Effects of short chain fatty acids and carbon dioxide on magnesium transport across sheep rumen epithelium

Short chain fatty acids (SCFAs) and CO2 have been shown to stimulate net Mg2+ efflux from the isolated reticulorumen in vivo. To investigate the underlying mechanisms of Mg2+ transport we performed Ussing chamber and microelectrode experiments and measured Mg-28(2+) fluxes across sheep rumen epithelium in vitro. In the presence of SCFAs mucosal-to-serosal Mg2+ flux (J(m-s)(Mg)) amounted to 82.3 +/- 7.8 nmol cm(-2) h(-1) and serosal-to-mucosal Mg2+ flux (J(s-m)(Mg)) to 3.2 +/- 0.7 nmol cm(-2) h(-1). Replacing SCFAs with gluconate caused a 50 % reduction of J(m-s)(Mg), whereas J(s-m)(Mg) was not affected. Among the SCFAs, n-butyrate was more effective in stimulating J(m-s)(Mg) than acetate, propionate or iso-butyrate. Eliminating HCO3--CO2 from SCFA-containing solutions did not affect Mg2+ fluxes, whereas the same replacement in SCFA-free solutions led to a further reduction in J(m-s)(Mg). J(m-s)(Mg) decreased after the addition of ethoxyzolamide to SCFA-free, bicarbonate buffered solutions. Decreasing mucosal pH from 6.4 to 5.4 increased J(m-s)(Mg) in SCFA-free, bicarbonate buffered solutions. SCFAs had no effect on the apical membrane potential of rumen epithelial cells. The experiments show that both SCFAs and CO2 stimulate Mg2+ transport through an increase in J(m-s)(Mg), most probably via stimulation of a Mg2(+)-2H(+) exchange mechanism. SCFAs may have additional metabolic effects on Mg2+ transport.