Effects of a change in phosphorus requirement on phosphorus kinetics in the sheep

An experiment was carried out to examine the effects of a change in P requirement, achieved by intravenous calcium loading, on intestinal phosphorus absorption, salivary phosphorus secretion and faecal endogenous phosphorus loss in adult sheep fitted with a rumen and duodenal cannula. Isotope dilution was used to measure faecal endogenous phosphorus loss while 103ruthenium phenanthroline and 51chromium-ethylenediamine tetra-acetic acid were used to measure duodenal phosphorus flow. The infusion of calcium chloride led to an increase in calcium and phosphorus retention, the increase in phosphorus retention being due to a reduction in faecal phosphorus excretion as a result of enhanced intestinal phosphorus absorption and reduced faecal endogenous phosphorus excretion. There was, however, no change in duodenal phosphorus flow or in the amount of phosphorus estimated to have been added to the digesta via the saliva. These results suggest that the increase in phosphorus retention seen in response to an increase in phosphorus requirement is largely achieved through an increase in intestinal absorptive efficiency and not through any reduction in endogenous phosphorus secretion into the gut.