Responses of the Transmembrane Potential Coupled to the ATP-evoked Catecholamine Release in Isolated Chromaffin Granules

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Using suspensions of chromaffin granules isolated from bovine adreno-medullae, the effects of Mg²⁺·ATP on membrane potential and catecholamine release across membranes of granules were investigated. The release of catecholamine was monitored by the measurement of changes in endogenous fluorescence of the amines, while the fluorometric method with 3, 3'-dipropylthiadicarbocyanine iodide (diS-C₃-(5)) was applied to the measurement of membrane potential changes. The catecholamine release brought about by Mg²⁺·ATP was found to obey Michaelis-Menten kinetics with K_m-value for Mg²⁺ of about 0.15 mM and to be inhibited by dicyclohexylcarbodiimide (DCCD), an inhibitor of membrane-bound H⁺-ATPase. Concomitantly with such an amine release, depolarization of the granule membrane was observed to occur. The magnitude of this depolarization closely paralleled with the rate of the amine release and followed a saturation kinetics of Michaelis-Menten type with K_m for Mg²⁺ of 0.21 mM and for ATP of 0.22 mM. Moreover, such a depolarization was competitively suppressed by DCCD. Based on these results, the role of the transmembrane potential change of granules in the catecholamine release was discussed.