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

  • Original Article
  • Molecular and Cellular Physiology
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Abstract

The presence of functional α 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 α 2-adrenoceptors by clonidine (an α 2-adrenoceptor agonist) or noradrenaline (a non-selective α-adrenoceptor agonist), both in the presence of 0.1 μM prazosin to block α 1-adrenoceptors, caused a slow and sustained increase in [Ca2+]i which was inhibited by 0.1 μM rauwolscine (an α 2-adrenoceptor antagonist). A similar Ca2+ response was obtained with oxymetazoline (a selective α 2A-adrenoceptor agonist) suggesting that the increase in [Ca2+]i resulted from activation of the α 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 α 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 μM prazosin. Taken together, these results indicate that α 2A-adrenoceptor activation does not stimulate phosphoinositide turnover and subsequent calcium release from intracellular stores. Wholecell patch-clamp experiments showed that α 2A-adrenoceptor activation promoted calcium influx through voltage-dependent L-type channels. Concomitant with calcium influx, α 2A-adrenoceptor activation induced a 10- to 15-mV depolarization. Similar effects on both calcium channel current and [Ca2+]i were obtained with mastoparan, an activator of Gi-proteins. Activation of calcium influx by both α 2A-adrenoceptors and mastoparan was reduced by treatment with pertussis toxin and GF 109203X (a protein kinase C inhibitor). These data suggest that activation of protein kinase C through a transduction pathway involving Gi-proteins phosphorylates voltage-activated L-type calcium channels and thus, increases their opening probability.

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Authors and Affiliations

  1. Laboratoire de Physiologie Cellulaire et Pharmacologie Moléculaire, URA CNRS 1489, Université de Bordeaux II, 3 place de la Victoire, F-33076, Bordeaux Cédex, France

    N. Leprêtre & J. Mironneau

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  1. N. Leprêtre
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  2. J. Mironneau
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Leprêtre, N., Mironneau, J. α 2-Adrenoceptors activate dihydropyridine-sensitive calcium channels via Gi-proteins and protein kinase C in rat portal vein myocytes. Pflugers Arch. 429, 253–261 (1994). https://doi.org/10.1007/BF00374320

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  • DOI: https://doi.org/10.1007/BF00374320

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