Renal function in water snakes

¹ Department of Physiology, School of Medicine, Tulane University, New Orleans, Louisiana

Electrolyte and osmolar concentrations of ureteral urine were compared with serum in several species of water snakes. Hyperosmotic urine was never observed, even after 48 hr of water deprivation. This finding is consistent with the countercurrent theory which precludes urinary concentration when Henle's loops are absent, as in reptilian kidneys. The creatinine/inulin clearance ratio may exceed 2.0, suggesting tubular secretion of creatinine. Tubular secretion is further suggested by the fact that the clearance ratio approaches unity when plasma creatinine concentration is elevated. Variations in glomerular activity are reflected in urine flow in snakes as shown by a direct relationship between clearance of inulin (GFR) and urine flow In snakes solute excretion is dependent on an increase in functional tubular population, with a concomitant increase in filtered load and not a decrease in tubular solute reabsorption. Evidence for this concept is presented and indicates that, over a tenfold increase in GFR, snakes continue to excrete about the same per cent of their filtered load. Water reabsorption, on the other hand, appears to be similar in character to that seen in higher vertebrates. Proximally, it appears to be dependent on solute reabsorption ("obligatory"), whereas distally it is influenced by ADH ("facultative").