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 c lamp technique. The kinetic changes in Ca channels induced either by p hosphorylation [1 mu M forskolin (FSK) plus 50 mu M isobutylmethylxant hine (IBMX)] or by lowering intracellular [Mg2+] ([Mg2+](i)) are quali tatively identical: an increase in both the open probability and avail ability 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 t he increase in activity of Ca channels shares a common pathway of kine tic change. The concentration/response curve for the Mg2+-evoked modif ication of calcium channels was altered greatly by channel phosphoryla tion. In the external medium containing 1 mu M FSK + 50 mu M IBMX, Ca channels recorded with pipettes containing okadaic acid (OA) lost thei r sensitivity to Mg2+ in the range 1 x 10(-6) M-1 x 10(-3) M and remai ned 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 chann els eliminated the blocking action of guanosine triphosphate observed under basal conditions. A model is proposed in which Ca channels are e quipped with regulatory gates for opening and closing the channels, an d their regulation is dependent on [Mg2+](i) and the degree of phospho rylation.