Role of endothelium in magnesium-induced relaxation of rat aorta.

The role of endothelium in the relaxation of rat aortic smooth muscle to raised extracellular magnesium concentration (Mg2+)o has been examined. Following contractile responses to norepinephrine (NE) or high-K+ in Mg2+-free media, cumulative increases in (Mg2+)o caused concentration-dependent relaxations in intact (+E) as well as endothelium-denuded (-E) strips. In NE-stimulated strips, Mg2+-induced relaxation was significantly greater in +E strips, whereas the reverse was the case in K+-stimulated strips. Bay K8644, a Ca2+ channel agonist, did not modify Mg2+-induced relaxation in NE-stimulated strips, but significantly attenuated the relaxation in K+-stimulated strips in the order: -E greater than +E. The results suggest that Mg2+-induced relaxation of rat aorta is associated, at least in part, with the release of an endothelium-derived relaxant factor in receptor-mediated, but not in depolarisation-dependent contractions.