Effect of low calcium and low phosphorus diets on the intestinal absorption of phosphate in intact and parathyroidectomized pigs.

The intestinal absorption of phosphate has been studied in conscious pigs, each prepared with a Thiry-Vella loop of jejunum. The feeding of diets low in either calcium or phosphorus caused a significant increase in the efficiency of absorption of phosphate from the solution used to perfuse the jejunal loop in both intact and parathyroidectomized (PTX) pigs. An intravenous infusion of parathyroid hormone (0.22 u. kg-1 h-1) into a PTX pig also enhanced the absorption of phosphate. The increase in the absorption of phosphate when the low phosphorus diet was fed was not caused by an increase in the concentration gradient of phosphate ions between the jejunal lumen and blood. It is concluded that the intestinal absorption of phosphate shows similar changes to those of calcium when diets low in calcium or phosphorus are fed and that parathyroid hormone, although capable of stimulating the absorption of phosphate, is not essential for this adaptation. These effects are probably brought about by changes in the renal production and mucosal uptake of 1,25-dihydroxycholecalciferol, the active metabolite of vitamin D3.