Alpha 2-adrenoceptors activate dihydropyridine-sensitive calcium channels via Gi-proteins and protein kinase C in rat portal vein myocytes.

The presence of functional alpha 2-adrenoceptors was investigated in isolated smooth muscle cells from rat portal vein using the nystatin-perforated patch-clamp technique. The free cytoplasmic calcium concentration ([Ca2+]i) was estimated using emission from the dye Fura-2. Activation of alpha 2-adrenoceptors by clonidine (an alpha 2-adrenoceptor agonist) or noradrenaline (a non-selective alpha-adrenoceptor agonist), both in the presence of 0.1 microM prazosin to block alpha 1-adrenoceptors, caused a slow and sustained increase in [Ca2+]i which was inhibited by 0.1 microM rauwolscine (an alpha 2-adrenoceptor antagonist). A similar Ca2+ response was obtained with oxymetazoline (a selective alpha 2A-adrenoceptor agonist) suggesting that the increase in [Ca2+]i resulted from activation of the alpha 2A-adrenoceptor subtype. The increase in [Ca2+]i did not occur in calcium-free solution or in the presence of oxodipine (a voltage-dependent calcium channel blocker), indicating that it depended on a calcium influx. The alpha 2A-adrenoceptor-activated calcium influx was unchanged after complete release of the stored calcium induced by applications of ryanodine and caffeine. In addition, no accumulation of inositol trisphosphate was detected in the presence of 0.1 microM prazosin. Taken together, these results indicate that alpha 2A-adrenoceptor activation does not stimulate phosphoinositide turnover and subsequent calcium release from intracellular stores. Whole-cell patch-clamp experiments showed that alpha 2A-adrenoceptor activation promoted calcium influx through voltage-dependent L-type channels.(ABSTRACT TRUNCATED AT 250 WORDS)