Regulation of Ca channel by intracellular Ca2+ and Mg2+ in frog ventricular cells

The effects of changing the intracellular concentrations of Ca2+ or Mg2+ ([Ca2+](i), [Mg2+](i)) on Ca current (I-Ca) was studied in frog ventricular myocytes using the whole-cell and cell-attached patch clamp techniques. In the physiological range of [Mg2+](i) an increase in [Ca2+](i) enhanced I-Ca whereas at lower [Mg2+](i) I-Ca was suppressed. The increase in I-Ca caused by Ca2+ loading was not mediated by phosphorylation since the kinase inhibitors H-8 {N-[2-(methylamino)-ethyl]-5-isoquinolinesulphonamide dihydrochloride}, staurosporine and KN-62 {1-[N,O-bis(5-isoquinoline-sulphonyl)-N-methyl-1-tyrosyl]-4-phenylpiperazine} and a non-hydrolysable adenosine 5'-triphosphate analogue beta,gamma-methyleneadenosine 5'-triphosphate did not prevent the Ca2+-induced I-Ca increase. I-Ca was dramatically nA/nF (n = 4) when [Mg2+](i) was lowered from 1.0 x 10(-3) to 1.0 x 10(-6) M at a [Ca2+](i) of 10(-8) M. The concentration response relation for inhibition of Ca channels by [Mg2+](i) is modulated by [Ca2+](i). To account for the experimental results it is postulated that competitive binding of Ca2+ or Mg2+ to the Ca channel accelerates the transition of the channel from an active to a silent mode. Single-channel recordings support this hypothesis. The regulation may have clinical relevance in cytoprotection during cardiac ischaemia.