Effect of reserpine on Mg++-induced calcium fluxes and reactivity of the rat aorta.

The effect of reserpine on 45Ca++ fluxes and reactivity of the rat aorta in Mg++-free and Mg++ (1.2 or 3.6 mM) media to various agonists was examined to gain further insight into mechanism(s) responsible for the altered sensitivities in vascular smooth muscle. Incubation of rat aorta in Mg++-free and high Mg++ (3.6 mM) media containing ethylene diamine tetra-acetic acid (EDTA 0.03 mM), respectively, increased and decreased the sensitivity to noradrenaline (NA), 5-hydroxytryptamine (5-HT) and KCl. Mg++-free medium significantly enhanced maximal response to 5-HT whereas high Mg++ (3.6 mM) medium reduced the maximal responses to KCl and 5-HT but not to NA. Mg++-free medium had no effect on 45Ca++ uptake, while it enhanced 45Ca++ efflux from the aorta. Reserpine treatment induced supersensitivity in the aorta to KCl, but not to either NA or 5-HT in normal medium, whereas in Mg++-free medium, it induced partial contraction and inhibited both maximal responses and sensitivity to NA and 5-HT but not to KCl. Reserpine in high Mg++ (3.6 mM) medium had no effect on sensitivity and maximal responsiveness to all these agents. Reserpine reduced 45Ca++ uptake by aorta in either Mg++-free or normal Mg++ medium and it reduced 45Ca++ efflux for aorta in Mg++-free but not in normal Mg++ medium. These observations suggest that reserpine may promote membrane permeability rather nonspecifically to divalent ions. However, the presence of EDTA in the media may have partially reduced the antagonistic effects of Mg++ due to reserpine treatment in this tissue.