Evidence for functional expression of TRPM7 channels in human atrial myocytes

Transient receptor potential melastatin-7 (TRPM7) channels have been recently reported in human atrial fibroblasts and are believed to mediate fibrogenesis in human atrial fibrillation. The present study investigates whether TRPM7 channels are expressed in human atrial myocytes using whole-cell patch voltage-clamp, RT-PCR and Western blotting analysis. It was found that a gradually activated TRPM7-like current was recorded with a K(+)- and Mg(2+)-free pipette solution in human atrial myocytes. The current was enhanced by removing extracellular Ca(2+) and Mg(2+), and the current increase could be inhibited by Ni(2+) or Ba(2+). The TRPM7-like current was potentiated by acidic pH and inhibited by La(3+) and 2-aminoethoxydiphenyl borate. In addition, Ca(2+)-activated TRPM4-like current was recorded in human atrial myocytes with the addition of the Ca(2+) ionophore A23187 in bath solution. RT-PCR and Western immunoblot analysis revealed that in addition to TRPM4, TRPM7 channel current, mRNA and protein expression were evident in human atrial myocytes. Interestingly, TRPM7 channel protein, but not TRPM4 channel protein, was significantly increased in human atrial specimens from the patients with atrial fibrillation. Our results demonstrate for the first time that functional TRPM7 channels are present in human atrial myocytes, and the channel expression is upregulated in the atria with atrial fibrillation.