Calcium movements during the release of catecholamines from the adrenal medulla: effects of methoxyverapamil and external cations

Abstract

Cortex-free adrenal glands previously labeled with the isotope 45Ca were perfused with Locke or modified Locke solution to assess Ca2+ movements under different conditions. Substitution of Na+ by either sucrose or choline during perfusion with Ca2+-free Locke solution induced a significant and sustained decrease in the 45Ca efflux. Concomitant with this effect there was an increase in the output of catecholamines from the perfused gland. In the presence of Ca2+ (2.2 mM) in the perfusion fluid, Na+ omission induced an increase in the 45Ca efflux. This increase was significantly reduced if 3 .times. 10-4 M methoxyverapamil (D-600) was present in the perfusion fluid. The increased catecholamine output in response to Na+ deprivation remained unchanged. Excess of Mg2+ (20 mM) in the extracellular medium blocked the increase in catecholamine output in response to Na+ omission. The decrease in the 45Ca efflux produced by Na+ deprivation in the presence of this high concentration of Mg2+ was similar to that observed in the presence of 1.2 mM-Mg2+. In the absence of Mg2+ in the extracellular medium, substitution of Na+ by either sucrose or choline induced a sharp and transient increase in the 45Ca efflux rate coefficient. This increased 45Ca efflux, which has similar time course as the enhanced catecholamine output, was not affected by the presence of 3 .times. 10-4 M methoxyverapamil. In the absence of Mg2+, the graded substitution of Na+ in the perfusion medium by sucrose enhanced the efflux of 45Ca. This incrase in the 45Ca outward movement was linearly related to the logarithm of the extracellular Na+ concentration. After perfusion of glands with Ca2+-free Locke solution, the re-introduction of Ca2+ (2.2 mM) into the perfusion fluid produced an increase in the 45Ca efflux. This was accompanied by a discharge of catecholamines. Although Mg2+ (20 mM) was effective in blocking catecholamine release, this diavalent cation did not modify the increase in the 45Ca efflux produced by Ca2+ re-introduction. In contrast to these later observations, methoxyverapamil (3 .times. 10-4 M) was effective in inhibiting both increases in catecholamine output and 45Ca efflux in response to Ca2+ re-introduction. Ca2+ movements in the adrenal medulla may involve both Na+-Ca2+ and Ca2+-Ca2+ exchange mechanisms. The omission of Na+ from the extracellular environment produces not only an increase in the output of catecholamines but it may increase the intracellular levels of Ca2+ and this may result in an increased Ca2+ efflux when Mg2+ is omitted from the perfusion fluids. The competition between Ca2+ and Mg2+ during the secretory process may involve an intracellular site.