by r. f. pitts, w. d. lotspeich, w. a. l. ayer of...

THE RENALREGULATIONOF ACID-BASE BALANCEIN MAN.I. THE NATUREOF THE MECHANISMFOR

ACIDIFYING THE URINE 1

By R. F. PITTS, W. D. LOTSPEICH, W. A. SCHIESS, AND J. L. AYERWITH THE TECHNICAL ASSISTANCE OF PHYLLIS MINER

(From the Department of Physiology, Syracuse University College of Medicine,Syracuse, N. Y.)

(Received for publication August 29, 1947)

Under normal conditions the urine is moreacid in reaction than the plasma from which it isformed, and some 100 to 300 ml. of one-tenth nor-mal alkali are required to titrate the 24-hour speci-men to pH 7.4 (1). When the metabolic acidload on the body is increased in diabetic ketosis, asmuch as 1200 ml. of one-tenth normal titratableacid may be excreted per day (2). Since the ex-cretion of acid in free titratable form makes avail-able to the body an equivalent quantity of fixedbase for conversion into bicarbonate, the renalmechanisms involved in acidifying the urine playa significant role in stabilizing the alkali reserve(3).

There are obviously 2 ways in which acid mightgain access to the urine. First, it might be presentas free buffer acid in the glomerular filtrate. Ifduring passage of the filtrate through the renaltubules, alkaline buffer components were pre-ferentially reabsorbed, the reaction of the urinewould shift toward the acid side. Second, theacid might be added to the filtrate by some activetransport mechanism resident in the renal tubularcells. The 4 possible permutations of these 2theories are presented in diagrammatic form inFigure 1.

Only 2 acids exist in the plasma in significantquantities in freely filtrable form: monobasicphosphate and carbonic acid. Each, associatedwith its alkaline counterpart, dibasic phosphateand bicarbonate, passes into the glomerular filtrate.If dibasic phosphate were reabsorbed by the renaltubules, the excreted monobasic phosphate couldbe titrated as acid in the urine (1). On the otherhand, if bicarbonate were reabsorbed from thefiltrate, and if the renal tubules were relativelyimpermeable to carbonic acid (in reality dissolved

I Aided by grants from the United States Public HealthService and the John and Mary R. Markle Foundation.

carbon dioxide), this acid could react with buffersalts and convert them into free titratable acid(4).

Similarly 2 hypothetical mechanisms dependingon secretary activities of the tubular epitheliumhave been proposed as explanations of urinaryacidification. The tubular secretion of molecularacid has been invoked to account for the conver-sion of the alkaline buffer components of the glo-merular filtrate into their acid forms (5). Onthe other hand a pseudosecretory or ionic-exchangemechanism accomplishing the same end has beensuggested (6). According to this latter view,hydrogen ions, formed within the tubular cells bythe dissociation of carbonic acid, are exchanged forions of fixed base bound by buffer salts in the tu-bular urine, thereby converting them into titratablebuffer acids.

Recent studies on the dog (7, 8, 9) have indi-cated that the induction of acidosis and the infusionof suitable buffers greatly facilitate the excretionof titratable acid. By measuring simultaneouslythe rate of filtration and reabsorption of phosphateand the rate of filtration of carbonic acid, it wasdemonstrated that the phosphate and carbonic acidfiltration-reabsorption concepts are inadequateto explain the origin of urinary acid. Thus, by ex-clusion, it was shown that the tubular cells mustadd acid to the urine. Indirect evidence favoredionic exchange as the more reasonable of the 2alternative mechanisms.

The present study of the mechanism of acidifica-tion of the urine by the normal human kidney wasundertaken with 2 purposes in mind: first, toidentify the nature of the mechanism; and second,to compare the acid excretory capacity of thehuman kidney with that of the dog. It has beenfound that the mechanisms are qualitatively simi-lar in man and dog. Quantitatively, the capacity

48

NATUREOF MECHANISMFOR ACIDIFYING URINE

of the normal human kidney to excrete acid ap-pears to exceed that of the dog.

