THE EFFECTS OF REDUCING DIETARY NITROGEN AND OF INCREASING SODIUM CHLORIDE INTAKE ON UREA EXCRETION AND REABSORPTION AND ON URINE OSMOLALITY IN SHEEP

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Abstract
Renal responses to reducing dietary N were studied in 4 ewes during i.v. infusion of arginine vasopressin. The fall in urea excretion and in plasma urea concentration was accompanied by significant reduction in GFR [glomerular filtration rate] and in urine osmolality. The fraction of filtered urea reabsorbed increased despite reduction in the urea U/P [urine/plasma] 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 [renal plasma flow] which in the absence of additional salt was significantly reduced on the low protein diet. The fall in GFR and the increase in the fraction of filtered urea reabsorbed may contribute to N economy, and 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.

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