Phosphate transport in the duodenum and jejunum of goats and its adaptation by dietary phosphate and calcium

Endogenous P-i recycling is a characteristic feature of the P homeostasis in ruminants. A pronounced salivary P-i secretion into the rumen is balanced by a high intestinal P-i absorption and an almost complete renal P-i reabsorption. In monogastric animals, the major P-i transport mechanism across the apical membrane of the enterocyte is an Na+-dependent transport mediated by NaPi cotransporter type IIb. In ruminants, an Na+-, as well as an H+-dependent, P-i transport system seems to exist in the small intestines. Therefore, morphological localization, type of ionic dependence, and ability to adapt to dietary P or Ca restriction of duodenal and jejunal P-i transport were characterized in goats. In the duodenum, there was an H+-dependent, Na+-sensitive P-i transport system that did not belong to the NaPi type II family and was not influenced by dietary P or Ca restriction. In contrast, in the jejunum, there was an Na+-dependent, H+-sensitive P-i transport mainly mediated by NaPi IIb. P restriction stimulated the NaPi IIb protein expression, resulting in higher P-i transport capacity.