Effects of potassium on magnesium transport across rumen epithelium

Increasing ruminal K concentration impairs active Mg transport across the forestomach epithelium of ruminants. We used Ussing chamber and microelectrode techniques to test the hypothesis that high mucosal K decreases the driving force for Mg2+ uptake by depolarizing the apical membrane. Serosal Ba enhanced net K-42 secretion. Under open-circuit conditions, rumen epithelial cells showed an apical membrane potential (V-a) of -67.3 +/- 1.5 mV. An increase in mucosal K concentration depolarized V-a and decreased mucosal-to-serosal Mg-28 flux (J(m-->s)(Mg)). An increase in the transepithelial potential difference (V-t) depolarized V-a, greatly decreased J(m-->s)(Mg), and slightly increased Mg serosal-to-mucosal flux (J(s-->m)(Mg)). J(m-->s)(Mg) exhibited an electrogenic and an electroneutral component. Mucosal verapamil depolarized V-a and reduced J(m-->s)(Mg), without affecting J(m-->s)(Mg). The study shows that rumen epithelial cells exhibit apical and basolateral K conductances and that transcellular Mg absorption has a distinct electrogenic component. This supports the assumption that a change in V-a, represents the link between increased ruminal K concentration and decreased Mg absorption. The results are discussed with regard to the development of hypomagnesemia in ruminants.