Abstract:
Animal production in Australia EVIDENCE FOR THREE SITES FOR PHOSPHORUS ABSORPTION IN SHEEP K. TOWNS*, C. SEVILLA**, H. DAVIES**, J. TERNOUTH** and R. BOSTON* Following the development of procedures for measurement of phosphorus (P) absorption in sheep (Towns 1979; Towns et al. 1980) we decided to use the technique on sheep fed different amounts of calcium (Ca) and P. Six sheep were used, all fed ad libitum a ration of 60% barley straw, 6.4% gluten, 1.7% urea, 31% sucrose and 0.7% vitamins and minerals. Each animal was also given 6.8 g Ca as CaC03 and three were given 3.9 g P as monosodium orthophosphate daily. Each sheep was dosed twice with 32P, via the abomasum and the jugular vein, and the plasma clearance curves were found. The animals on the low P diet absorbed large amounts of P with a delay of 2-3 minutes; the initial peak of 32P in the blood was high and then decreased sharply, sometimes showing a second peak at about 200 minutes. In contrast, the animals on high P diets showed a greatly reduced initial peak, but showed (Fig-l.) two other peaks of increasing height at 150 and 275 minutes Because the sheep given the high P intakes have three peaks in their clearance curves following abomasal injection, three discrete sites for P absorption may occur in the gastro-intestinal tract. Also since the first peak in the clearance curve is reduced by the high P intake, the site for control of absorption is probably in the duodenum or proximal small intestine, i.e. close to the abomasum where the isoto-pe was administered. Finally, if absorption rate is reduced at this proximal site then more absorption may occur further down the gastro-intestinal tract, even though the overall absorption rate for P is lower than in the sheep given a low P intake. TOWNS, K.M. (1979). Proc.Nutr.Soc.Aust. = 4:151. BOSTON, R.C. AND LEAVER, D.D. (1980). TOWNS, K-M., 450. Proc.Aust.Soc.Anim.Prod. 13: s * ** School of Agriculture, La Trobe University, Bundoora, Vic. 3083. Dept. of Animal Production, University of Queensland, St. Lucia, Qld 4067. 446