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

1 Acetylcholine (ACh) evoked secretion by the calcium ionophore, ionom ycin, was studied at frog motor nerve endings. 2 Bath application of i onomycin stimulated an irreversible increase in the rate of spontaneou s, quantal ACh release in the presence of extracellular Ca2+. In contr ast, local application of ionomycin stimulated a rapid, reversible acc eleration of spontaneous ACh release. 3 The magnitude of the secretory response to ionomycin was dependent both upon the concentration of io nophore and the concentration of extracellular Ca2+. 4 Adenosine or 2- chloroadenosine inhibited ionomycin-stimulated ACh release with the sa me potency and efficacy observed previously for these adenosine analog ues as inhibitors of ACh secretion evoked by nerve impulses. 5 These r esults support the conclusion that adenosine receptor activation inhib its quantal ACh secretion at a site distal to that of Ca2+ entry at fr og motor nerve endings.