Modulation of distinct isoforms of L-type calcium channels by Gq-coupled receptors in Xenopus oocytes Antagonistic effects of Gβγ and protein kinase C

L-type voltage dependent Ca2+ channels (L-VDCCs; Ca(v)1.2) are crucial in cardiovascular physiology. In heart and smooth muscle, hormones and transmitters operating via G(q) enhance L-VDCC currents via essential protein kinase C (PKC) involvement. Heterologous reconstitution studies in Xenopus oocytes suggested that PKC and G(q)-coupled receptors increased L-VDCC currents only in cardiac long N-terminus (NT) isoforms of alpha(1C), whereas known smooth muscle short-NT isoforms were inhibited by PKC and G(q) activators. We report a novel regulation of the long-NT alpha(1C) isoform by G beta gamma. G beta gamma inhibited whereas a G beta gamma scavenger protein augmented the G(q)-but not phorbol ester-mediated enhancement of channel activity, suggesting that G beta gamma acts upstream from PKC. In vitro binding experiments reveal binding of both G beta gamma and PKC to alpha(1C)-NT. However, PKC modulation was not altered by mutations of multiple potential phosphorylation sites in the NT, and was attenuated by a mutation of C-terminally located serine S1928. The insertion of exon 9a in intracellular loop 1 rendered the short-NT alpha(1C) sensitive to PKC stimulation and to G beta gamma scavenging. Our results suggest a complex antagonistic interplay between G(q)-activated PKC and G beta gamma in regulation of L-VDCC, in which multiple cytosolic segments of alpha(1C) are involved.