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To ensure optimal fermentation of ruminal content and prevent electrolyte and water imbalances, ruminal transport has to be tightly regulated. Thus, when dietary intake of potassium is high, the rumen epithelium responds by increasing the rate of sodium absorption (Lang & Martens, 1999) while uptake of magnesium is reduced. Patch-clamp experiments on cultured ruminal epithelial cells showed that removal of divalent cations from the external solution depolarized the cells and induced a highly significant increase in inward current at negative potential levels in the presence of Na and K, but not choline. Removal of external magnesium alone had a smaller effect. Relative permeability P(K)/P(Na) did not depend on the concentration of internal or external divalents in otherwise identical solutions, but varied between potassium gluconate and choline chloride pipette solution (2.1 versus 1.4), suggesting interaction between Na+ and K+ in the channel pore. Both concentrations of divalents and conductance level increased significantly when magnesium, or both calcium and magnesium were removed from the internal solution. We propose that ruminal sodium transport involves a non-selective cation channel that is regulated by changes in cytosolic magnesium, such as those observed when external potassium levels are elevated.