Camp. Biochem. P/r@/. Vol. 90A, No. I. pp. 121-126, 1988 Printed in Great Britain

0300-9629/88 $3.00 + 0.00 Pergamon Press plc

STUDIES ON RENAL EXCRETION OF POTASSIUM IN THE DIK-DIK ANTELOPE

B. M. RUGANGAZI and G. M. 0. MALOIY Department of Animal Physiology, Comparative Animal Physiology Research Unit,

University of Nairobi, Kenya

(Received 8 September 1987)

Abstract-l. In a study on the renal handling of potassium by the dik-dik antelope, plasma and urine samples were analysed for potassium, sodium and creatinine concentrations and osmolality during dehydration and intra-ruminal loading of potassium solutions.

2. The fractional excretion of potassium was 0.64 during the control period and rose up to as high as 2.3 during potassium loading. Urinary osmolality and potassium concentration decreased as the urine volume increased but the total amounts of potassium excreted were independent of urine volume.

3. Potassium loading led to a steady increase in its urinary excretion but a decrease in plasma potassium concentration was observed. This observation casts doubt on the hypothesis that alterations in potassium intake produce parallel alterations in plasma potassium concentration (which supposedly stimulates or depresses potassium excretion) and thereby maintain potassium homeostasis.

4. A possible alternative signal for increased potassium excretion following increased intake is discussed.

INTRODUCTION

The dik-dik antelope (Rhynchotrugus kirkii) inhabits arid and semi-arid areas of eastern, central and southwest Africa (Dorst and Dandelot, 1970). It feeds selectively on succulent leaves and seeds with high energy content (Hofmann, 1973). It is generally accepted that wild and domestic herbivores such as sheep, cattle, deer and antelopes feed on diets rich in potassium and poor or low in sodium, as opposed to the diets containing lower total potassium content typically ingested by non-herbivores. Despite the comparable large potassium dietary intake by both domestic and wild ungulates, there have been no studies done on the renal excretion of potassium in the latter group, especially in the East African species.

A renal tubular mechanism for potassium secretion has been demonstrated in man and dog (Berliner et al., 1950), cow and sheep (Anderson and Pickering, 1962; Scott, 1969b) and rat (Malt& et al., 1964, 1966). These studies showed that unlike in the dog, where potassium loading leads to a progressive rise in plasma potassium concentrations (Berliner et al., 1950), the bovine plasma potassium levels did not rise. This observation was the beginning of what came to be a broadly accepted concept on renal potassium excretion, namely, that whereas ruminants respond to increased potassium intake by promptly activating the tubular secretory mechanism, simple stomach species need a prior exposure period to higher levels of potassium intake before high levels of secretion are obtained without an elevated plasma potassium concentation. However, recently,

Address all correspondence to: Professor G. M. 0. Maloiy, Comparative Animal Physiology, Research Unit, Uni- versity of Nairobi, PO Box 30197, Nairobi, Kenya,

Rabinowitz et al. (1984) have shown that the rat will rapidly increase its potassium excretion in response to a high potassium diet.

Besides the delay in increased potassium tubular secretion by simple stomach species following in- creased potassium intake, there are other aspects of renal potassium excretion differences between this group and ruminants, including difference in urinary K/Na concentration ratio which is lower in the former group. Total quantities of the electrolyte excreted by the kidney have also been investigated (Keynes and Harrison, 1967; Anderson and Picker- ing, 1962; Dewhurst and Harrison, 1966; Dewhurst, Harrison and Keynes, 1968). As expected, however, both urinary potassium concentration as well as urine osmolality have a close relation with urine volume but the amount of potassium excreted is not related to the urine volume (Scott, 1969a; Maloiy and Scott, 1969). This is true for both groups of animal species.

This study was undertaken to examine the renal handling of potassium in the dik-dik antelope. Simi- larly, our results are compared with the published data for both ruminants and non-ruminants studied under comparable conditions.

