Modulation of Mg2+ influx and cytoplasmic free Mg2+ concentration in rat ventricular myocytes
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To examine whether TRPM7, a member of the melastatin family of transient receptor potential channels, is a physiological pathway for Mg2+ entry in mammalian cells, we studied the effect of TRPM7 regulators on cytoplasmic free Mg2+ concentration ([Mg2+]i) of rat ventricular myocytes. Acutely isolated single cells were AM-loaded with the fluorescent indicator furaptra, and [Mg2+]i was estimated at 25 °C. After [Mg2+]i was lowered by soaking the cells with a high-K+ and Mg2+-Ca2+-free solution, [Mg2+]i was recovered by extracellular perfusion of Ca2+-free Tyrode’s solution that contained 1 mM Mg2+. The initial rate of increase in [Mg2+]i was analyzed as the Mg2+ influx rate. The Mg2+ influx rate was increased by the TRPM7 activator, naltriben (2–50 μM), in a concentration-dependent manner with a half maximal effective concentration (EC50) of 24 μM. This EC50 value is similar to that reported for the activation of recombinant TRPM7 overexpressed in HEK293 cells. Naltriben (50 μM) caused little change in basal [Mg2+]i (~ 0.9 mM) in Ca2+-free Tyrode’s solution, but significantly raised [Mg2+]i to 1.31 ± 0.03 mM in 94 min after the removal of extracellular Na+. Re-introduction of extracellular Na+ lowered [Mg2+]i back to the basal level even in the presence of naltriben. Application of 10 μM NS8593, an inhibitor of TRPM7, significantly lowered [Mg2+]i to 0.72 ± 0.03 mM in 50-60 min independent of extracellular Na+. The results suggest that Mg2+ entry through TRPM7 significantly contributes to physiological Mg2+ homeostasis in mammalian heart cells.
KeywordsMagnesium influx TRPM7 Naltriben NS8593 Rat ventricular myocyte
We thank Shinobu Tai for technical assistance and Mary Shibuya for reading the manuscript. This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP15K08188, and the Institute of Seizon and Life Sciences.
All authors conceived and designed the study. MT performed the experiments, analyzed data, and wrote the initial draft of the manuscript. MK contributed to analysis and interpretation of data, and wrote the manuscript. HI contributed to data interpretation, and critically reviewed the manuscript. All authors approved the final version of the manuscript.
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Conflict of interest
The authors declare that they have no conflict of interest.
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