infusions of potassium salts even when their diet has not previously been

Transcription

1 Q. Jl exp. Phy8iol. (1969) 54, TH FFCTS OF INTRAVNOUS INFUSION OF KCl OR HCl ON TH RNAL XCRTION OF POTASSIUM IN SHP. By D. SCOTT. From the Physiology Department, Rowett Research Institute, Bucksburn, Aberdeen, AB2 9SB. (Received for publication 15th May 1968) Intravenous infusion of potassium chloride in sheep led to a rise in the concentration of potassium in the plasma, and to an increase in the rate of excretion of potassium in the urine, which rose rapidly until the rate of excretion of potassium was about equal to the rate of infusion of potassium. During potassium infusion the rate of excretion of potassium in the urine in some experiments exceeded the rate of filtration of potassium across the glomerulus, indicating net secretion of potassium by the renal tubules. Intravenous infusion of HC1 did not modify the renal response to intravenous infusion of potassium, but only a small proportion of the acid infused appeared in the urine. Chronic acidosis resulting from intra-ruminal infusion of HCI led to an increase in the rate of net acid excreted but no change in the rate of excretion of potassium in the urine. A large proportion of the net acid excreted was in the form of the ammonium ion and little was present as titratable acid. This result suggests that the excretion of large amounts of potassium in the urine of the sheep may limit the rate of excretion of titratable acid. WHN potassium salts are infused intravenously into dogs, secretion of potassium by the renal tubules is demonstrated by the rate of excretion of potassium in the urine rising to exceed the rate of filtration of potassium at the glomerulus [Berliner et al., 1950; Mudge et al., 1950]. These authors agree that this occurs readily only if the dog is given supplements of potassium in its diet for several days prior to infusion. This development of a tolerance to potassium as a result of increased potassium intake has also been shown in the rat [Thatcher and Radike, 1947] but cattle can tolerate intravenous infusions of potassium salts even when their diet has not previously been supplemented with potassium [Anderson and Pickering, 1962]. The diet of cattle is normally rich in potassium and Anderson and Pickering [1962] suggested that the response of the cow to intravenous potassium infusion may be associated with this high potassium intake. The purpose of the experiments described below was to examine the renal response of sheep to intravenous infusions of potassium salts and to see whether this response was modified during acidosis caused by administration of hydrochloric acid. MTHODS Animals and Diets. - Seven adult Scottish Blackfaced ewes averaging 39*5 kg. in weight were used in these experiments. All were fitted with permanent ebonite cannulas into the rumen several months before observations were begun. During experiments the sheep were kept in metabolism cages and were fed 800 g. per day 25