MATERIALS AND METHODS

Animals

Four male and three female dik-diks bought from local trappers and weighing between 2.5 and 4.7 kg were used in this study. They were transferred to the Comparative Phys- iology Unit animal house at Chiromo Campus, University of Nairobi where the study was carried out. They were allowed a 3-week acclimatization period before the experi- ments commenced. Their diet, which was available ad libitum, was comprised of Grewia similis leaves and early calf weaning pellets, and their electrolyte composition as deter- mined by us is shown in Table 1. During experimentation, each animal was put in a metabolism cage in a “climatic” room where ambient temperature and humidity were con-

121

122 B. M. RUCANGAZI and G. M. 0. MALOIY

Table I. Nutrient values and composition of the experimental diets. (Values are percent of dry weight)

Early calf weaning Grewia similis

Nutrient pellets leaves

Dry matter (%) 91.3 92.3 Crude fibre (%) 6.3 II.8 Crude protein (%) 20. I 14. I Total ash (%) 8.1 10.4 Potassium (%) 0.1 I.7 Sodium (%) 0.3 0.2

trolled at 22°C and 30%, respectively. The metabolism cages were constructed in such a way as to allow complete separation of faeces and urine without cross contamination. Inside the cage, the animal could not turn around, but there was enough room to stand, he and move short distances forwards and backwards.

Surgery

To vary the amount of potassium intake independent of food consumption; it was necessary to intra-ruminally in- fuse solutions and this required surgical implantation of a permanent infusion line. This was done 4 weeks before the experiment; giving the animals time to recover from the surgery completely. The infusion lines were locally made and the animals were fasted for 18-24 hr before surgery. The operation was performed under local infiltration anaesthesia using Xylotox (Willows Francis, Bolton, London). The animal was placed in the right lateral recumbency position and the left sublumbar triangle prepared aseptically for surgery.

A surgical incision about 4.5 cm long was made through the skin and subcutaneous tissues midway between the last rib and the tubar coxae and about 3.4 cm below the line of the transverse processes of the lumbar vertebrae. The ab- dominal muscles and peritoneum were perforated bluntly with surgical scissors and the muscle split along the lines of their fibres for the length of the skin incision. The rumen wall was identified and a relatively avascular portion was exteriorized through the incision and held in position at the corners of the incision with two pairs of peritoneal forceps. A purse string suture was made into the ,rumen wall using a non-absorbable suture material. An incision of about I .5 cm was made through the rumen wall between the purse string taking care not to spill out the contents. The infusion line was then manipulated into the rumen through this incision. The purse string was tightened and knotted around the line. The line was exteriorized to the skin via a stab incision made through the abdominal wall about 3 cm in front of or behind the main incision line, with the latter incision being sutured routinely.

Experimental procedures

A sequential design involving a set of three experiments was used. Animals that came out of one experiment par- ticipated in the subsequent one after a rest period of 1-2 weeks. The procedure followed in all the three experiments is described by Rugangazi and Maloiy (1987). Experiment 1 was a control to establish the baseline levels of the different parameters. Four animals (three males and one female) were used in the experiment. Urine and plasma samples were analysed for osmolality, creatinine and potas- sium concentrations. Food and faecal samples were ana- lysed for potassium content. In experiment 2, animals were dehydrated by restricting their water intake and weight loss was used as an index of dehydration, Plasma samples were collected when dehydration was thought to have been achieved. Water restriction lasted for 3 weeks from the beginning of experimental treatment. In experiment 3, freshly prepared KC1 solutions of 0.3 and 0.5 mol/l concen- trations were loaded intra-ruminally. Each concentration was loaded in single doses daily starting with 20 ml. The

volume was then increased progressively in 6 steps of 20 ml up to 120 ml. Each volume load was given for 2 days so that loading lasted for 12 days for each concentration.

