IONOMYCIN-INDUCED ACETYLCHOLINE-RELEASE AND ITS INHIBITION BY ADENOSINE AT FROG MOTOR-NERVE ENDINGS

1 Acetylcholine (ACh) evoked secretion by the calcium ionophore, ionomycin, was studied at frog motor nerve endings. 2 Bath application of ionomycin stimulated an irreversible increase in the rate of spontaneous, quantal ACh release in the presence of extracellular Ca2+. In contrast, local application of ionomycin stimulated a rapid, reversible acceleration of spontaneous ACh release. 3 The magnitude of the secretory response to ionomycin was dependent both upon the concentration of ionophore and the concentration of extracellular Ca2+. 4 Adenosine or 2-chloroadenosine inhibited ionomycin-stimulated ACh release with the same potency and efficacy observed previously for these adenosine analogues as inhibitors of ACh secretion evoked by nerve impulses. 5 These results support the conclusion that adenosine receptor activation inhibits quantal ACh secretion at a site distal to that of Ca2+ entry at frog motor nerve endings.