RENAL CONTROL OF PLASMA UREA LEVEL IN SHEEP: THE DIURETIC EFFECT OF UREA, POTASSIUM AND SODIUM CHLORIDE

¹ Department of Physiology, University of New England, Armidale, New South Wales 2351 Australia

Two merino ewes were fed 800 and then 1000 g of lucerne chaff per day and renal responses and extracellular fluid volume (E.C.F.) were determined. Urine urea nitrogen, glomerular filtration rate (G.F.R.), urine volume, effective renal plasma flow (E.R.P.F.) and E.C.F. increased in both animals with the greater nitrogen intake. In a second experiment, five merino wethers and four merino ewes were infused intraruminally with increasing quantities of urea, whilst being fed a mainly roughage ration, and the above parameters were determined. No significant changes in G.F.R., E.R.P.F. or E.C.F. were noted. Urinary excretion maintained a linear response to nitrogen input until 20·6 g of urea nitrogen per day were infused. At this level of infusion, an increased fractional reabsorption of urea resulted in a large increase in plasma urea concentration, but when 500 mmol of NaCl or KCl were added to the infusate, urine urea nitrogen excretion increased from 10·4 to 11·4 and 11·9 g/d respectively and plasma urea concentration decreased from 68·0 to 35·2 and 37·3 mg nitrogen/100 ml. Urea clearance virtually trebled with both electrolyte infusions and E.C.F. and G.F.R. increased by aproximately 10 and 25% respectively. It is concluded that urea alone has limited diuretic ability and that the increased electrolyte content of higher protein dry roughage diets aids urine urea excretion by increasing G.F.R., and the urine flow rate by an osmotic diuretic effect.