The animals were weighed using a Salter spring balance (Model 235). Daily urine volume was recorded and a 20 ml aliquot sample was taken and frozen at -20°C for later analysis. All chemical analyses were done within a period of 1 week after collection, starting with creatinine. On alternate days, 6lOml of blood was taken from the jugular vein, between 09.30 and 10.30 hr, using a heparinized syringe. This was immediately centrifuged at 6000 rpm using M.S.E. centrifuge (Model GF-6). The plasma was then separated and frozen at -20°C for later chemical analysis. Urinary and plasma creatinine concentrations were determined using the alkaline picrate method (Varley, 1967), with spectro- photometer readings taken using a Beckman Spec- trophotometer (Model DB-GT) at wavelength 500 pm. Urinary and plasma osmolality was determined on diluted samples by the freezing point depression method using the Knauer Microosmomenter (Herbert Knauer & Co. GMbH, Berlin 37, West Germany). The Wet-Ashing Technique (Lindner and Harley, 1942; Miller and Miller, 1948) was used to analyse the concentration of both potassium and sodium in food and faeces. The Endogenous True Creat- inine (ETC) clearance was used for the determination of GFR of the dik-dik antelopes. This was calculated using the corresponding values of plasma creatinine concentrations. urine creatinine concentrations and urine flow rates ob- tained in the different experiments. The following equation was used:

where

GFR = Glomerular filtration rate (ml/min) V, = Urinary creatinine concentration (mg/lOO ml) Vu = Urine flow rate (ml/min) P, = Plasma and hence ultra-filtrate creatinine concen-

tration (mg/lOO ml).

Statistical analysis involved calculation of the mean values for individual dik-dik and the standard error of the mean (+ SEM) for the experimental data. The Student r-test was used to analyse the significance of difference between the control mean and the means of the various treatments.

RESULTS

During the control experiment, each dik-dik consumed, on average, 115.0 + 2.6 g/day of early calf-weaning pellets and 33.9 f 0.6 g/day of Grewia similis leaves (mean + SEM). This diet provided 50.7 _+ 0.9 mmol of potassium and 17.0 _+ 0.9 mmol of sodium daily. The mean daily water intake was 251.0 f 8.2 ml/day over 21 days. The daily urine volume excreted was 121.4 & 9.1 ml/day over 20 days. There were differences in urine osmolality be- tween individual animals. The highest osmolality averaging 3461.5 f 86.0 mosm/kg Hz0 was observed in dik-dik C, while the lowest osmolality of 1469.0 f 85.0 mosm/kg Hz0 was observed in dik-dik B. The mean overall osmolality for the four dik-diks was 1989.3 _+ 103.0 mosm/kg H,O.

Individual animal variations were observed in urinary potassium concentration. The overall average potassium concentration was 319.0 f 11.8 mmol/l and potassium was the principal cation excreted in urine. The highest concentrations were observed in dik-dik C with an average of 586.8 f 27.1 and the lowest values recorded in dik-dik B had a mean

Potassium excretion in the dik-dik

Table 2. Daily excretion of potassium and sodium in urine and faeces of four dik-diks receiving the maintenance ration (mean + SEM)

123

Potassium excretion Sodium excretion

Urine Farces Urine Faeces Dik-dik No. of days (mmol/day) (mmol/day) % In urine (mmol/day) (mmoliday) % In urine

A 21 34.6 f 1.0 12.6 + 0.4 73.3 * 0.9 2.9 + 0.2 8.7 f 0.3 25.0 + 1.5 B 14 35.7 * 1.4 13.1 f0.6 73.2 + 1.2 3.7 * 0.2 9.4 f 0.7 28.2 f I .7 C 15 24.4 + 1.2 12.3 + 0.3 66.5 + 2.0 0.5 + 0.3 11.7+0.6 4.1 + 0.3 D 16 32.4 IO.9 9.3 IO.2 77.7 I 1.0 2.8 : 0.2 5.0 ? 0.2 35.8 ; 2. I

of 196.5 + 23.9 mmol/l. Despite the variations in urinary potassium concentration between animals, the proportion of the urine osmolality represented by potassium was hardly variable and averaged 16 + 1.2% for all the animals. Plasma potassium concentration was 3.3 f 0.1 mmol/l. Urinary potas- sium excretion accounted for approximately 73.0 f 1.0% of the combined total potassium re- covered in urine and faeces while faecal sodium represented 89.2 + 1.2% of the ‘total sodium re- covered in urine and faeces. Data ;on the mean daily output of electrolytes in urine and faeces of the individual animals are given in Table 2.

