Catecholamine binding complex in bovine adrenal medulla: Possible involvement of calcium

Abstract

Seventy per cent of the calcium of chromaffin granules of bovine adrenal medulla was estimated to be nonexchangeable by incubation of chromaffin granules with 45Ca for 19 hr at 4°. By contrast, only 34 per cent of the calcium of a preparation from bovine adrenal medulla containing lysosomes and mitochondria, as well as chromaffin granules, was nonexchangeable. These results indicate that a relatively large amount of calcium is held in a nondiffusible form within chromaffin granules and may be part of the binding complex for catecholamines in adrenal medulla. Stimulation of bovine adrenals with acetylcholine in the presence of DOPA and 45Ca resulted in a greater accumulation of 45Ca in granule fractions than stimulation with acetylcholine in the absence of DOPA. This finding suggests that calcium is incorporated into chromaffin granules during catecholamine synthesis and supports the hypothesis that calcium is part of the catecholamine storage complex in adrenal medulla.