Abstract
The intestinal absorption of phosphate (P(i)) takes place transcellularly through the active NaPi-cotransporters type IIb (NaPiIIb) and III (PiT1 and PiT2) and paracellularly by diffusion through tight junction (TJ) proteins. The localisation along the intestines and the regulation of P(i) absorption differ between species and are not fully understood. It is known that 1,25-dihydroxy-vitamin D(3) (1,25-(OH)(2)D(3)) and phosphorus (P) depletion modulate intestinal P(i) absorption in vertebrates in different ways. In addition to the apical uptake into the enterocytes, there are uncertainties regarding the basolateral excretion of P(i). Functional ex vivo experiments in Ussing chambers and molecular studies of small intestinal epithelia were carried out on P-deficient goats in order to elucidate the transepithelial P(i) route in the intestine as well as the underlying mechanisms of its regulation and the proteins, which may be involved. The dietary P reduction had no effect on the duodenal and ileal P(i) transport rate in growing goats. The ileal PiT1 and PiT2 mRNA expressions increased significantly, while the ileal PiT1 protein expression, the mid jejunal claudin-2 mRNA expression and the serum 1,25-(OH)(2)D(3) levels were significantly reduced. These results advance the state of knowledge concerning the complex mechanisms of the P(i) homeostasis in vertebrates.