FLUX OF CALCIUM 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.
² Veterinary Faculty, University of Konya, Turkey

Experiments were carried out in vivo and in vitro to study the flux of Ca²⁺ across the sheep rumen wall. Buffer solutions with five different Ca²⁺, concentrations in the range 0·2-3·6 mmol l^-1 were introduced into the isolated, emptied and washed reticulo-rumen for 1 h to determine Ca²⁺ net flux. Phosphate concentrations were either kept constant at 8 mmol l^-1, or varied so as to maintain a constant phosphate/Ca²⁺ ratio of 2:1. Under both conditions there was a Ca²⁺ net secretion into the rumen at Ca²⁺ concentrations below approximately 1 mmol l^-1 whereas Ca²⁺ net absorption occurred at higher Ca²⁺ concentrations and linearly increased with increasing Ca²⁺ concentrations. When phosphate concentrations were reduced to 4·3 and 0·6 mmol l^-1 (Ca²⁺, 3 mmol l^-1) Ca²⁺ net absorption was reduced by about 50% and completely abolished, respectively. The unidirectional (mucosal-to-serosal or serosal-to-mucosal) Ca fluxes in vitro, J^Ca_ms and J^Ca_sm, were measured in Ussing chambers with computer-controlled voltage clamp. Under short-circuit current conditions (Ca²⁺ and phosphate, 2 mmol l^-1) there was a significant net flux of Ca²⁺ from the mucosal to the serosal side. This net flux of Ca²⁺ was abolished when ouabain was added to the serosal solution, indicating that the unidirectional Ca²⁺ flux J^Ca_ms and, hence, the net flux of Ca²⁺ was dependent upon active Na⁺ transport accomplished by the Na, K-ATPase system. Under voltage clamp conditions, the net flux of Ca²⁺ was greatly increased at a potential difference PD = -25 mV (serosa negative) but disappeared at PD = +25 mV (serosa positive). Analysis of the unidirectional Ca²⁺ fluxes J^Ca_ms and J^Ca_sm under varying potential differences indicated that both Ca²⁺ fluxes consisted of a PD-dependent and a PD-independent component. The results show that the reticulo-rumen participates in Ca²⁺ absorption in sheep.

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