Transmitter release from the cytoplasm is of physiological importance but not subject to presynaptic modulation

Ca2+-dependent release of [3H] noradrenaline ([3H] NA) evoked by electrical stimulation of the isolated mouse vas deferens was subject to negative feedback modulation by idazoxan an alpha 2-adrenoceptor blocking agent. Both the resting release and that evoked by 1-phenylephrine proved to be Ca0-independent and unaffected by idazoxan. Ouabain-evoked release of [3H] acetylcholine from the myenteric plexus of ileal longitudinal muscle strips in the presence of eserine was not affected by atropine, but that evoked by electrical stimulation was enhanced. Since the release of NA or ACh by 1-phenylephrine and ouabain respectively is mainly of cytoplasmic origin, it is concluded that the release of transmitter from the cytoplasm is not subject to negative feedback modulation.