MECHANISMS OF INTESTINAL PHOSPHATE-TRANSPORT IN SMALL RUMINANTS

In order to study the localization and mechanisms of intestinal phosph ate transport in sheep and goats, unidirectional inorganic phosphate ( P-i) flux rates across isolated stripped epithelial tissues were measu red in vitro by applying the Ussing-chamber technique, In the first ex periment the tissues were obtained from animals which had been kept on an adequate dietary P supply, In the second experiment the animals ha d either been kept on an adequate Ca and P supply or were Ca- and/or P -depleted. Significant net P-i absorption was measured in all segments of the small intestine and in the proximal colon of sheep and in the duodenum and jejunum of goats. Since the experiments were carried out in the absence of any electrochemical gradient, this clearly indicates the presence of active mechanisms for P-i transport in the intestinal tract of small ruminants, In sheep jejunum, reduction of mucosal Na c oncentration to 1.8 mM or serosal application of ouabain (0.1 mM) resu lted in significant decreases of net P-i absorption of the same order of magnitude, indicating that about 65% of active P-i transport in she ep jejunum is mediated by a Na-dependent active transport mechanism. T he mechanism for the remaining Na+-independent active P-i transport ha s not yet been identified, Dietary P depletion caused hypophosphataemi a and induced a significant stimulation of net P-i absorption in goat duodenum and jejunum, This increase was independent of dietary Ca supp ly and was not associated with increased plasma calcitriol concentrati ons, This suggests substantial differences in hormonal regulation of P -i transport in small ruminants in comparison with single-stomached sp ecies.