Abstract
A significant release of catecholamines within the heart has been observed during myocardial ischemia. Because this can be markedly inhibited by amine-uptake-blocking agents, it has been suggested that its mechanism is a carrier-mediated efflux from neurons, which is not operative under normal conditions. The present work examined this release process in chromaffin cells isolated from the bovine adrenal medulla, a model system for studying the sympathetic nervous system. Chromaffin cells in primary culture retained normal secretory responses for up to 7 days. Conditions designed to mimic ischemia, that is, anoxia or metabolic inhibition, resulted in a significant release of catecholamines. This release was shown to be independent of extracellular calcium but, in contrast to the release observed in ischemic hearts, was not inhibited by amine-uptake blockers. Electrophoresis with immunoblotting demonstrated that significant levels of the chromaffin granule protein, chromogranin A, were released during metabolic inhibition, indicative of an exocytotic mechanism. However, there was no release of the cytosolic protein, lactate dehydrogenase, indicating that there was no concomitant breakdown of the cell membrane. These results provide evidence for an exocytotic release of catecholamines mediated by the direct action of conditions of metabolic inhibition.
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