METHODS

The 8 experiments forming the basis of this report,were performed on 4 healthy adult male subjects. Acido-sis was induced by the ingestion of 20 grams of ammoniumchloride on the day preceding the experiment. The saltwas divided into 10 equal doses and was taken well dilutedwith water at hourly intervals throughout the day. Nofood was taken following the evening meal. Two glassesof water were ingested the morning of the experiment.Urines were collected by catheter and the bladder waswashed with distilled water at the end of each collectionperiod. Blood was drawn from 1 of the superficial veinsof the forearm after soaking the limb in water at 470 to480 C. for 5 to 7 minutes to render the composition ofthe venous blood essentially the same as that of arterialblood. For measurement of pH and carbon dioxide con-tent, the blood was handled anaerobically (10). Analyti-cal procedures for thiosulfate, phosphate, creatinine andurinary titratable acid and ammonia have been describedin previous communications (7, 11).

Glomerular filtration rate was assessed by the thiosul-fate clearance (12). Plasma thiosulfate concentrationswere maintained between 30 and 40 mgm. per cent bycontinuous intravenous infusions.' Both urine and plasmablanks were determined and appropriate corrections ap-plied in all calculations. Neutral sodium phosphate 2(pH 7.4) was incorporated in the infusions and volumewas adjusted by the addition of distilled water to renderall solutions isotonic. The rate of infusion was variedbetween 3 and 13 ml. per minute to attain the desiredplasma concentrations. No adverse reactions were ob-served from the infusion of phosphate; nausea and emesisfollowed the infusion of creatinine. At the highest plasmaphosphate levels (6 to 7 mMper liter) minimal signs oftetany were observed which subsided when the infusionswere stopped.

RESULTS

Experiments znith phosphate

Our plan of attack in identifying the nature ofthe renal mechanism for acidifying the urine hasbeen to increase the rate of excretion of tittatableacid by the induction of acidosis and by the in-fusion of large quantities of sodium phosphate. Bymeasuring the quantities of phosphate filtered, re-absorbed and excreted and the quantity of car-bonic acid filtered it has been possible to calculateexactly the maximum contribution which the hy-

2We are indebted to the William R. Warner Co. ofNew York for the preparation of a generous supply ofampoules of the sterile pyrogen-free sodium phosphatesolution of pH 7.4 used in this work.

THEORIES TO ACCOUNTFOR THE EXCRETIONOFACID URINE

URINARY ACID PRESENT IN ORIGINAL FILTRATEPHOSPHATEREABSORPTIONTHEORY

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URINARY ACID SECRETEDINTO TUBULAR URINEMOLECULARACID SECRETION THEORY

BA ( .BA- BCL*HA -HA

HCL BCL

IONIC EXCHANGETHEORY

BA 4 7 23HA-. HA

Ho HCOj

FIG. 1. FOR EXPLANATION, SE TEXT

pothetical phosphate and carbonic acid filtration-reabsorption mechanisms could possibly make tothe excretion of titratable acid. Since monobasicphosphate and carbonic acid are the only acidspresent in significant amounts in a protein-freeultrafiltrate of plasma, it is obvious that if thesesubstances cannot account for the observed rateof excretion of titratable acid, acid must be addedto the urine by one or the other of the 2 activecellular transport mechanisms. Experiments pre-sented below are conclusive in showing that tubularmechanisms are responsible for the addition of acidto the urine under our conditions of stress.

Experiment 1 in Table I illustrates both thecharacter of the measurements and the relativesignificance of acidosis and plasma phosphate con-centration in the experimental procedures em-ployed. A moderately severe yet well compensatedacidosis was induced by the ingestion of ammoniumchloride. The extent of the acidosis is apparent inthe plasma bicarbonate concentration of 14.2 mMper liter, and the degree of compensation is evi-dent both in the plasma pH of 7.37 and in theplasma carbonic acid concentration of 0.77,3 shownin the first 2 periods of Experiment 1. In these

3 Normal values for R. F. P. are bicarbonate, 24 to 25mMper liter; pH, 7.39 to 7.41; carbonic acid, 1.17 to1.23 mMper liter.

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2 control periods the concentration of phosphatein the plasma ranged between 1.10 and 0.92 mMper liter, or 0.00110 and 0.00092 mMper milliliter.If these latter figures are multiplied by the glo-merular filtration rate, one obtains the quantityof phosphate filtered per minute. The productof the urine concentration and urine flow yieldsthe quantity of phosphate excreted per minute.The quantity of phosphate reabsorbed, equal tothe difference between the quantities filtered andexcreted, amounted initially to 0.083 and 0.061mMper minute. Assuming this reabsorbed phos-phate to be the dibasic form, the residual excessof monobasic phosphate left in the urine could ac-count for the excretion of 0.017 and 0.016 mEqoftitratable acid per minute, or 56.6 to 48.1 per centof that actually observed. Thus, acidosis alonerendered the phosphate reabsorption concept un-tenable in this experiment. In contrast, the fil-tration of carbonic acid is capable of explaining100 per cent of the titratable acid eliminated in the2 control periods.

