Mechanisms and regulation of epithelial phosphate transport in ruminants: approaches in comparative physiology.

Ruminants have a unique utilization of phosphate (P) based on the so-called endogenous P recycling to guarantee adequate P supply for ruminal microbial growth and for buffering short-chain fatty acids. Large amounts of P enter the gastrointestinal tract by salivary secretion. The high saliva P concentrations are generated by active secretion of P from blood into primary saliva via basolateral sodium (Na)-dependent P transporter type II. The following subsequent intestinal absorption of P is mainly carried out in the jejunum by the apical located secondary active Na-dependent P transporters NaPi IIb (SLC34A2) and PiT1 (SLC20A1). A reduction in dietary P intake stimulates the intestinal P absorption by increasing the expression of NaPi IIb despite unchanged plasma 1,25-dihydroxyvitamin D concentrations, which modulate P homeostasis in monogastric species. Reabsorption of glomerular filtrated plasma P is mainly mediated by the P transporters NaPi IIa (SLC34A1) and NaPi IIc (SLC34A3) in proximal tubule apical cells. The expression of NaPi IIa and the corresponding renal Na-dependent P capacity were modulated by high dietary phosphorus (P) intake in a parathyroid-dependent manner. In response to reduced dietary P intake, the expression of NaPi IIa was not adapted indicating that renal P reabsorption in ruminants runs at a high level allowing no further increase when P intake is diminished. In bones and in the mammary glands, Na-dependent P transporters are able to contribute to maintaining P homeostasis. Overall, the regulation of P transporter activity and expression by hormonal modulators confirms substantial differences between ruminant and non-ruminant species.