Intracellular Ca2+ antagonist TMB-8 blocks catecholamine secretion evoked by caffeine and acetylcholine from perfused cat adrenal glands in the absence of extracellular Ca2+.

Unlike acetylcholine, caffeine was much more effective in releasing catecholamine in the absence of extracellular Ca2+ than in its presence in perfused cat adrenal glands. The intracellular Ca2+ antagonist, TMB-8 (10(-4) M), inhibited reversibly the catecholamine secretion evoked by caffeine (40 mM) and that induced by acetylcholine (10(-4) M) in the presence of hexamethonium (10(-3) M) during perfusion with Ca2+-free Locke solution containing EGTA (10(-5) M). These results support our view that muscarinic receptor activation causes catecholamine secretion by mobilizing Ca2+ from an intracellular pool just as caffeine does.