The effect of ryanodine on catecholamine secretion induced by caffeine and muscarinic receptor activation was investigated in perfused adrenal glands of the guinea-pig. 2. Caffeine (40 mM) caused only a small increase in catecholamine secretion during perfusion with standard Locke solution. Caffeine-induced catecholamine secretion was markedly enhanced after removal of CaCl2 together with replacement of NaCl with sucrose. 3. In the absence of CaCl2 and NaCl, 50 microM ryanodine had no effect on the resting catecholamine secretion. Caffeine (40 mM) administered 15 min after treatment with ryanodine caused an increase in catecholamine secretion similar to that prior to application of ryanodine, but failed to have any effect thereafter. Combined application of ryanodine and caffeine also prevented catecholamine secretion induced by caffeine applied subsequently. 4. Catecholamine secretion induced by 100 microM acetylcholine (ACh) was only partially inhibited after treatment with ryanodine plus caffeine under Ca(2+)-free, Na(+)-deficient conditions. 5. Preferential influence of ryanodine on the response to caffeine was also confirmed in catecholamine secretion evoked by paired stimuli with caffeine and ACh alternately, during perfusion with either Ca(2+)-free Locke or sucrose-substituted solutions. 6. These results indicate that caffeine increases catecholamine secretion by mobilizing Ca2+ from intracellular Ca2+ stores through ryanodine-sensitive mechanisms in guinea-pig adrenal chromaffin cells. Ca2+ stores sensitive to caffeine and muscarinic receptor activation may not overlap entirely.
