Evoked secretion of [³H]noradrenaline and ATP from nerve varicosities isolated from the myenteric plexus of the guinea pig ileum

Neuronal varicosities, isolated from the myenteric plexus of guinea pig ileum longitudinal muscle, were incubated with [³H]noradrenaline to label the contents of the noradrenergic secretory vesicles. Exposure of these varicosities to KCl, nicotine, or acetylcholine resulted in the Ca²⁺-dependent release of [³H]noradrenaline. Veratridine also evoked a large efflux of [³H] from this preparation, but this release was only partially Ca²⁺ dependent. The α₂-adrenoceptor agonist, clonidine, inhibited the K⁺, nicotine-, and acetylcholine-induced release of [³H]noradrenaline. Similarly, exogenously administered (-)noradrenaline was an effective inhibitor of the K⁺-evoked release of [³H]noradrenaline. The α₂-adrenoceptor antagonist, yohimbine, antagonized the inhibitory actions of both clonidine and (-)noradrenaline on the K⁺-evoked release of [³H]noradrenaline from myenteric varicosities. Nicotine, acetylcholine, KCl, and veratridine also released ATP from these guinea pig ileal myenteric varicosities. However, the evoked release of ATP was unaffected by clonidine. These results indicate that myenteric varicosities can take up and release [³H]noradrenaline and that they possess presynaptic α₂-adrenoceptors which, when activated, inhibit the release of [³H]noradrenaline. These receptors may play a role in modulating the release of noradrenaline in the myenteric plexus in vivo. In addition, the present results suggest that ATP and [³H]noradrenaline may not be released from the same population of secretory vesicles in neuronal varicosities isolated from guinea pig ileum longitudinal muscle.