to the organic phase inthe red cells, such that at five hours, 91 per centof the total P32 iS in the inorganic, while at 120hours, this percentage has decreased to 71, and theorganic has risen concomitantly from 9 per centto 24 per cent (Table VIII). All of the P82 presentin the organic fraction was present in the acidsoluble phase five hours after injection. In thenext few days, some appeared in the acid insolubleorganic phase, but the major part still was foundin the acid soluble fraction.

SUMMARY

1. The distribution of radiophosphorus hasbeen studied in various plasma, red cell, and urinefractions of normal young men following the rapidintravenous administration of 100 to 200 micro-curies of p32 as Na2HP3204. The subjects wereon normal intakes prior to study. Observationswere made after a 12 hour fast.

TABLE VIII

Per cent Pa1 in various red blood cell phosphorus fractions

Time (hrs.)after I.V.injection

5

244872

120

TotalInorganic acid

P soluble P

91 10083 9879 9376 9271 95

Acidsoluble

organic P

915

141624

Acidinsoluble

organic P

0

2785

2. Most of the labeled phosphate leaves theblood within a few minutes after injection, indi-cating a very rapid transcapillary movement ofphosphate.

3. Following this initial rapid decrease in plasmaradioactivity its disappearance slows. This mayrepresent entrance of phosphate into cells. Mathe-matical analysis of this transfer shows it to be ofa double exponential nature. The double exponen-tial takes the form:

P = 0.85e0-08t + 0.27e-°01t + 0.03,

where P = per cent injected p32 per 100 ml.plasma found t minutes after injection. The rateconstants for the two processes are 38 x 10- permin. and 7.9 x 103 per min., respectively.

4. The uptake of p32 by the red cells in vivowas found to be represented by the equation:

Q = 0.07(1 - e30t) + 0.13(1 - e- 02t),where Q= per cent injected P32 per 100 ml. redcells found t minutes after injection.

5. Glucose (25 gm.), administered intravenouslyone hour after injection of Na2HP3204, acceleratesthe movement of p32 out of the plasma by a factorof 3, while intravenous insulin (0.1 unit per Kg.body weight) accelerates it by a factor of 4.

6., Excretion of p32 in the urine during the pe-riod one to seven hours after injection was shownto follow the process:

R = Roa(l - e-bt),

where R = per cent injected P32 found in urine att minutes after injection and a and b are con-stants. The excretion of P32 was 0.8 per cent permin. of the injected P32.

7. From the above data and measurement ofurinary phosphorus as such, certain average body"constants" were calculated. These are the rapidlyexchangeable body "phosphorus pool" (about 1.2gm.) and the rate at which phosphorus leaves thispool (about 8.5 mg. per min.). These data sug-gest that the rapidly exchangeable body phosphorusturns over approximately 10 times per day underthe conditions of this study.

8. Over a four hour period following the in-jection of labeled phosphate approximately 95 percent of the PB2 iS in the inorganic phosphorus frac-tion. Almost all the remaining p32 is in the acidinsoluble organic fraction. No definite changes

508

STUDIES IN PHOSPHORUSMETABOLISM

were found in the p32 distribution in the plasmaphosphate fractions as a result of glucose or insu-lin administration.

9. The distribution of P32 was also studied invarious red cell phosphorus fractions. At peakuptake (about five to seven hours after injection)about 85 per cent was found in the inorganic phos-phorus fraction. The specific activities of P32 inthe inorganic fractions of plasma and red cells areidentical at this time. There is a gradual shift ofp32 from the inorganic to the organic fraction sothat at 120 hours after injection about 30 per centof the PS2 is present as organic phosphorus, chieflyas acid soluble compounds.

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Biol. Chem., 1929, 81, 629.2. Adams, M. A., Levenson, S. M., Fluharty, R. G., and

Taylor, F. H. L., Methods for the determinationsof radioactive phosphorus (P') in body fluids. J.Lab. & Gin. Med., 1949, 34, 1301.

3. Gourley, D. R. H., and Gemmill, C. L., The effect oftemperature upon the uptake of radioactive phos-phate by human erythrocytes in vitro. J. Cell. &Comp. Physiol., 1950, 35, 341.

4. Erf, L. A., and Lawrence, J. H., Clinical studies withthe aid of radioactive phosphorus. I. The absorptionand distribution of radio-phosphorus in the blood

and its excretion by normal individuals and patientswith leukemia. J. Clin. Invest, 1941, 20, 567.

5. Cardozo, R. H., and Edelman, I. S., The volume ofdistribution of sodium thiosulfate as a measure ofthe extracellular fluid space. J. Clin. Invest., 1952,31, 280.

6. Pappenheimer, J. R., Renlin, E. M., and Borrero,L. M., Filtration, diffusion and molecular sievingthrough peripheral capillary membranes. A con-tribution to the pore theory of capillary perme-ability. Am. J. Physiol., 1951, 167, 13.

7. Kleiber, M., Smith, A. H., and Ralston, N. P., Mix-ing rate of phosphate between plasma and inter-stitial body fluid of cows. J. Gen. Physiol., 1950,33, 525.

8. Walker, W. G., and Wilde, W. S., Kinetics of radio-potassium in the circulation. Amer. J. Physiol.,1952, 170, 401.

9. Solomon, A. K., Equations for tracer experiments.J. Gin. Invest., 1949, 28, 1297.

10. Sprinson, D. B., and Rittenberg, D., The rate of inter-action of the amino acids of the diet with the tissueproteins. J. Biol. Chem., 1949, 180, 715.

11. Widdowson, E. M., McCance, R. A., and Spray, C. M.,The chemical composition of the human body.Clin. Sc., 1951, 10, 113.

12. Taylor, F. H. L., Levenson, S. M., and Adams, M. A.,Studies of phosphorus metabolism in man: II. Astudy of the permeability of the human erythrocyteto inorganic phosphate in vitro by the use of radio-active phosphate (P). Blood, 1948, 3, 1472.

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