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

Abstract: The temporal resolution of carbon-fiber microelectrodes has been exploited to examine 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 m M K⁺ in the presence of Ca²⁺ or exposure to 3 m M Ba²⁺ in solutions of different pH, with 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⁺ (～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 larger 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 with 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.