The amount of potassium filtered across the glomerulus was 26.8 + 9 ~mol/min, with 16.1 2 0.4 pmol/min being excreted in urine. Thus, of the filtered potassium, a minimum of 40% was re- absorbed during the control experiment. All potas- sium clearances during control were, observed to be lower than the corresponding creatinine clearance. There was, therefore, no evidence (as far as clearance observations could reveal) of tubular potassium secretion.

Following dehydration, plasma potassium concen- tration rose from 3.4 + 0.1 to 4~4 f 0.1 mmol/l and plasma osmolality increased from a control value of 322.0 f 1.4 to 332.4 f 2.2 mosm/kg H,O. At the same time, the amount of potassium filtered across the glomerulus decreased from a mean of 26.8 f 1.9 to 20.8 f 1.7 pmol/min. This decrease in the amount of potassium filtered was a result of a decrease in

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Fig. 1. The effect of intra-ruminal infusion of KC1 on the rate of excretion of potassium. Note that infusion of 50mmol led to excretion of potassium over 5 times the control rate. Symbols 0 and + represent dik-diks A and D,

respectively.

glomerular filtration rate following dehydration since there was an observed increase in plasma con- centration of the electrolyte. During dehydration, there was a rapid fall in urine volume and this decrease in urine volume was accompanied by a significant increase (P < 0.05) in its osmolality. The osmolality rose from the normal hydration value of 1989.2 + 103.0 to 2738.1 f 36.6 mosm/kg H,O during dehydration. The amount of urinary sodium excretion was decreased from a control value of 1.6 f 0.2 to 0.7 + 0.1 pmol/min. Dehydration also led to a decrease in the amount of sodium filtered from a control value of 833.5 + 36.4 to 711.4f 19.4pmol/min.

During the intra-ruminal potassium loading, plasma and urine osmolalities were 327.0 f 1.1 and 1328.6 f 129.7 mosm/kg H,O, respectively and urinary potassium and sodium concentrations were 309.6 f 20.6 and 42.0 It 5.5 mmol/l. The effects of intra-ruminal infusion of KC1 on the urinary ex- cretion of potassium are shown in Fig. 1 and Table 3. The intra-ruminal loading of KC1 resulted in an increase of the rate of urinary potassium excretion. The increase was from a control value of 16.1 to a maximum of about 50.3 pmol/min during intra- ruminal of 0.5 mol/l KCI solution. The increased amount of potassium excretion was approximately equal to the amount infused. Infusion of 30 and 45 mmol/day led to an excretion rate of 45 and 54 pmol/min, respectively.

The effects of intra-ruminal potassium loading on the ratio of potassium to creatinine clearances (K,:Cr,) is shown in Fig. 2. The ratio steadily increased from a control value of 0.4 to 060.8 during the 0.3 mol/l KC1 infusion. On increasing the potassium load, the ratio further increased to 1 B-1.1. Throughout the infusion of over 30mmol/day, the K,:Cr, ratio exceeded unity. This was evidence of potassium secretion by the renal tubules. The maxi- mum ratio attained was 2.4 during 55 mmol/day of potassium infusion.