2 26 Scott of a grass-cube diet from a continuous belt feeder. The diet provided from 361 to 381 m.equiv. of potassium per day. Clearance procedure. - Urine samples were collected and glomerular filtration rate measured in experiments lasting from 4 to 6 hr. At the beginning of each experiment two polyethylene catheters were introduced into the right jugular vein. One catheter was used subsequently for the infusion of inulin while the other was used for the infusion of experimental solutions. A single polyethylene catheter was introduced into the left jugular vein from which blood samples were drawn. Urine was collected from a self-retaining 18 gauge urethral catheter. The sheep were given 2 or 3 litres of water into the rumen before urine collections started so that urine flows of 5-10 ml./min. were produced throughout the experiment. The solution of inulin used and the procedure for measuring glomerular filtration rate was the same as that described in the preceding paper, [Scott, 1969]. In calculating the amount of potassium filtered across the glomerulus a Gibbs-Donnan factor of 0 95 was used [Berliner et al., 1950]. Intravenous infusion of KCl and HCI. - After two or three clearance periods during which the inulin solution alone was infused, intravenous infusion of a sterile solution of from 0-5 to 2 N-KCI or 0-5 N-HCI was begun. These infusions were continued for periods up to 3 hr. at infusion rates varying from 0-5 to 1-0 ml./min. This produced rates of infusion of KCI varying from 0-27 to 1-2 m.mole/min. and rates of infusion of HC1 varying from 0-32 to 0-52 m.mole/min. Intra-ruminal infusions of HCI. - In these experiments a solution of 0 5 N-HCl was infused continuously at rates up to 300 m.mole of HCI per day into the rumen through the fistula. Urine was collected daily at 9.30 a.m.; its volume and ph were recorded and a sample was stored at C for analysis. Analytical methods. - The concentrations of sodium and potassium in plasma and urine were measured by flame photometry as described by Scott [1966]. The concentration of inulin in plasma and urine was determined by the direct resorcinol method of Schreiner [1950]. This method was modified for use on the Technicon Autoanalyser using a procedure described by Wilson, Stacy and Thorburn [1968]. The ph and PC02 of blood taken during either intravenous or intra-ruminal infusion of HCI were measured using an Astrup Micro-electrode and tonometer system (Radiometer: Copenhagen) as described by Siggaard Andersen, ngel, J0rgensen and Astrup [1960]. From these measurements the base excess of the blood was calculated using the Siggaard Andersen curve nomogram [Siggaard Andersen and ngel, 1960]. Net excretion of acid in the urine was determined by the method of J0rgensen [1957]. In this method net excretion of acid = HH,+ + Htitr acid - HCO3 - OHtjtr base- The rate of excretion of ammonium ion in the urine was measured using the method of Conway [1955] and the total CO2 content of urine was measured using the method of Peters and Van Slyke [1932]. The amount of titratable acid excreted in the urine during intraruminal infusion of HCI was calculated from the relationship Titratable acid =Net acid - HN,+ + HCO 3 RSULTS Intravenous infusion of KC1. - The effects of intravenous infusion of KCI on the urinary excretion of potassium are shown in fig. 1. Intravenous infusion of KC1 resulted in an increase in the rate of excretion of potassium in the urine. This increase reached a maximum min. after the onset of infusion and at this time the increased rate of excretion of potassium was

3 Renal xcretion of Potassium and Acid in Sheep 27 approximately equal to the rate of infusion of potassium. Intravenous infusion of KCl resulted in an increase in the rate of excretion of sodium in the urine. This sodium diuresis was most marked at the higher rates of infusion of KC1. The effects of intravenous infusion of KCI on the concentration of 3 'a _ D D c C3 Cp 40 a# Duration of infusion (min) FIG. 1. The effects of intravenous infusion of KCI on the renal excretion of potassium and sodium. Symbols 0 -sheep 1374, 12 m.mole KCl/min.; 0 -sheep 1374, 0 55 m.mole KCl/min.; l -sheep 1372, 100 m.mole KCl/min.; + -sheep 403, 0 27 m.mole KCl/min.; * - sheep 2866, 0 62 m.mole KCl/min. potassium in the plasma, the rate of clearance of inulin from the plasma and the amount of potassium filtered at the glomerulus are shown in fig. 2. Intravenous infusion of KCI in the sheep led to a rise in the concentration of potassium in the plasma min. after starting the infusion. This increase in potassium concentration in the plasma was most marked at the higher rates of infusion of KCI when concentrations between 8-9 m.equiv./l. were achieved. At the lower rates of infusion of KCI ( m.mole/min.) there was no change in the rate of clearance of inulin from the plasma. At the higher rates of infusion of KCI ( m.mole/min.) the rate of clearance

4 28 Scott of inulin from the plasma increased by 10 to 20 ml./min. above pre-infusion values. Intravenous infusion of KCI led to an increase in the amount of potassium filtered at the glomerulus. At the lower rates of infusion of KCI this increase in filtered potassium was entirely due to an increase in plasma concentrations of potassium, but at the higher rates of infusion of KCI the 780 I Duration *of infusion (min) Fia. 2. The effects of intravenous infusion of KOl on the concentration of potassiulm in the plasma, the clearance of inulin from the plasma and the amnount of potassiulm filtered across the glomerulus. Symbols as in Fig. 1. increase in filtered potassium was due both to a marked increase in the concentration of potassium in the plasma and to a moderate increase in the glomerular ifiltration rate. Intravenous infusion of HOl. - Intravenous infusion- of hydrochloric acid by itself at either 0-32 m.mole/min. into sheep 1372 or 0S52 m.mole/min. into sheep 1374 (fig. 3) had no effect on the rate of clearance of inulin from the plasma or on the concentration of potassium in the plasma. In consequence there was no effect on the calculated rate of filtration of potassium across the glomerulus. The excretion of potassium in the urine was not affected by intravenous infusion of hydrochloric acid. The effects of intravenous infusion of hydrochloric acid on the excretion of potassium during intravenous potassium loading is shown in fig. 4. In

