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The present study was performed to show the molecular identity of functionally characterized Mg transport pathways in rumen epithelial cells (REC) and to investigate the effects of extracellular [Mg] changes on their expression and activity. By using RT- PCR, Western blot, flow cytometry and immunocytochemistry, TRPM7, MagT1 and a Na+/Mg2+ exchanger were found in REC. Compared with control conditions ([Mg]e = 1.2 mM), a decreased or increased MagT1 (-30%; 20%) and Na+/Mg2+ exchanger (-25%; 40%) protein abundance was observed after a 24-h incubation of REC in low (0.12 mM)- and high (5 mM)-Mg medium, respectively. To determine the Mg transport capacity, [Mg2+]i changes were measured by use of mag-fura 2. The basal [Mg2]i (0.43 +/- 0.03 mM) was not influenced by the [Mg] of the pre-incubation medium. However, compared to control cells, REC incubated in low- or high-Mg medium showed significantly reduced (59%) and elevated (57%) Mg extrusion rates, respectively. In addition, they were characterized by an increased influx capacity (30-40%). In low-Mg cells the latter results mainly from a strong TRPM7 related transport component whereas in high-Mg cells the imipramine-sensitive, the Na+/Mg2+ exchanger-mediated transport component causes this effect. In conclusion, TRPM7, MagT1 and a Na+/Mg2+ exchanger are shown to be the main Mg transport proteins in REC and their expression and functional activity is influenced by the cellular Mg status. The latter responses permit adaptation of epithelial Mg absorption and enable REC to maintain a physiological [Mg2+]i which is a prerequisite for various cell functions.