Calcium and Stimulus-secretion Coupling in the Adrenal Medulla: Contrasting Stimulating Effects of the Ionophores X-537A and A23187 on Catecholamine Output

1. The ionophores X-537A and A23187, which are known to transfer calcuim across cell membranes, stimulated catecholamine release from perfused cat adrenal glands. 2. These stimulant effects persisted in the presence of hexamethonium and atropine and are therefore attributable to direct actions of the ionophores on the adrenal chromaffin cells. 3. Perfusion with calcium-free Locke abolished responses to A23187 and reduced those to X-537A. 4. Responses to X-537A were consistently large and comparable with those produced by 56 mM potassium. By contrast, responses to A23177, over the wide range of concentrations tested, were variable and much smaller. 5. That the two ionophores can stimulate through calcium-dependent mechanisms is considered fresh support for the calcium hypothesis of stimulus-secretion coupling. That they differ in effectiveness may mean that factors besides calcium are important. The greater potency of the less specific ionophore, X-537A, may be attributable to its ability to depolarize as well as carry calcuim, while the relatively small effects of A23187, a generally more effective ionophore for calcuim, may indicate that inward movement of calcium without a background of membrane perturbation such as may be produced by depolarization, is insufficient to elicit strong secretory responses.