Different inhibitory sites of action for magnesium and verapamil in the bovine adrenal medulla.

The effects of verapamil and Mg2+ on catecholamine release induced by the substitution of NaCl by sucrose in the extracellular medium were studied in the perfused bovine adrenal medulla. Verapamil (3 X 10(-4) M) totally blocked the acetylcholine-evoked release of amines, but failed to antagonize the secretory response promoted by NaCl-omission. Perfusing glands with a Locke solution containing 10 mM Mg2+, produced a 61.4 and 62.8% inhibition of catecholamine release induced by acetylcholine and by NaCl-deprivation respectively. A similar inhibitory effect of Mg2+ (10-20 mM) was obtained in glands exposed to a NaCl-free solution in the absence of extracellular Ca2+. In adrenal glands previously perfused with verapamil (3 X 10(-4) M) and then exposed to a Locke solution containing verapamil (3 X 10(-4) M) and Mg2+ (10 or 20 mM) the inhibition of the secretory responses promoted by NaCl-omission was of the same magnitude as that obtained when the glands were perfused with Mg2+ (10 or 20 mM) in the absence of verapamil. A similar lacking effect of verapamil on the blocking action of Mg2+ on catecholamine release evoked by NaCl-deprivation was observed in the absence of extracellular Ca2+. These results are compatible with the idea that in the bovine adrenal medulla verapamil and Mg2+ could have different inhibitory sites of action. Verapamil would exert its blocking effect at an extracellular site, while the inhibitory action of Mg2+ may be mediated by an intracellular site. An additional extracellular locus of action for Mg2+ at the external surface of the chromaffin cell cannot be discarded. These results also suggest that calcium channels may not be involved in the inward transport mechanism of Mg2+ to its intracellular locus of action.