Faster kinetics of quantal catecholamine release in mouse chromaffin cells stimulated with acetylcholine, compared with other secretagogues

Adrenal chromaffin cells (CCs) have been used extensively in studies aimed at revealing the intricacies of the Ca2+‐dependent early and late steps of regulated exocytosis. They have also served as invaluable models to study the kinetics of single‐vesicle exocytotic events to infer the characteristics of opening and closing of the exocytotic fusion pore. We have here tested the hypothesis that stimulation at room temperature of CCs from mice C57BL/6 with physiological acetylcholine (ACh) and with other secretagogues (dimethylphenylpiperazinium, high K+, muscarine, histamine, caffeine), alone or in combination, could trigger amperometric spike events with different kinetics. We found that mean secretory spike events in CCs stimulated with ACh had a fast rise rate of 25 pA/ms and a rapid decay time of 6.2 ms, with a small quantal size (0.31 pC). Surprisingly, these parameters considerably differed from those found in CCs stimulated with all other secretagogues that triggered secretory responses with spike events having smaller rise rates, longer decay times and higher quantal sizes. ACh spikes were unaltered by atropine but mitochondrial protonophore carbonyl cyanide‐4‐(trifluoromethoxy)phenylhydrazone markedly slowed down the rate rise and decay time, and augmented the quantal size of mean secretory events. We conclude that the physiological neurotransmitter ACh triggers a fast and efficient exocytotic response that cannot be mimicked by other secretagogues; such response is regulated by the mitochondrial circulation of calcium ions. Both Ca2+ entry through voltage‐activated Ca2+ channels (VACCs) and Ca2+ release from the endoplasmic reticulum augment [Ca2+]c and trigger the exocytotic release of catecholamine. However, the secretagogues that stimulate Ca2+ entry through VACCs such as acetylcholine (ACh), dimethyl‐phenylpiperazinium (DMPP) or high‐K+, as well as those that promote endoplasmic reticulum Ca2+ release such as muscarine, caffeine or histamine trigger secretion with quite different fusion pore kinetics, as illustrated in the bottom prototype averaged amperometric spike for each secretagogue.