ATP-activated exchange of catecholamines by isolated intact chromaffin granules

Isolated chromaffin granules were found to accumulate exogenous (R)-[³H]-norepinephrine at 37 C in the presence of Mg²⁺-ATP by a process involving one-to-one exchange for endogenous epinephrine. By contrast, omission of Mg²⁺-ATP from the incubation medium resulted in a substantial unidirectional efflux of the endogenous epinephrine independent of external catecholamine concentrations. However, the effects of ATP on the spontaneous efflux and the exchange process were found to occur with different kinetic parameters and differential sensitivity to a stilbene disulfonate inhibitor of the granule ATPase, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. These data lead us to conclude that ATP catalyzes bidirectional catecholamine flux across the granule membrane (not net accumulation) by an ATPase-dependent mechanism. In addition, ATP has the ability to suppress unidirectional efflux of catecholamines from granules by a mechanism independent of the ATPase or the exchange mechanism. The data also allow us to distinguish among three possible models for how these processes might be occurring.