5 Renal xcretion of Potassium and Acid in Sheep 29 Sheep 1372 Infusion OSM MCI at 032 mmote/min She*p 1374 Infusion OSM HMC at 0.52 mrmote/mmn Inulin 100 _ o= I 60L _- 0*t _ K*fitterd domwukis 0.3 o0.o K-xafted 0_ in urne 0@1 Ouwm_ Plasm n Kconcn. honp.) ih so * * a I Duration of inhuion (min) 05 _ 04._ _ v s0 100 FIG. 3. The effects of intravenous infusion of HCI on the renal excretion of potassium. Sheep 2866 Inulin 100 r cleirance ml/min a K excreted in urine m.mole HC/min I 05 m.mole KClt/min I K filtered 0-8 at glomerulus 0 040/. - m.equiv/min r Plasma K 701- concn. 6'0 m.equiv/i- 50 L 4 0 I 1*2 Na - excreted in 08 urine m.equiv/mrin 0.4 _ 0 I I a II II. Ị Duration of infusion (min) FIG. 4. The effects of intravenous infusion of KCl and HCl on the renal excretion of potassium.

6 30 Scott response to intravenous infusion of 0 5 N-KCI at 0 5 m.mole/min. excretion of potassium in the urine rose to reach a plateau at min. after onset of infusion. Intravenous infusion of hydrochloric acid at this stage at a rate of 0 5 m.mole/min. resulted in no change in the rate of excretion of potassium which remained approximately equal to the rate of infusion of potassium chloride. The concentration of potassium in the plasma and the amount of sodium in the urine increased in response to infusion of potassium chloride but were not affected by infusion of hydrochloric acid. K ktak 361 m.equiv/day. Sheep 1372 K tloke 12 m.eq*4day. krso of N.HCI at l-0 m.mole/min Inf%mion d KHCI at l-ommdle/min f0 ' *_ 7.0-6* o 5.0 c 02[ S ] t~~~ <-004 /% S0OO Duration ot infusion (min) FIG. 5. The effects of intravenous infusion of HCI on the renal excretion of potassium and net acid. The relationship between the rates of excretion of sodium and potassium in the urine and the net excretion of acid in the urine during intravenous infusion of hydrochloric acid in sheep 1372 is shown in fig. 5. In one experiment the sheep was receiving 800 g./day of a grass-cube diet providing 361 m.equiv. of potassium while in a second experiment the sheep was fed 800 g. day of a pelleted barley diet which provided 112 m.equiv. of potassium. In both experiments 10 N-HCI was infused at 1-0 m.mole/min. Before the infusion of hydrochloric acid and when receiving 361 m.equiv. of potassium per day the ph of the urine was alkaline and net excretion of acid averaged m.equiv./min. This rate rose to a mean of - 0*010 m.equiv./min., from 100 to 200 min. after starting the intravenous infusion of hydrochloric acid, an increase representing 7-5 per cent of the rate of infusion of acid. The ph of urine fell from 7-3 to 6-3 in response to infusion of hydrochloric

