Flux of inorganic phosphate across the sheep rumen wall in vivo and in vitro.

Experiments were done in vivo and in vitro to study the flux of inorganic phosphate (P1) across the rumen wall in sheep. The technique of the temporarily isolated and washed reticulo-rumen was applied to measure P1 net flux at nine different P1 concentrations in the buffer solutions introduced into the reticulo-rumen for 1 h. Six sheep were either P replete (4.25 g P d-1) or in a P-depleted state (1.05 g P d-1). Under both dietary treatments P1 net secretion into the reticulorumen was found at P1 concentrations between 0.1 and 2.2 mmol l-1 whereas P1 net absorption was observed at P1 concentrations between 4.1 and 15.8 mmol l-1. There were positive linear relationships between P1 concentration and P1 net flux for the whole concentration range studied in repleted and in depleted sheep. There were no indications of active transport components or saturation phenomena. Unidirectional P1 fluxes were then measured in Ussing chambers with 32P using a total of 100 rumen wall epithelia obtained from eighteen sheep of different ages. No P1 net flux was found under short-circuit conditions. Experiments with changed potential differences (+25 or -25 mV at serosal side) showed that P1 flux was influenced by the electrical gradient indicating electrogenic phosphate ion diffusion without involvement of active transport processes. Critical evaluation of the obtained unidirectional flux ratios suggests that part of the P1 flux was electroneutral, e.g. by ion exchange or cotransport. The results from these studies show that the reticulo-rumen substantially contributes to phosphate absorption from the sheep gastrointestinal tract.