This study examines the routes by which Mg 2ϩ leaves cultured ovine ruminal epithelial cells (REC). Mg 2ϩ -loaded (6 mM) REC were incubated in completely Mg 2ϩ -free solutions with varying Na ϩ concentrations, and the Mg 2ϩ extrusion rate was calculated from the increase of the Mg 2ϩ concentration in the incubation medium determined with the aid of the fluorescent probe mag-fura 2 (Na ϩ salt). In other experiments, REC were also studied for the intracellular free Mg 2ϩ concentration ([Mg 2ϩ ]i; using magfura 2), the intracellular Na ϩ concentration (using Na ϩ -binding benzofuran isophthalate), the intracellular cAMP concentration ([cAMP]

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