Renal excretion of urea and electrolytes in a small wild ruminant: the dik dik antelope (rhynchotragus kirkii)

Abstract

A study to examine the glomerular filtration rate, renal mechanisms for potassium, sodium and urea excretion, urine osmolar concentration as well as saline drinking abilities of the dik dik antelope was carried out under controlled laboratory experiments. Observations were made on the daily intake and excretion of electrolytes (K and Na) by dik diks fed only once daily and housed in metabolism cages. Food and faeces were analysed for dry matter, ash and electrolytes contents. Urine and plasma samples were analysed for osmolality, K, Na, Urea -N, NH3-N and Endogenous true creatinine concentrations during control and various treatments. In the control state, 73% of the total K recovered in both urine and faeces was excreted in urine while urine Na represented only 11% of the total Na recovered in urine and faeces. Renal tubular reabsorption for K, urea and Na was 39.9%, 55.3% and 99.9% of the filtered quantities,respectively .. The glomerular filtration rate varied from 3.4 to 16.8 with a mean of 7.2 ml/min. Urine osmolality, K, Na, urea and creatinine concentrations varied inversely with the urine volume. Dehydration was accompanied by significant de..crease of glomerular filtration rate (P< 0.05) and urine flow rate and an increased tubular urea reabsorption. Solutes loading however tended to have opposite effects. Potassium loading on the other hand increased the ratio of potassium clearance to creatinine clearance to over unity thereby reaching a maximum of 2.0. This provided evidence for potassium secretion by the antelope's renal tubules. Similarly1as solute excretion rate increased, urine osmolality decreased, resulting with the highest urine osmolality being observed at the lowest rates of solute excretion. Dik diks offered saline solutions as the only source of drinking water ranging from 0.1 to 0.5 mole/l NaCI could tolerate only a concentration of 0.3 mole/l NaCl. The percentages and pattern of urinary potassium excretion as well as faecal sodium excretion were similar to the pattern found in both domestic and wild ruminants such as sheep, cattle and red deer thus far studied. Tubular potassium secretion was evidenced in the dik dik antelope and explains its tolerance for rich-potassium diets without any adverse effects. It was further observed that by increasing tubular water reabsorption and decreasing glomerular filtration rate, the dik dik antelope is able to excrete a very highly concentrated urine (4200 mOsm/kg ri20) and conserve water. In the dik dik an te lopey as in the case of the mammalian kidney in general, 99.9% of filtered Na was reabsorbed along the renal tubules. The ability of their kidneys to increase urea reabsorption during dehydration was of interest and probably explains this species· ability to live in dry areas and withstand prolonged periods of drought mostly encountered in the East and Central African regions.