Modulation of Mg Influx and Cytoplasmic Free Mg Concentration in Rat Ventricular Myocytes

To examine whether TRPM7, a member of the melastatin family of transient receptor potential channels, is a physiological pathway for Mg entry in mammalian cells, we studied the effect of TRPM7 regulators on cytoplasmic free Mg concentration ([Mg]) of rat ventricular myocytes. Acutely isolated single cells were AM-loaded with the fluorescent indicator furaptra, and [Mg] was estimated at 25 °C. After [Mg] was lowered by soaking the cells with a high-K and Mg-Ca-free solution, [Mg] was recovered by extracellular perfusion of Ca-free Tyrode's solution that contained 1 mM Mg. The initial rate of increase in [Mg] was analyzed as the Mg influx rate. The Mg influx rate was increased by the TRPM7 activator, naltriben (2-50 μM), in a concentration-dependent manner with a half maximal effective concentration (EC) of 24 μM. This EC value is similar to that reported for the activation of recombinant TRPM7 overexpressed in HEK293 cells. Naltriben (50 μM) caused little change in basal [Mg] (~ 0.9 mM) in Ca-free Tyrode's solution, but significantly raised [Mg] to 1.31 ± 0.03 mM in 94 min after the removal of extracellular Na. Re-introduction of extracellular Na lowered [Mg] back to the basal level even in the presence of naltriben. Application of 10 μM NS8593, an inhibitor of TRPM7, significantly lowered [Mg] to 0.72 ± 0.03 mM in 50-60 min independent of extracellular Na. The results suggest that Mg entry through TRPM7 significantly contributes to physiological Mg homeostasis in mammalian heart cells.