Inhibitory effects of caffeine on secretagogue-induced catecholamine secretion from adrenal chromaffin cells of the guinea-pig.

1. The inhibitory action of caffeine on catecholamine secretion induced by secretagogues was investigated in perfused adrenal glands and dispersed chromaffin cells of the guinea-pig. 2. Caffeine (10 mM) caused a reversible inhibition of catecholamine secretion evoked by acetylcholine (ACh, 50 microM), KCl (56 mM, high K+) and veratridine (100 microM) and that induced by muscarinic receptor activation in the absence of extracellular Ca2+ in perfused adrenal glands. 3. In dispersed chromaffin cells, caffeine caused a dose-dependent inhibition of the secretory responses to 100 microM ACh and veratridine. Forskolin (30 microM), dibutyryl cyclic AMP (1 mM) and 8-bromo cyclic AMP (1 mM) did not mimic the action of caffeine. 4. In the voltage-clamp, whole-cell recording mode (at a holding potential of -60 mV or -70 mV), ACh (100 microM) evoked an inward current, and depolarizing pulses elicited inward Na+, Ca2+ and outward K+ currents. All these responses were partially inhibited by caffeine (20 mM). 5. ACh rapidly increased the intracellular concentration of Ca2+ ([Ca2+]i) in fura-2-loaded cells in either the presence or the absence of external Ca2+, though its magnitude was decreased by about 50% in Ca(2+)-free conditions. Caffeine (20 mM) inhibited these ACh-induced increases in [Ca2+]i. 6. In permeabilized chromaffin cells, caffeine (20 mM) caused an inhibition of catecholamine secretion evoked by Ca2+ (10 microM). 7. These results suggest that caffeine inhibits evoked catecholamine secretion through mechanisms such as the blockade of voltage-dependent Na+ and Ca2+ currents and ACh receptor current, and reduction of the release of intracellularly stored Ca2+ and/or Ca(2+)-sensitivity of the secretory apparatus.