Both alpha 1A- and alpha 2A-adrenoreceptor subtypes stimulate voltage-operated L-type calcium channels in rat portal vein myocytes. Evidence for two distinct transduction pathways.

Previously, we have shown that myocytes from rat portal vein express alpha 1A-adrenoreceptors that couple with a Gq/G11-protein to stimulate phosphoinositide turnover and release of calcium from intracellular stores. The purpose of this study was to investigate the contribution of both alpha 1- and alpha 2-adrenoreceptor subtypes in inducing stimulation of voltage-operated calcium channels. Norepinephrine (a nonselective alpha-adrenoreceptor agonist), phenylephrine (an alpha 1-adrenoreceptor agonist), clonidine, and oxymetazoline (alpha 2-adrenoreceptor agonists) stimulated the calcium channel current by a similar extent. Using subtype-selective antagonists we showed that both alpha 1A- and alpha 2A-adrenoreceptors modulated voltage-operated calcium channels through two distinct transduction pathways. alpha 1A-Adrenoreceptors coupled with a pertussis toxin-insensitive G-protein whereas alpha 2A-adrenoreceptors coupled with a pertussis toxin-sensitive G-protein. Portal vein myocytes expressed G-proteins that were recognized by anti-alpha q/alpha 11, -alpha i(1-2), and -alpha i(3) antibodies. As internal applications of anti-phosphatidylinositol and anti-alpha q/alpha 11 antibodies had no effect on the alpha 2A-adrenoreceptor-induced enhancement of calcium channel current, these findings suggest that phosphatidylinositol hydrolysis and Gq/G11-protein are not involved in the alpha 2A-adrenoreceptor-induced coupling process. A protein kinase C inhibitor, GF 109203X, and a long term (24 h) treatment with phorbol dibutyrate to decrease the activity of protein kinase C blocked the alpha 1A- and alpha 2A-adrenoreceptor-induced stimulation of calcium channels as well as that stimulation induced by phorbol dibutyrate. Moreover, activation of alpha 2A-adrenoreceptors did not induce a significant calcium release from intracellular stores. These data suggest that two distinct G-proteins, probably Gq/G11 and Gi, coupled to alpha 1A- and alpha 2A-adrenoreceptors regulate calcium influx through voltage-operated calcium channels by two different transduction pathways leading to activation of protein kinase C.