Renal handling of urea in Ram catesbeiana

LONG, WILLIAM SCOTT. Renal handing of urea in Rana catesbeiana. Am. J. Physiol. 224(Z) : 482-490. 1973.-The processes involved in renal urea excretion in the bullfrog (Rana catesbeiana) were studied by clearance, tissue, and micropuncture methods. According to micropuncture data with urea-14C and inulin-3H, secretion of urea against a concentration gradient from plasma to tubular fluid occurs across the proximal and probably the distal tubules, Tissue studies examining the cellular compartment separating plasma and tubular fluid show that secretion across the basal membranes of the cells is dependent on plasma urea concentrations but independent of flow along the nephron. The fact that urea secretion rates are linear with urine flow rates in clearance studies indicates that movement of urea from cellular stores into tubular fluid is passive. The collecting duct, ureter, and bladder show no significant reabsorption or secretion of urea and so insure excretion of the filtered and secreted urea loads (as well as the accuracy of the clearance studies’ results.) An osmoregulatory function for different modes of urea excretion is proposed.