The absorption of calcium ions from the ovine reticulo-rumen

Net Ca2+ and Mg2+ absorption rates were measured in vivo from buffer solutions placed in the washed reticulo-rumen, isolated in situ in 30 conscious, trained sheep. An increase in concentration of short chain fatty acids (SCFA) in the buffer, over the range 0-50 mM, was shown to stimulate the net rates of absorption of Ca2+ and Mg2+ ions from the rumen. Similarly, the results of in vitro experiments, carried out with ovine rumen epithelium mounted in short-circuited Ussing chambers, showed that the absence of SCFA from the chamber fluid resulted in a reduction in Jnet Ca2+ caused by reduced flux of Ca2+ ions in the mucosal to serosal direction (Jms Ca2+). The addition of 1 mM acetazolamide, an inhibitor of carbonic anhydrase, to the ruminal buffer used in the in vivo experiments led to significant reductions in the net absorption rates of Ca2+ and Mg2+ ions in the presence of SCFA (50 mmol x l(-1)) but not in the absence of SCFA. However, in the in vitro experiments, the addition of 60 microM ethoxyzolamide had no significant effect on Jnet Ca2+. A reduction in pH of the intraruminal buffer in vivo from 6.8 to 5.4 led to significant increases in the net absorption rates of Ca2+ and Mg2+ ions, an effect which was duplicated for Ca2+ in preliminary in vitro experiments in which the pH of the mucosal buffer was reduced from 7.4 to 5.4. This stimulatory effect was confined to Jms Ca2+ and Jnet Ca2+. Ussing chambers were also used to demonstrate that Jnet Ca2+ was reduced by a high transmural potential difference (PD), caused by voltage clamping, independently of the mucosal K+ concentration. Both unidirectional Ca2+ fluxes consisted of a PD-dependent and a K+-insensitive PD-independent component. The latter may be represented by a Ca2+/ 2H+ antiporter. It is postulated that SCFA, and to a lesser extent H2CO3, can stimulate Jms Ca2+ by activation of an apical Ca2+/2H+ antiporter through the provision of protons within the ruminal epithelial cell. A mild reduction in ruminal pH may also lead to a similar stimulation of this putative electroneutral exchange.