Research Unit Nutritional Physiology “Oskar Kellner”, Research Institute for the Biology of Farm Animals (FBN), Dummerstorf, Department of Veterinary Physiology, Free University of Berlin, Berlin, Germany
A protein of ~ 70-kDa was identified as a candidate Na
2+ exchanger in rumen epithelial cells (REC). Melastatin-related Transient Receptor Potential 7 (TRPM7) and Magnesium Transporter 1 (MagT1) transcripts and, from them, encoded proteins were also detected. The regulation of these Mg transport pathways by extracellular [Mg] changes was the main focus of this study. Therefore, a 24-h pre-incubation of ovine REC in control (1.2 mM), low (0.12 mM)-Mg, and high (5 mM)-Mg medium was performed. Na
2+ exchangers, TRPM7 and MagT1 abundance and activity were investigated by Western blot analysis, flow cytometry, immunocytochemistry and fluorescence spectroscopic measurements of [Mg
i changes. Inhibitors were employed to differentiate Na
2+ exchanger-mediated (imipramine) and channel-mediated (cobalt(III)hexaammine, nitrendipine) Mg transport. Basal [Mg
i (0.40 ± 0.02 mM) was not influenced by pre-incubation in low- or high-Mg medium. However, compared with control REC (4.1 ± 0.7 μM/min), such cells showed reduced (2.8 ± 0.6 μM/min) or elevated (6.4 ± 0.9 μM/min) Mg extrusion rates that correlated with a decreased (25%) and increased (38%) expression of the putative Na
2+ exchanger protein, respectively. Low- and high-Mg pre-incubated REC were both characterized by an increased (30-40%) influx capacity. In low-Mg REC, the latter resulted mainly from a strong activation of the TRPM7-related transport component. The data thus clearly demonstrate the intrinsic regulation of REC transmembrane Mg transport.