In the succeeding 6 periods the plasma phos-phate concentration was elevated in stepwise fash-ion to a maximum of 6.52 mMper liter by theinfusion of neutral sodium phosphate. The re-absorption of phosphate reached a maximum of0.120 to 0.130 mMper minute and the excess fil-tered, over and above this quantity reabsorbed,was excreted in the urine. The observed rate ofexcretion of titratable acid increased in linearfashion with the increased rate of excretion ofphosphate despite the relatively constant rate ofreabsorption. Accordingly, the phosphate re-absorption concept became progressively a lessadequate explanation of urinary acidity, until inthe final periods it could account for only 8 percent of the observed acid. The quantity of car-bonic acid filtered remained essentially constantthroughout the experiment, varying only between0.068 and 0.086 mEq per minute. Accordinglyfrom the fourth period on, the carbonic acid fil-tration concept likewise became an inadequate ex-planation of urinary acidity and in the final pe-riod could account for only 21.6 per cent of theacid excreted. It is apparent that titratable acidexcretion can be increased to levels greatly abovenormal by increasing the quantity of phosphateexcreted. In fact in the eighth period of Experi-ment 1, a rate of excretion of 0.333 mEqper min-

ute is equivalent to the elimination of 4750 ml. ofone-tenth normal acid per day. This is some 3to 4 times the highest ever observed in diabeticketosis. At plasma phosphate concentrations of6 to 7 mMper liter, the 2 hypothetical filtration-reabsorption mechanisms together can accountfor only one-third of the titratable acid excreted.Since these are the only significant sources of acidin the filtrate it is apparent that acid must be addedto the tubular urine by some active cellular trans-port mechanism. It seems reasonable to us toassume that this cellular mechanism accounts foracid excretion under normal conditions as wellas under conditions of acidosis and hyperphos-phatemia, although it is impossible to identify itsactivity except by exceeding the limited acid ex-cretory capacity of the 2 filtration-reabsorptionmechanisms. We likewise feel from theoreticalconsiderations discussed at length in a previouspaper (7), that under no circumstances do phos-phate reabsorption and carbonic acid filtrationcontribute significantly to the titratable acid ofthe urine.

With the information derived from this pre-liminary experiment, identical 4-period experi-ments were performed on 4 subjects, the resultsof which are presented in Table I. The extentof the acidosis varied considerably in the severalsubjects, as did the plasma concentration of phos-phate. Variations in filtration rate, phosphatereabsorptive capacity and plasma carbonic acidconcentration combined to alter the relative ade-quacies of the phosphate reabsorption conceptand the carbonic acid filtration concept as explana-tions of the observed rates of excretion of titra-table acid. In these latter 4 experiments phosphatereabsorption could account for only 8.0 to 11.7per cent of the excreted acid. Carbonic acid filtra-tion could account for only 21 to 40 per cent ofthe excreted acid. Both mechanisms togethercould account for only one-third to one-half ofthe acid. Thus, it is apparent that acid must beadded to the urine by the renal tubules.

The columns in the center of the table headedrate of excretion of titratable acid calculated fromphosphate excreted serve as a check on the ac-curacy of the chemical determinations, at least inthose instances in which the rate of excretion ofphosphate was high. In Experiments 2 to 5,values for titratable acid calculated from the pH

51

R. F. PITTS, W. D. LOTSPEICH, W. A. SCHIESS, AND J. L. AYER

of the urine and the rate of excretion of phos-phate should agree with the observed valueswithin reasonably small limits. The calculatedvalues average 101.84 per cent of the observedvalues. Actually the calculated values shouldaverage slightly less than 100 per cent, for there isa small moiety of non-phosphate titratable acidin the urine made up of creatinine and organicacid. However, the agreement is sufficiently closeto warrant confidence in conclusions drawn fromthe more rigorous analysis of the data. In theinitial 2 periods of Experiment 1, phosphate con-

stituted between 50 and 60 per cent of the titra-table acid; about. half of the remainder was cre-atinine, about half was organic acid. As the ex-

cretion of phosphate and titratable acid increased,this nonphosphate moiety became a less and lesssignificant proportion of the total. Whether itremained constant or diminished cannot be stated,because the range of experimental error of thedetermination at high phosphate levels is greaterthan this moiety.

