Abstract
Isolated superfused guinea-pig vas deferens, in which the neuronal stores of noradrenaline (NA) were labelled by preincubation with 3H-(−)-NA, was used to study the relative importance of calcium and of cyclic AMP (cAMP) for the regulation of 3H-NA secretion from the nerves, evoked by electrical field stimulation. Secretion of 3H-NA was found to be completely dependent on the presence of calcium in the medium, even when drugs (theophylline or dibuturyl-cAMP) were added to elevate intraneuronal cAMP levels. Furthermore, the same drugs only marginally, if at all, enhanced 3H-NA secretion at 0.5 or 1.8 mM calcium in the medium. The results thus do not support the concept that the secretory mechanism is to any important extent dependent on the intraneuronal levels of cAMP. Moreover, the potency of prostaglandin E 2, which is known to alter intracellular cAMP levels in many tissues, as inhibitor of 3H-NA secretion, was found to be identical in the presence or absence of theophylline or dibuturyl-cAMP. Evidence is quoted that the inhibitory effect of PGE 2 on 3H-NA secretion, rather than being related to change in intraneuronal cAMP levels, may be due to interference with the availability of external calcium for the secretory mechanism.
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