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.
Yamaoka
K
K
Department of Physiology, School of Medicine, Hiroshima University, Kasumi 1-2-3, Minami-Ku, Hiroshima 734, Japan.
Seyama
I
I
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology; D003570:Cytidine Triphosphate / Q000494:pharmacology; D004553:Electric Conductivity; D006321:Heart / Q000502:physiology*; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008274:Magnesium / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D019319:Okadaic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010766:Phosphorylation; D011892:Rana catesbeiana; D014357:Trypsin / Q000494:pharmacology; D016276:Ventricular Function
D015220:Calcium Channels; D005576:Colforsin; D019319:Okadaic Acid; D003570:Cytidine Triphosphate; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D014357:Trypsin; D008274:Magnesium; D015056:1-Methyl-3-isobutylxanthine