Role of calcium in the release of noradrenaline induced by sodium deprivation from the guinea-pig vas deferens

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Abstract
The ability of Na-free media containing isotonic sucrose or equimolar choline chloride to increase the spontaneous release of noradrenaline from adrenergic nerve terminals and the dependence of the increase on extracellular calcium have been examined in the isolated guinea-pig vas deferens. The increase in noradrenaline output induced by the Na deprivation was gradual in contrast to the rapid response produced by excess potassium. The release of noradrenaline induced by the Na-free solution containing sucrose was not significantly reduced by the removal of calcium with or without EGTA in the first 1 h period, but was reversibly inhibited in the second 1 h period. On the other hand, the effect of the Na-free solution containing choline was significantly but reversibly decreased by exposure to the Ca-free media without any chelating agent in both periods of incubation. Lanthanum (0.25 mM) moderately inhibited the response induced by calcium reintroduction following exposure to the Ca-free, Na-free choline medium, but magnesium (10 or 20 mM) did not. It is suggested that Na deprivation may cause an increase in Ca influx, a decrease in Ca efflux or a release of intracellulary stored calcium resulting in an increase in the level of intracellular free calcium which then stimulates the output of noradrenaline.