The rate of ammonia excretion was followed ineach experiment and averaged 0.066 mEq per

minute in the series presented in Table I. Al-though considerably increased over the normal,this rate of excretion is not high in comparisonwith that observed in diabetic ketosis, no doubtbecause the acidosis was not especially severe andwas of short duration (less than 24 hours). Innormal individuals and in patients in diabeticketosis as well, the ratio of ammonia to titratableacid in the urine varies within limits of 1.0 and2.5 (13). In the initial 2 periods of Experiment1 the ratio amounted to 2.03 and 2.22, well withinthe usual limits. Following the infusion of phos-phate the ratio dropped progressively to a lowof 0.16. This drop was brought about for themost part by the increase in rate of titratable acidexcretion, not by diminution in ammonia elimina-tion. Thus, the magnitude of this ratio in thepresence of normal renal function is conditionedlargely by the quantity and properties of the uri-

nary buffers.It is obviously possible to analyze the data in

another way, namely in terms of the pH of theurine. The pH of the urine was calculated from

'The pK' for phosphate has been taken to be 6.80.Obviously variation in this value in different urine samplesis a possible source of error.

the measured rate of buffer excretion assumingfirst that the only source of acid is filtered carbonicacid, and second that the source of acid is theexcess monobasic phosphate remaining in thetubules following reabsorption of dibasic phos-phate. The results of this analysis are presentedin the last 3 columns of Table I. In order toperform this calculation it has been necessary toassume a pK' for the non-phosphate moiety of thetitratable acid. This has been taken to be thatof creatinine, namely 4.97, an assumption whichis only partially justified. Except in the initial 2periods of Experiment 1, no serious error is in-troduced in the calculations if this assumed pK'is erroneous. It is apparent from Experiment 1that the phosphate reabsorption concept and thecarbonic acid filtration concept are adequate toexplain the pH of the urine only when the rateof buffer excretion is low. As the buffer con-tent of the urine is increased by the infusion ofphosphate, the deviations between observed andcalculated urine pH values become large. It isinteresting that 2 subjects, R. F. P. and W. D. L.,were able to excrete urines containing large quan-tities of phosphate, yet of an acidity considerablygreater than that assumed to be possible by thehuman kidney. The generally accepted limit ofacidity of human urine is pH 4.7 (14). Actuallyin 3 experiments on these 2 subjects all urinesranged in acidity between pH 4.48 and 4.60.

Experiments with creatinineIn order to test further the thesis that the urine

is acidified by tubular secretary processes, we re-peated the experiments substituting creatinine forphosphate as the buffer administered. Three ex-periments were performed on 3 different subjects.In the first experiment neither the quantity ofcreatinine administered nor the degree of acidosiswas sufficiently great to yield conclusive results.The 2 more significant experiments are presentedin Table II.

In Experiment 7 the acidosis was moderate; inExperiment 8, somewhat more severe; in both,the acidosis was well compensated. Sufficientcreatinine was administered to raise the plasmaconcentration to 20 to 33 mgm. per cent and toeffect the excretion of 0.296 to 0.466 mMof thisbuffer per minute. The rate of elimination ofendogenous phosphate varied between 0.016 and

52

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0.026 mMper minute. The values for titratableacid calculated from the urine pH and the sum ofthe creatinine and phosphate eliminated variedfrom 0.161 to 0.205 mEq per minute. The ob-served rates of titratable acid excretion variedfrom 0.167 to 0.203 mEqper minute, values muchlower than those observed following phosphate ad-ministration. Nevertheless, calculated and ob-served values for titratable acid agreed withinreasonable limits.