7 Renal xcretion of Potassium and Acid in Sheep 31 acid. Infusion of hydrochloric acid produced no change in the rate of excretion of potassium in the urine. On the barley diet the urine was acid and net excretion of acid averaged m.equiv./min. Intravenous infusion of hydrochloric acid resulted in an increase in the net excretion of acid in the urine to an average value of m.equiv./min., 160 to 200 min. after onset of infusion, an increase representing 4*4 per cent of the rate of infusion of acid. There was no detectable change in the ph of the urine or in the amount of potassium excreted in the urine in response to infusion of hydrochloric acid. K Ukine Sheep 2665 Sheep 403 i260m.mae MCI/iday l 300m.mcIe CI/day r ph eced 600 LdOO- urintw min 300- t 300- mfequrv/day 200[ L Net 100 [ 1 [00 acid in urine 0 _ mequiwday <200 -^200 Nao exacreted in urie 100 oo[ 0 _ I 2 3 * 56s TIme (days) FIG. 6. The effects of intra-ruminal infusion of HCI on the renal excretion of potassium and net acid. Intra-ruminal infusion of HCl. - The results of these experiments are shown in fig. 6. In both experiments the sheep were receiving 361 m.equiv. of potassium per day. In sheep 2865 before infusion of hydrochloric acid the ph of the urine was alkaline (8.1 to 8.4) and net excretion of acid in the urine averaged m.equiv./day. In response to infusion of 260 m.mole of hydrochloric acid per day the ph of the urine fell over 2 days to between 5'9 to 6.8, and net excretion of acid in the urine increased steadily so that between 5 and 8 days after starting the infusion it averaged m.equiv./day. This increase of m.equiv./day in the net excretion of acid in the urine occurred over a period during which there was no change in the rate of excretion of potassium in the urine. The rate of excretion of sodium over the period was variable although the rate may have tended to fall during acid infusion.

8 32 Scott Infusion of 300 m.mole of hydrochloric acid per day into the rumen of sheep 403 produced essentially similar results. The ph of the urine fell over 2 to 3 days from 8-4 to a mean of 5.5, and net excretion of acid in the urine increased from a pre-infusion mean of m.equiv./day up to m.equiv./day 6 to 10 days after onset of infusion. There was throughout the experiment little day to day change in the rate of excretion of potassium in the urine. xcretion of sodium was variable. In both experiments intra-ruminal infusion of hydrochloric acid produced moderate to severe non-respiratory acidosis. The ph and base excess of the blood of sheep 2865 fell from 7x446 and m.equiv./l. before infusion to and m.equiv./l. respectively 5 days after onset of acid infusion. The ph and base excess of the blood of sheep 403 fell from 7x399 and m.equiv./l. to and m.equiv./l. respectively 6 days after the onset of acid infusion. In both cases there was little change in blood pco2. TABL I. TH FFCTS OF INTRA-RUMINAL INFUSIONS OF HYDROCHLORIC ACID ON TH CLARANC (mn./min.) OF INULIN FROM TH PLASMA. Sheep 1374 Control 729 ± m.mole HCl/day into rumen 67A4± ±771 Table I gives values for the rate of clearance of inulin from the plasma before and during the infusion of acid into the rumen and it can be seen that the acid infusion was without effect. Fig. 7 gives the results of experiments in which three sheep were given infusions of between 200 to 330 m.mole of hydrochloric acid into the rumen each day. In these experiments the amount of potassium and the amount of net acid excreted in the urine were measured and in addition the form in which acid was present in the urine during HCl infusion was examined by measuring the total CO2 and ammonium ion content of the urine. Titratable acidity in the urine was then obtained by calculation. As in the previous experiments (fig. 6) intra-ruminal infusion of hydrochloric acid produced a fall in the ph of the urine, an increase in net acid excreted and no change in the amount of potassium excreted in the urine each day. The amount of titratable acid increased in response to acid infusion but the amount excreted was small relative to the total net acid excreted and the great proportion of the net acid in the urine appeared to be in the form of the ammonium ion. Disc-ussioN Intravenous infusion of potassium chloride in sheep led to a rise in the concentration of potassium in the plasma and to an increase in the rate of excretion of potassium in the urine. This rose rapidly until the rate of excretion of potassium was about equal to the rate of infusion of potassium.

