FLUX OF INORGANIC PHOSPHATE ACROSS THE SHEEP RUMEN WALL IN VIVO AND IN VITRO

¹ Department of Physiology, School of Veterinary Medicine, Bischofsholer Damm 15, D-3000 Hannover, F.R.G.

Experiments were done in vivo and in vitro to study the flux of inorganic phosphate (P_i) across the rumen wall in sheep. The technique of the temporarily isolated and washed reticulo-rumen was applied to measure P_i net flux at nine different P_i concentrations in the buffer solutions introduced into the reticulo-rumen for 1 h. Six sheep were either P replete (4·25 g P d^-l) or in a P-depleted state (1·05 g P d^-1). Under both dietary treatments P_i net secretion into the reticulo-rumen was found at P_i concentrations between 0·1 and 2·2 mmol 1^-1 whereas P_i net absorption was observed at P_i concentrations between 4·1 and 15·8 mmol 1^-1. There were positive linear relationships between P_i concentration and P_i 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 P_i 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 P_i net flux was found under short-circuit conditions. Experiments with changed potential differences (+25 or -25 mV at serosal side) showed that P_i 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 P_i flux was electroneutral, e.g. by ion exchange or co-transport. The results from these studies show that the reticulo-rumen substantially contributes to phosphate absorption from the sheep gastrointestinal tract.