It is evident from the calculations of the maxi-mum possible contributions of phosphate reab-sorption and carbonic acid filtration to titratableacid excretion that neither concept adequately ex-plains acidification of the urine. Thus, carbonicacid filtration could account for only 35 to 56 percent, and phosphate reabsorption for only 5.4 to8.7 per cent of the excreted acid. Similarly ananalysis of the possible contributions of the 2 mech-anisms to the excretion of urine of low pH, indi-cates their essential incapacity to account for thedegree of acidity actually obtained. Although thedeficiencies of the 2 hypothetical filtration-reab-sorption mechanisms are somewhat less striking inthese experiments with creatinine than in thephosphate ones, they do confirm the thesis thatactive tubular processes must add acid to theurine. This confirmation is especially significantin view of the differences in the mechanisms bywhich the human kidney handles phosphate andcreatinine: the former by filtration and reabsorp-tion, the latter by filtration and secretion (15).

DISCUSSION

The data presented above are conclusive indemonstrating that under our experimental condi-tions the normal human kidney can excrete moreacid per unit of time than could gain access to theurine in the glomerular filtrate. Therefore, by ex-clusion, acid must be added to the urine by somemechanism of active tubular transport. As pointedout above, this mechanism might be one of tubu-lar secretion of molecular acid, or one of exchangeof hydrogen ions formed within the renal tubularcells for ions of fixed base in the tubular urine.Our experimental methods cannot distinguish be-tween these 2 possibilities. Indeed, as pointedout in previous work on the dog (7) the questionis largely an academic one. If molecular acid,

e.g., hydrochloric acid, were secreted by the renaltubules, the sodium chloride formed in its reactionwith buffer salts would be largely reabsorbedfurther along the tubules. Therefore, in effect,an exchange of hydrogen ions for sodium ionswould be brought about by secretion of molecularacid followed by reabsorption of neutral salt.Because the various salts in plasma, glomerularfiltrate and urine are dissociated, it seems reason-able to assume that the kidney operates on ionsrather than on molecular species, within the limi-tations imposed by the necessity for maintainingthe electroneutrality of these several fluids. Anionic exchange mechanism would accomplish in 1step the conservation of base and the excretion ofacid, and incidentally satisfy requirements ofelectroneutrality.

Our experimental results on normal humansubjects are so similar to those previously pub-lished on dogs that it seems legitimate to assumethat the renal mechanisms are identical. One maytherefore be permitted to extrapolate from certainexperiments performed on dogs, too rigorous tobe applicable to man, to fill in gaps in knowledgeof the human renal mechanism. Obviously car-bonic acid, formed by the hydration of carbondioxide, is the only source of hydrogen ions avail-able to the body in quantities sufficient to satisfythe demands imposed by the observed rates ofurinary acid excretion. Kidney tissue containslarge amounts of carbonic anhydrase, an enzymewhich markedly increases the rate of hydration ofcarbon dioxide to carbonic acid (16). Sulfanila-mide, an inhibitor of carbonic anhydrase (17),when present in the plasma in high concentration(30 to 80 mgm. per cent) greatly reduces orabolishes the excretion of titratable acid in thedog (7). From these facts it was concluded thatcarbonic anhydrase is an essential link in thetubular ionic exchange mechanism, and that itsrole is to speed the hydration of carbon dioxide tocarbonic acid within the cells of the renal tubules.The dissociation of this carbonic acid provideshydrogen ions for exchange with base bound bybuffers of the tubular urine. The base in associ-ation with bicarbonate ions is then returned tothe renal venous blood. We feel that these con-clusions regarding the nature of the tubular mech-anism in so far as they are valid for the dog, are

54


equally applicable in man, especially in view ofthe disturbances in acid-base balance described inthe early clinical use of sulfanilamide (18). In-deed we feel that one of the most significant as-pects of the present investigation lies in its justifi-cation of the use of the dog for analysis of prob-lems of acid-base regulation.

