Modulation of Cardiac Ca²⁺ Channel by G_q-activating Neurotransmitters Reconstituted in Xenopus Oocytes

L-type dihydropyridine-sensitive voltage dependent Ca²⁺ channels (L-VDCCs; α_1C) are crucial in cardiovascular physiology. Currents via L-VDCCs are enhanced by hormones and transmitters operating via G_q, such as angiotensin II (AngII) and acetylcholine (ACh). It has been proposed that these modulations are mediated by protein kinase C (PKC). However, reports on effects of PKC activators on L-type channels are contradictory; inhibitory and/or enhancing effects have been observed. Attempts to reproduce the enhancing effect of AngII in heterologous expression systems failed. We previously found that PKC modulation of the channel depends on α_1C isoform used; only a long N-terminal (NT) isoform was up-regulated. Here we report the reconstitution of the AngII- and ACh-induced enhancement of the long-NT isoform of L-VDCC expressed in Xenopus oocytes. The current initially increased over several minutes but later declined to below baseline levels. Using different NT deletion mutants and human short- and long-NT isoforms of the channel, we found the initial segment of the NT to be crucial for the enhancing, but not for the inhibitory, effect. Using blockers of PKC and of phospholipase C (PLC) and a mutated AngII receptor lacking G_q coupling, we demonstrate that the signaling pathway of the enhancing effect includes the activation of G_q, PLC, and PKC. The inhibitory modulation, present in both α_1C isoforms, was G_q- and PLC-independent and Ca²⁺-dependent, but not Ca²⁺-mediated, as only basal levels of Ca²⁺ were essential. Reconstitution of AngII and ACh effects in Xenopus oocytes will advance the study of molecular mechanisms of these physiologically important modulations.