? 2-Adrenoceptors activate dihydropyridine-sensitive calcium channels via Gi-proteins and protein kinase C in rat portal vein myocytes

Abstract

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)

Signaling pathway underlying stimulation of L-type Ca2+ channels in rabbit portal vein myocytes by recombinant Gβγ subunits

“…In previous studies, we have shown that the actions of ACh and G␤␥ on Cav currents involve specifically a novel PKC (6,42). In contrast, PdBu activates both classical and novel PKCs, and both of these classes of PKC can enhance Cav currents (6,10,15,19,22,29,36). It is therefore possible that the greater effect of PdBu on Cav currents observed in this study is due to the involvement of multiple PKC-dependent pathways.…”

Acute and chronic NOS inhibition enhances α2- adrenoreceptor-stimulated RhoA and Rho kinase in rat aorta

“…The potential role of RhoA and ROK in ␣ 2 -AR contraction is suggested by studies in other systems. For example, norepinephrine contraction in human small omental arteries and rabbit aorta is sensitive to inhibition by Y-27632 (27) and C3 botulinum toxin (24), respectively, suggesting that RhoA and ROK contribute to adrenergic arterial contraction. Carbachol-induced actin formation in human airway smooth muscle requires G i activation of RhoA (39).…”

“…ROK phosphorylates and deactivates myosin light chain (MLC) phosphatase, resulting in accumulation of phosphorylated MLC at constant intracellular calcium concentrations (32,38). Rho and its target ROK participate in contraction to phenylephrine, endothelin, angiotensin, and norepinephrine (27,28,35,40,41). The study of Rho and subsequently ROK has revealed a new paradigm for the regulation of vascular smooth muscle contraction.…”

Endothelin-1 Regulates Tone of Isolated Small Arteries in the Rat

“…Activation of smooth muscle ␣ 2 -adrenergic receptors triggers contraction, whereas endothelial ␣ 2 -adrenergic receptor occupation usually mediates relaxation of preconstricted arteries in vitro. 14,[22][23][24][25] The data obtained in treated rats, however, suggest that in the absence of preconstricting tone, activation of endothelial ␣ 2 -adrenergic receptors mediates a contraction that is sensitive to bosentan but not BQ123. It seems therefore that activation of ␣ 2 -adrenergic receptors triggers the release of ET-1 that stimulates smooth muscle ET B receptors.…”