Effects of Phospholipase C Inhibitors on Ca2+ Channel Stimulation and Ca2+ Release from Intracellular Stores Evoked by Alpha 1A- and Alpha 2A-adrenoceptors in Rat Portal Vein Myocytes

The ability of phospholipase C inhibitors to inhibit Ca2+ channel stimulation and Ca2+ release from intracellular stores evoked by norepinephrine in single rat portal vein myocytes was investigated in the aim of identifying the type of phospholipase C involved in the transduction pathways activated by alpha 1A- and alpha 2A-adrenoceptors. U73122 (an inhibitor of phosphatidylinositol-phospholipase C) inhibited in a concentration-dependent manner the release of Ca2+ from the intracellular stores induced by activation of alpha 1A-adrenoceptors and related to inositol phosphate production whereas U73343 was ineffective. Both compounds had no effect on the release of Ca2+ induced by caffeine. However, U73122 and U73343 inhibited the L-type Ca2+ channel. D609 (an inhibitor of phosphatidylcholine-phospholipase C) had no direct inhibitory effects on the L-type Ca2+ channel but it inhibited concentration dependently the alpha 2A-adrenoceptor-induced stimulation of Ca2+ channels, which had been shown to be independent of phosphatidylinositol hydrolysis. Therefore, these results suggest that alpha 2A-adrenoceptors activate a phosphatidylcholine-phospholipase C in vascular myocytes. However, D609 had other sites of action as it blocked norepinephrine- and caffeine-induced Ca2+ release from the intracellular stores.