CONTRIBUTION OF INTRACELLULAR AND EXTRACELLULAR CA2+ TO NORADRENALINE EXOCYTOSIS INDUCED BY OUABAIN AND MONENSIN FROM GUINEA-PIG VAS-DEFERENS

1 Contributions of intra- and extracellular Ca2+ to noradrenaline (NA) release evoked by increasing intracellular Na+ concentrations (ouabain plus monensin) from adrenergic nerves of guinea-pig vas deferens were evaluated under conditions eliminating carrier-mediated NA release (with 100 mu M cocaine). 2 Ouabain (100 mu M) plus monensin (10 mu M), unlike 100 mM KCl, produced a marked NA release which was unchanged by Ca2+-removal. 3 In normal solution but not in Ca2+-free solution, the release of NA evoked by ouabain plus monensin was reduced by adenosine, clonidine and neuropeptide Y, and by Ca2+-channel blockers such as omega-conotoxin GVIA and nifedipine. The release of NA was also decreased by cromakalim in a glibenclamide-sensitive fashion. 4 In contrast, in the absence but not in the presence of Ca2+, the drug-evoked NA release was inhibited by mitochondrial inhibitors (carbonylcyanide-m-chlorophenylhydrazone and oligomycin) and further by immobilizers of intracellular Ca2+ (TMB-8 and BAPTA-AM) and calmodulin antagonists (W-7 and trifluoperazine). 5 These findings suggest that the release of NA evoked by elevation of [Na+](i) from adrenergic nerves in the presence and absence of Ca2+ involves, in part, exocytotic processes which are triggered by depolarization-induced Calf influx and by utilization of Ca2+ from intracellular Ca2+ store sites such as mitochondria, respectively.