Phosphorylation modulates L-type Ca channels in frog ventricular myocytes by changes in sensitivity to Mg2+ block

The effect of phosphorylation on the intracellular Mg2+ concentration-dependent change in Ca channel activity was examined using the patch clamp technique. The kinetic changes in Ca channels induced either by phosphorylation [1 muM forskolin (FSK) plus 50 muM isobutylmethylxanthine (IBMX)] or by lowering intracellular [Mg2+] ([Mg2+]i) are qualitatively identical: an increase in both the open probability and availability of channels, as well as a decrease in the closed time without a change in the mean open time. This suggests that the mechanism for the increase in activity of Ca channels shares a common pathway of kinetic change. The concentration/response curve for the Mg2+-evoked modification of calcium channels was altered greatly by channel phosphorylation. In the external medium containing 1 muM FSK + 50 muM IBMX, Ca channels recorded with pipettes containing okadaic acid (OA) lost their sensitivity to Mg2+ in the range 1 x 10(-6) M-1 x 10(-3) M and remained in a fully active state. On the contrary, under basal conditions, the activity of Ca channel was strongly dependent on the internal Mg2+ over the same range of [Mg2+]. Similarly, phosphorylation of Ca channels eliminated the blocking action of guanosine triphosphate observed under basal conditions. A model is proposed in which Ca channels are equipped with regulatory gates for opening and closing the channels, and their regulation is dependent on [Mg2+]i and the degree of phosphorylation.