9 Renal xcretion of Potassium and Acid in Sheep 33 During potassium chloride infusion the rate of excretion of potassium in the urine exceeded the rate of filtration of potassium across the glomerulus indicating net secretion of potassium by the renal tubules. Similar results were obtained by Anderson and Pickering [1962] who infused N-KCl into cattle at a rate of 7-9 m.mole/min. They observed no change in the inulin clearance rate; although an increase was observed in the present experiments at the higher rates of infusion of potassium chloride, this may only reflect a difference in the rate of infusion relative to body weight. 9-0 Urine 80 0 ph 7.Oj 6 0 SHP 2750 SHP mmole HCI/day i:1[ mrmosle HCI/day 0.0! SHP 2755 HCS/-day m.quiv/day _ % Not acid in urin [ SO- Titratable acid in ursh o -ao 0 m. equis/day Total C02 in urir 5 0 [ m.mol/day O1 Urine NH: qssi,/day * ll 200k SO f I ISI [ _ 50 O 2 l I Time (days) FIG. 7. The effects of intra-ruminal infusions of HCI on the renal excretion of potassium and acid. The ability to tolerate intravenous infusion of large amounts of potassium seems to depend upon the kidney being able to rapidly increase its rate of excretion of potassium. This ability seems well developed in cattle [Anderson and Pickering, 1962] but less well developed in dogs unless they are rendered tolerant through a progressive increase in dietary potassium intake prior to infusion [Berliner et al., 1950]. Improved tolerance to potassium after increased potassium intake has also been shown in rats [Thatcher and Radike, 1947] while the experiments of Bergman and Sellers [1954] suggest that the young milk-fed calf may respond to potassium loading more like the dog than the mature cow. The present experiments show that sheep are like cattle in their ability to withstand intravenous infusions of potassium salts, and it seems that this ability in ruminants may be related to the fact that their diet is normally rich in potassium. The evidence that the renal tubular cells can secrete potassium is convincing and stop-flow studies indicate that distal regions of the nephron may be responsible [Black and mery, 1957; Pitts et al., 1958; Sullivan VOL. JLTV, NO

10 34 Scott et al., 1960; Berliner, 1961]. The mechanism by which this potassium is secreted has been studied by Berliner et al. [1950] who proposed that potassium ions may compete with hydrogen ions in the cell in an exchange process for sodium ions in the tubular urine. This theory represents an extension of that put forward by Pitts et al. [1945, 1948] to account for the acidification of the urine. Berliner et al. [1950] supported it by demonstrating that carbonic anhydrase inhibitors, which interfere with the production of hydrogen ions in cells, lead to an increased excretion of potassium in the urine. As a procedure for examining the effects of acidosis on potassium excretion in the sheep intravenous infusion of hydrochloric acid proved unsuccessful, since the increase in the rate of net excretion of acid in the urine represented only a small proportion of the acid infused. It seems likely that extracellular and cellular buffering were providing a much greater immediate degree of compensation against acidosis than the kidney. It is commonly believed that the renal response to acidosis resulting in an increased excretion of acid in the urine is slow to develop [Pitts, 1965; Davenport, 1965] and this may be particularly true in the sheep which normally excretes a markedly alkaline urine. Intra-ruminal infusions of hydrochloric acid produced chronic acidosis during which net excretion of acid in the urine increased until it approximately equalled the rate of infusion of acid with no change in the rate of excretion of potassium in the urine. Most of this increase in acid excretion appeared to be in the form of ammonium ions and the amount of titratable acid excreted was very small relative to the amount of potassium excreted in the urine. This low rate of excretion of titratable acid may indicate that when the rate of excretion of potassium is high, as it usually is in herbivores, the rate of excretion of titratable acid may be limited through competition with potassium in the mechanism proposed by Berliner et al. [1950]. This conclusion may be supported by the experiments of Roberts et al. [1953] who observed that in dogs excreting an acid urine, infusions of potassium chloride increased the cellular concentration of potassium and resulted in increased potassium excretion and a urine which became progressively more alkaline. The main feature of the present experiments was that the sheep was able to excrete large amounts of both potassium and net acid in the urine and that this excretion of acid was largely in the form of the ammonium ion, the excretion of which would not involve competition with potassium. Intravenous infusion of potassium chloride in sheep led to a marked sodium diuresis. Anderson and Pickering [1962] reported a similar observation in cattle and they suggested that this may relate in part to the increased solute load in the glomerular filtrate. This, they suggested, could result in an osmotic diuresis which would sweep sodium out of the proximal convoluted tubules more rapidly than it could be absorbed distally.

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