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

Ruminants have a unique utilization of phosphate (P-i) based on the so-called endogenous P-i recycling to guarantee adequate P-i supply for ruminal microbial growth and for buffering short-chain fatty acids. Large amounts of P-i enter the gastrointestinal tract by salivary secretion. The high saliva P-i concentrations are generated by active secretion of P-i from blood into primary saliva via basolateral sodium (Na+)-dependent P-i transporter type II. The following subsequent intestinal absorption of P-i is mainly carried out in the jejunum by the apical located secondary active Na+-dependent P-i transporters NaPi IIb (SLC34A2) and PiT1 (SLC20A1). A reduction in dietary P-i intake stimulates the intestinal P-i absorption by increasing the expression of NaPi IIb despite unchanged plasma 1,25-dihydroxyvitamin D-3 concentrations, which modulate P-i homeostasis in monogastric species. Reabsorption of glomerular filtrated plasma P-i is mainly mediated by the P-i 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-i capacity were modulated by high dietary phosphorus (P) intake in a parathyroid-dependent manner. In response to reduced dietary P-i intake, the expression of NaPi IIa was not adapted indicating that renal P-i 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-i transporters are able to contribute to maintaining P-i homeostasis. Overall, the regulation of P-i transporter activity and expression by hormonal modulators confirms substantial differences between ruminant and non-ruminant species.