There was a slight increase in glomerular filtration rate and urine flow rate increased during infusion of both 0.3 and 0.5 mol/l KC1 solutions. Intra-ruminal loading of potassium led to a natriuresis (Fig. 3). Sodium excretion rate during the control period was very low (about 1 pmol/min). It increased to 2.5 f 0.3 pmol/min during intra-ruminal loading of 1Ommol KC1 and then to 10 pmol/min when 40mmol were loaded (a rate of 10 times the control excretion rate). Of interest was an observed drop in plasma potassium concentration during potassium loading. Plasma potassium concentration dropped from 3.9 to 2.9 mmol/l following 0.5 mol/l KC1 load- ing. The urinary sodium concentration during potas-

124 B. M. RUGANGAZI and G. M. 0. MALOIY

Table 3. The relation between the amount of dietary potassium intake, potassium filtered across the glomerulus and the amount excreted in the urine (mean + SEM)

Potassium Potassium Dietary Cl~~~~nC~ filtered at excreted Fractional Plasma

potassium the in the potassium potassium intake Creatinine Potassium glomerulus urine excretion concentration

Condition (mmol/day) (ml/min) (ml/min) (pmol/min) (flmol/min) (U,Y/P, GFR) (mmol/l)

Control 50.7 * 0.5 7.1 + 0.1 4.1 f 0.1 26.8 k I .9 16.1 k 0.4 0.64 3.9 Potassium loading

(0.3 mol/l) Potassium loading

CO.5 molil)

73.2 + I .2 8.9 & 0.7 7.5 * 0.7 38.7 ? 3.8 32.0+3.1* 0.84. 4.2

100.7 + 1.3 9.0 + 0.6 17.1 + 1.8 22.3 + I .5 50.3 + 2.2. 2.3* 2.9

*Indicates means that are significantly different from the control mean. Data from 4 experimental animals.

sium infusion rose from 19.7 to 42.0 mmol/l. There was a decrease in the fractional sodium reabsorption from a control value of 99.9 to 98.8% during intra- ruminal loading of 0.5 mol/l KCl. Potassium loading led to a decrease in urine osmolality. The most dilute urine was excreted during potassium loading and had an overall osmolality of 1328.6 + 60.0 mosm/kg H,O.

Urine osmolality decreased as the volume in- creased. The lowest volume of 23 ml/day was excreted by dik-dik C and it had the highest osmolality of 4200 mosm/kg H,O. The highest volume of 214 ml/ day was excreted by dik-dik B and its osmolality was 850 mosm/kg H,O. The urine osmolality thus ranged between 4200 and 850 mosm/kg H,O with a re- spective urine volume range of 23 and 214ml/day. Potassium concentration also had an inverse relation- ship with the urine volume. The concentration of potassium fell as the urine volume increased from 2 l-2 14 ml/day. The highest potassium concentration attained was 763 mmol/l and the lowest concen- tration was 108 mmol/l. As is shown in Fig. 4, however, there was no apparent relation between the total amount of daily potassium excreted and urine volume except in dik-dik C, which excreted very little urine even when water was given ad libitum and tended to increase the daily potassium excretion from 12.0 to 33.1 mmol/day as the urine volume increased from 20 to 45 ml/day. Potassium excretion by the

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Fig. 2. The effect of intra-ruminal infusion of KCI on the ratio of potassium to creatinine clearances. Note the evidence of potassium secretion from the loading of about 20 mmol when the ratio exceeds one. It remained above one throughout the rest of the potassium loading. Symbols

represent same animals as in Fig. 1.

other animals was 23.3 + 1.2 mmol/day or 60.1% of the daily potassium intake. This amount did not vary much despite the variation in urine volume from 30 to 214ml/day (Fig. 4).

DISCUSSION

The dik-dik antelopes are browsers, selecting fruits and protein-rich dicotyledonous foliage (Hofmann, 1973). Like other ruminants, they consume far larger amounts of potassium than that which could be accounted for on the basis of their body weight. Urinary potassium excretion accounted for 73% of the total potassium excreted, while faecal sodium excretion represented 89% of the total sodium re- covered in urine and faeces. The urinary K/Na ratio was 16.8. These results closely resemble those ob- served in sheep (Dewhurst et al., 1968). Unlike the simple stomach species, which have a mean ratio of K/Na of 0.43, the dik-dik’s ratio of 16.8 is within the range that has been found in both wild and domestic ruminants so far studied.