In a quantitative sense, we feel that the capacityof the human kidney to excrete acid probably ex-ceeds that of the dog. This statement may not begenerally valid for it is based only upon a com-parison of 4 healthy young adults with 4 healthymongrel dogs. But the fact that the results are soconsistent seems to justify this tentative state-ment. In experiments on dogs (7) similar tothose reported above, the highest rate of titratableacid excretion which was observed amounted to0.380 mEq per minute. This was attained ata plasma phosphate level of 9.84 mMper literand at a rate of excretion of phosphate of 0.613mMper minute. In attaining this rate of acidexcretion, the kidney elaborated a urine of pH6.07; in essence the kidney could establish a con-centration gradient for hydrogen ions betweentubular urine and plasma of somewhat less than20 to 1. The 4 human subjects excreted from0.374 to 0.451 mEq of titratable acid at plasmaphosphate concentrations which were substantiallylower than those of the dogs. Furthermore, atcomparable rates of phosphate excretion, urinesof much lower pH (4.48 to 5.54) were elaboratedby the human subjects. Thus the human kidneycan transfer the same or larger quantities of hy-drogen ions against higher concentration gradients.Subjects W. A. S. and J. L. A. could establish agradient of roughly 100 to 1 while transferring ap-proximately 0.400 mEq of hydrogen ions perminute; subjects R. F. P. and W. D. L. couldestablish a gradient of 800 to 1 while transferringequivalent quantities of hydrogen ions. The diffi-culties of comparing acid excretory capacity indog and man not only lie in differences in bodyand kidney size, but likewise in the fact that inneither dog nor man was any absolute maximumfor acid excretion attained. Doubtless further in-creases in the rate of phosphate excretion in bothforms would have led to further increases in acidexcretion.

SUMMARY

In a series of 8 experiments on 4 healthy humansubjects it was found that the induction of acidosisby the ingestion of ammonium chloride and thepromotion of buffer excretion by the infusion ofphosphate or creatinine greatly increased the rateof excretion of titratable acid. Simultaneousmeasurement of the rate of filtration, reabsorptionand excretion of mono- and dibasic phosphate andcarbonic acid demonstrated that the quantity ofacid excreted far exceeded that which entered theurine in the glomerular filtrate. Therefore, acidmust have been added to the filtrate as it passedalong the nephron by some mechanism of activetransport resident in the renal tubular cells. Itis suggested that this addition of acid is effectedby the exchange of hydrogen ions formed withinthe tubular cells for ions of fixed base in the tu-bular urine. Carbonic acid is doubtless the intra-cellular source of hydrogen ions.

This mechanism for acid excretion in the humankidney is qualitatively similar to that previouslydescribed in the dog. Quantitatively, the humankidney has a greater acid excretory capacity thanthe dog kidney.

BIBLIOGRAPHY1. Peters, J. P., and Van Slyke, D. D., Quantitative

Clinical Chemistry, Vol. 1, Interpretations. Wil-liajns and Wilkins Co., Baltimore, 1932.

2. Stillman, E., Van Slyke, D. D., Cullen, G. E., andFitz, R., Studies on acidosis. VI. The blood, urineand alveolar air in diabetic acidosis. J. Biol.Chem., 1917, 30, 405.

3. Henderson, L. J., A critical study of the process ofacid excretion. J. Biol. Chem., 1911, 9, 403.

4. Sendroy, J., Jr., Seelig, S., and Van Slyke, D. D.,Studies on acidosis. XXIII. The carbon dioxidetension and acid-base balance of human urine. J.Biol. Chem., 1934, 106, 479.

5. Macallum, A. B., and Campbell, W. R., The secretionof acid by the kidney. Am. J. Physiol., 1929, 90,439.

6. Smith, H. W., The Physiology of the Kidney. Ox-ford University Press, New York, 1937.

7. Pitts, R. F., and Alexander, R. S., The nature ofthe renal tubular mechanism for acidifying theurine. Am. J. Physiol., 1945, 144, 239.

8. Pitts, R. F., and Lotspeich, W. D., Factors governingthe rate of excretion of titratable acid in the dog.Am. J. Physiol., 1946, 147, 481.

9. Pitts, R. F., The renal regulation of acid-base bal-ance with special reference to the mechanism foracidifying the urine. Science, 1945, 102, 49 and 81.

55


10. Pitts, R. F., and Lotspeich, W. D., Bicarbonate andthe renal regulation of acid-base balance. Am. J.

Physiol., 1946, 147, 138.11. Pitts, R. F., and Lotspeich, W. D., Use of thiosulfate

clearance as a measure of glomerular filtration ratein acidotic dogs. Proc. Soc. Exper. Biol. andMed., 1947, 64, 224.

12. Newman, E. V., Gilman, A., and Philips, F. S., Therenal clearance of thiosulfate in man. Bull. JohnsHopkins Hosp., 1946, 79, 229.

13. Van Slyke, D. D., Linder, G. C., Hiller, A., Leiter, L.,and McIntosh, J. F., The excretion of ammoniaand titratable acid in nephritis. J. Clin. Invest.,1926, 2, 255.

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by r. f. pitts, w. d. lotspeich, w. a. l. ayer of...

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