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

Endogenous [P.sub.i] recycling is a characteristic feature of the P homeostasis in ruminants. A pronounced salivary [P.sub.i] secretion into the rumen is balanced by a high intestinal [P.sub.i] absorption and an almost complete renal [P.sub.i] reabsorption. In monogastric animals, the major [P.sub.i] transport mechanism across the apical membrane of the enterocyte is an [Na.sup.+]-dependent transport mediated by NaPi cotransporter type IIb. In ruminants, an [Na.sup.+]-, as well as an [H.sup.+]-dependent, [P.sub.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.sub.i] transport were characterized in goats. In the duodenum, there was an [H.sup.+]-dependent, [Na.sup.+]-sensitive [P.sub.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.sup.+]-dependent, [H.sup.+]-sensitive [P.sub.i] transport mainly mediated by NaPi IIb. P restriction stimulated the NaPi IIb protein expression, resulting in higher [P.sub.i] transport capacity. NaPi cotransporter type IIb; proton-sensitive phosphate transport; ruminants