Transfer of Phosphate in the Digestive Tract III. Dairy Cattle

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Abstract
The appearance of intravenously injected radiophosphate in the contents of the gastrointestinal tract of cows and calves indicates a large secretion of endogenous phosphorus. In the mature cow, as in mature sheep, the main site of secretion is the rumen, although most of the phosphorus in the bovine rumen seems to enter in the saliva. In the calves, however, the ruminal secretion is smaller and a large part of the endogenous phosphorus appears to be secreted into the small intestine, though not as much as that previously observed in swine. The rate of equilibration of tissue phosphorus with circulating inorganic phosphorus was calculated for some organs. A slight aging effect on tissue phosphorus metabolism was observed with a lower uptake rate in older animals. The phosphorus exchange of intestinal tissues is generally less in the cow than in sheep. Four calves, which varied widely in body size and time of fasting, were sacrificed 24-hours after injection of radiophosphate. An analysis of variance revealed no significant difference in chemical composition between corresponding tissues of these animals; however, significant differences in phosphorus secretion rates were observed.