Renal response to intravenous phosphate infusion in the sheep

Renal clearance experiments were performed on six Merino ewes in which plasma phosphate concentrations were increased by the intravenous infusion of isohydric sodium phosphate. As the phosphate load to the kidney increased, the renal tubular reabsorptive capacity became saturated and a definite tubular maximum for phosphate reabsorption (Tmp) was demonstrated. The Tmp was directly related to the glomerular filtration rate and had a mean value of 333-1+/-27-0 (s.e.m.) mumol/min or 416-6+/-13-5 mumol/100 ml glomerular filtrate. Calcium infused concurrently with phosphate in order to maintain plasma total calcium levels did not alter the Tmp. Ultrafilterability of calcium and phosphate in the plasma decreased with phosphate infusion and this was accentuated by an accompanying calcium infusion. The Tmp in sheep's kidney is higher than in non-ruminant animals and the implications of this are discussed.