Extracellular Ionic Composition Alters Kinetics of Vesicular Release ofCatecholamines and Quantal Size During Exocytosis at Adrenal Medullary Cells

摘要

The temporal resolution of carbon-fiber microelectrodes has been exploited toexamine the plasticity of quantal secretory events at individual adrenal medullary cells. The size of individual quantal events monitored by amperometric oxidation of released catecholamines, was found to be dependent on the extracellular ionic composition, the secretagogue, and the order of depolarization delivery. Release was observed with either exposure to 60 mM K(+) in the presence of Ca2(+) or exposure to 3 mM Ba2(+) in the solutions of different pH, and without external Ca2(+). Ba2(+) was demonstrated to induce Ca2(+) -independent exocytotic release for an extended period of time (> 4 min) relative to release induced by K(+) (approx. 30 s), which is Ca2(+) -dependent. In all cases, simultaneous changes of intracellular divalent cations, monitored by fura-2 fluorescence, accompanied quantal release and had a similar time course. Exocytosis caused by Ba2(+) in Ca2(+) -free medium had a large mean spike area at pH 8.2 than at pH 7.4. When Ba2+ -induced spikes measured at pH 7.4 were compared, the spikes in Ca2(+) -free medium were found to be broader and shorter, but had the same area. Release induced by K(+) after exposure to Ba2(+) was comprised of larger quantal events when compared to preceding K(+) stimulations. Finally, spikes obtained with Ba2(+) exposure at an extracellular pH of 5.5 had a different shape than those obtained in more basic solutions. These changes in spike size and shape are consistent with the interactions between catecholamines and other intravesicular components.