Recent electrophysiological evidence from studies of sympathetic nerves in the guinea-pig vas deferens [1, 2] indicate that the transmitter content of a vesicle is the quantum secreted, that nerve impulses only intermittently evoke transmitter secretion from the average individual varicosity (P about 0.01) (cf. [3]), that transmitter secretion from a varicosity is monoquantal, and that release of both quanta of a ‘pair’ is followed by long secretory silence in that site. The sympathetic transmitter responsible for these effects is apparently not noradrenaline but adenosine 5'-trisphosphate (ATP) [4], presumably secreted as a co-transmitter with noradrenaline [4a, 4b]. If both transmitters are secreted in parallel, then on the evidence quoted above the maximal rate of secretion would be that at which every nerve impulse releases the noradrenaline contents of one vesicle from each varicosity. In the present study of the overflow of [3H]noradrenaline, selected pharmacological agents are used to examine this possibility. The results are interpreted in the light of the above-mentioned electrophysiological evidence, in an attempt to evaluate the scope and mechanisms of α-adrenoceptor-mediated control of stimulus-secretion coupling in the average individual varicosity of the sympathetic nerves of vas deferens.

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