Magnesium-inhibited, TRPM6/7-like channel in cardiac myocytes: permeation of divalent cations and pH-mediated regulation

Cardiac tissue expresses several TRP proteins as well as a Mg2+-inhibited, non-selective cation current (I-MIC) that bears many characteristics of TRP channel currents. We used the whole-cell voltage clamp technique in pig and rat ventricular myocytes to characterize the permeation, blockage properties and regulation of the cardiac I-MIC channels in order to compare them with TRP channels, in particular with Mg2+-sensitive TRPM6 and TRPM7. We show that removing extracellular divalent cations unmasks large inward and outward monovalent currents, which can be inhibited by intracellular Mg2+. Inward currents are suppressed upon replacing extracellular Na+ by NMDG(+). Divalent cations block monovalent I-MIC and, at 10-20 mm, carry measurable currents. Their efficacy sequence in decreasing outward I-MIC (Ni2+ = Mg2+ > Ca2+ > Ba2+) and in inducing inward I-MIC (Ni2+ much greater than Mg2+ = Ca2+ approximate to Ba2+), and their permeabilities calculated from reversal potentials are similar to those of TRPM6 and TRPM7 channels. The trivalent cations Gd3+ and Dy3+ also block I-MIC in a voltage-dependent manner ( = 0.4-0.5). In addition they inhibit the inward current carried by divalent cations. I-MIC is regulated by pH. Decreasing or increasing extracellular pH decreased and increased I-MIC, respectively (pH(0.5) = 6.9, n(H) = 0.98). Qualitatively similar results were obtained on I-MIC in rat basophilic leukaemia cells. These effects in cardiac myocytes were absent in the presence of high intracellular buffering by 40 mm Hepes. Our results suggest that I-MIC in cardiac cells is due to TRPM channels, most probably to TRPM6 or TRPM7 channels or to their heteromultimeres.