Magnesium withdrawal and contraction of arterial smooth muscle: effects of EDTA, EGTA, and divalent cations.

The divalent cation chelators, CaEDTA and EGTA, were demonstrated to exert opposite effects on contractions of rat aortic smooth muscle induced by withdrawal of external magnesium ([Mg2+]0). Addition of CaEDTA potentiated such contractions more than 100%, while EGTA promoted rapid relaxation. Rapid relaxation of contractions induced by withdrawal of [Mg2+]0 could also be induced by Mn, Ni, and Cd but not Sr. Using EC50's, a relative descending order of contractile inhibition was noted for the divalent cations: Mn greater than Cd greater than Mg greater than Ni. The ability of CaEDTA to potentiate contractions produced by withdrawal of [Mg2+]0, as well as the ability of divalent cations to relax contractions of aortic smooth muscle, appears to be related to actions on the transmembrane flux of Ca2+. These findings thus lend support to the view that Mg ions either play an important role in regulating membrane permeability to [Ca2+]0 or occupy membrane sites which are exchangeable with membrane-bound Ca in certain types of aterial smooth muscle.