The effects of reducing dietary nitrogen and of increasing sodium chloride intake on urea excretion and reabsorption and on urine osmolality in sheep.

Renal responses to reducing dietary nitrogen were studied in four ewes during intravenous infusion of arginine vasopressin. The fall in urea excretion and in plasma urea concentration was accompanied by significant reduction in GFR and in urine osmolality. The fraction of filtered urea reabsorbed increased despite reduction in the urea U/P concentration ratio and this increase was sustained when the urea U/P ratio was further reduced at higher urine flows observed when the drinking water was replaced with saline. This procedure also sustained the RPF which in the absence of additional salt was significantly reduced on the low protein diet. It is suggested that the fall in GFR and the increase in the fraction of filtered urea reabsorbsed may contribute to nitrogen economy and that the increase in fractional reabsorption and the reduction in urine osmolality on the low protein diet provided evidence of active reabsorption of urea by renal tubules.