Potassium and sodium concentrations in urine decreased as the urine volume increased, but there was no relation between the total daily potassium excreted and the urine volume (Fig. 4). For any urine volume, urinary potassium excretion accounted for approximately 60% of the daily intake. There was diuresis and natriuresis following potassium loading (Fig. 3). Faecal samples were not analysed for sodium

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Fig. 3. The et&t of intra-ruminal infusion of KC1 on the rate of sodium excretion. Note the steady increase in sodium excretion rate with increase in KCI infusion. Symbols

represent the same animals as in Fig. 1.

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URINE VOLUME (ml /day)

Potassium excretion in the dik-dik 125

the glomerulus are relatively unimportant factors; wide variations in potassium may be found at any level of plasma potassium or filtered load”.

These observations, together with the results ob- tained in this study where plasma potassium is noted to decrease, probably imply that a rise in plasma potassium concentration cannot be the signal for increased potassium excretion during higher potas- sium intake. This statement is not, however, in agreement with the widely accepted hypothesis for the control system that maintains potassium balance. This hypothesis states that alterations in potassium intake produce parallel alterations in plasma potas- sium. This, in turn, directly influences potassium excretion by the action of plasma potassium at the distal nephron secretory sites and indirectly by modulating secretion of aldosterone, a well known stimulant of potassium excretion. The increased renal potassium excretion serves both to maintain external balance and to maintain constancy of plasma potas- sium (Yong, 1985; Laragh and Sealey, 1973).

If plasma potassium concentration is not the signal for increased potassium excretion, then it is probable that there are other sensors elsewhere in the body that detect increased potassium load and effect, through kaliuretic regulatory factors which are not yet under- stood (Rabinowitz ef af., 1984), increase potassium excretion as observed in this study on the dik-dik. The presence of enteric potassium-sensitive receptors in the rumen, hepatic portal vein, or liver, serving as components of an afferent limb of the excretory control system has been suggested by Rabinowitz et al. (1984). This hypothesis is in agreement with results obtained here and would probably give a clear expla- nation as to how the kidney detects an increased potassium intake before this intake may raise the plasma potassium level. Results of this study showed that the dik-dik kidney qualitatively handles potas- sium and sodium in the same way as other ruminants thus far investigated.

Fig. 4. The relation between the daily renal excretion of potassium and urine volume. The broken line indicates the daily potassium intake. Symbols 0, 0, l and + represent

dik-diks A, B, C and D respectively.

content in this experiment and hence it is not possible to indicate whether this natriuresis was associated with a negative sodium balance or with a decreased faecal sodium excretion.

Intra-ruminal potassium loading increased potas- sium excretion rate. The potassium creatinine ratio rose from a control value of 0.42 to exceed unity. This was suggestive of renal tubular potassium secretion by the dik-dik. It is now commonly thought that about 90% of filtered potassium is reabsorbed proxi- mal to the distal tubules and the final excreted load represents either further reabsorption or distal se- cretion (Brenner and Berliner, 1973). During the control experiment, it was observed that the frac- tional excretion of potassium was 60%. This would imply a modest tubular potassium secretion in the dik-dik during normal dietary potassium intake. Following intra-ruminal potassium loading, a frac- tional excretion of potassium of about 230% was observed. This would imply a very high secretory state in this species of ruminants and is responsible for the large quantities of potassium excreted in the urine during KC1 loading.

Acknowledgements-This study was supported by a re- search grant from NORAD (The Norwegian Agency for International Development), KEN 046 Project. We are grateful to Mrs F. Frost for her excellent secretarial services.

REFERENCES

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