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 (ICa) 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 ICa whereas at lower [Mg2+]i ICa was suppressed. The increase in ICa 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-isoquinolinesulphonyl)-N-methyl-1-tyrosyl]-4- phenylpiperizine¿ and a non-hydrolysable adenosine 5'-triphosphate analogue beta,gamma-methyleneadenosine 5'-triphosphate did not prevent the Ca2+-induced ICa increase. ICa was dramatically increased from 10+/-6 (n=4) to 71+/-7 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.