Transmitter secretion from individual varicosities of guinea-pig and mouse vas deferens: highly intermittent and monoquantal.

A modification of "classical" electrophysiological techniques was used to characterise the secretory activity of individual release sites of the sympathetic nerves of guinea-pig and mouse vas deferens. The rising phases of the intracellularly recorded excitatory junction potentials of a smooth muscle cell, were electrically differentiated, and fluctuations in the rate of rise recorded as phasic peaks (in the dV/dt of excitatory junction potentials), termed "discrete events". Experimental factors which may influence discrete events were examined systematically, and criteria established to recognize a discrete event as the "image" of transmitter secreted from a particular release site. To determine the quantal content of a stimulus-evoked discrete event, it was compared with discrete events occurring spontaneously in the same cell. Furthermore, the frequency dependence of the probability of occurrence of discrete events was compared with that of the evoked fractional secretion of tritium-labelled noradrenaline, to find out if release sites from which noradrenaline is secreted share the characteristics of those secreting the (still unknown) transmitter causing discrete events. The results obtained permit the following tentative conclusions: Both in guinea-pig and mouse vas deferens stimulus-induced secretion of transmitter from a single varicosity of the sympathetic nerves is highly "intermittent". Transmitter secretion from a varicosity is basically monoquantal. Spontaneous secretion of transmitter quanta occurs from "random" sites, but a nerve impulse causes secretion of a quantum from a "preferred site" of a varicosity: during a stimulus train quanta are secreted in "complementary pairs". Secretion of the first quantum in a "pair" does not lead to "autoinhibition" of the site from which it was released, but induces a short-lasting facilitation. (5) Some characteristic features of the secretion of the neurotransmitter causing discrete events, seem to apply to the secretion of noradrenaline from noradrenergic nerves also.