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 mi croelectrode techniques to test the hypothesis that high mucosal K dec reases the driving force for Mg2+ uptake by depolarizing the apical me mbrane. Serosal Ba enhanced net K-42 secretion. Under open-circuit con ditions, rumen epithelial cells showed an apical membrane potential (V -a) of -67.3 +/- 1.5 mV. An increase in mucosal K concentration depola rized V-a and decreased mucosal-to-serosal Mg-28 flux (J(m-->s)(Mg)). An increase in the transepithelial potential difference (V-t) depolari zed V-a, greatly decreased J(m-->s)(Mg), and slightly increased Mg ser osal-to-mucosal flux (J(s-->m)(Mg)). J(m-->s)(Mg) exhibited an electro genic 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 co nductances and that transcellular Mg absorption has a distinct electro genic component. This supports the assumption that a change in V-a, re presents the link between increased ruminal K concentration and decrea sed Mg absorption. The results are discussed with regard to the develo pment of hypomagnesemia in ruminants.