Mechanism of Ca2+ release and entry during contraction elicited by norepinephrine in rat resistance arteries

Laboratoire de Pharmacologie et Physiopathologie Cellulaires, Université Louis Pasteur de Strasbourg, Unité de Recherche Associée Centre National de la Recherche Scientifique 600, Faculté de Pharmacie, 67401 Illkirch Cedex, France The intracellular Ca 2+ stores and the mechanisms of Ca 2+ entry produced by norepinephrine (NE) were investigated in small mesenteric resistance arteries of the rat. In Ca 2+ -free medium, NE (10 µM) elicited a transient increase in both intracellular free Ca 2+ concentration ([Ca 2+ ] i ) and tension that were both drastically reduced by caffeine and only partially reduced by the two sarco(endo)plasmic reticulum Ca 2+ -ATPase (SERCA) blockers thapsigargin and cyclopiazonic acid, despite the presence of SERCA2a and SERCA2b isoforms in the medial smooth muscle layer of the artery. After depletion of intracellular Ca 2+ stores with 10 µM NE, addition of exogenous CaCl 2 (2.5 mM) produced large and sustained increases in both [Ca 2+ ] i and contraction of the arteries provided that the agonist was continuously present. In these conditions, the responses to CaCl 2 were inhibited by the voltage-dependent Ca 2+ entry blocker nitrendipine (1 µM), the putative inhibitor of receptor-operated Ca 2+ entry SKF-96365 (30 µM), and NiCl 2 (1 mM). The inhibition produced by SKF-96365 and NiCl 2 was greater than that of nitrendipine. Also, the responses to CaCl 2 were greatly reduced or abolished in the presence of the Na + /Ca 2+ exchanger inhibitors 1,3-dimethyl-2-thiourea, 3',4'-dichlorobenzamil, MgCl 2 , and amiloride or after omission of NaCl in the medium. Also, protein kinase C inhibitors, calphostin C and staurosporine, and tyrosine kinase inhibitors, genistein and tyrphostin 23, both reduced the responses to CaCl 2 . The inhibitory effect of protein kinase C inhibitor and tyrosine kinase were additive. These results suggest that NE releases Ca 2+ from intracellular stores that are caffeine sensitive and partially sensitive to SERCA inhibitors. They indicate that in addition to Ca 2+ influx via nitrendipine-sensitive and SKF-96365-sensitive channels, Na + /Ca 2+ exchanger participates in the CaCl 2 -induced contraction produced in NE-exposed vessels. The pathway leading to Ca 2+ entry probably involves tyrosine kinase and protein kinase C. All the above mechanisms require ongoing receptor stimulation. calcium entry; intracellular calcium stores