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

Abstract

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 28Mg2+ fluxes across sheep rumen epithelium in vitro. In the presence of SCFAs mucosal-to-serosal Mg2+ flux (Jm-sMg) amounted to 82.3 +/- 7.8 nmol cm-2 h-1 and serosal-to-mucosal Mg2+ flux (Js-mMg) to 3.2 +/- 0.7 nmol cm-2 h-1. Replacing SCFAs with gluconate caused a 50% reduction of Jm-sMg, whereas Js-mMg was not affected. Among the SCFAs, n-butyrate was more effective in stimulating Jm-sMg 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 Jm-sMg. Jm-sMg decreased after the addition of ethoxyzolamide to SCFA-free, bicarbonate buffered solutions. Decreasing mucosal pH from 6.4 to 5.4 increased Jm-sMg 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 Jm-sMg, most probably via stimulation of a Mg(2+)-2H+ exchange mechanism. SCFAs may have additional metabolic effects on Mg2+ transport.