Magnesium Withdrawal and Contraction of Arterial Smooth Muscle: Effects of EDTA,EGTA,and Divalent Cations 1

Abstract

Summary

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⁺]_o). Addition of CaEDTA potentiated such contractions more than 100%, while EGTA promoted rapid relaxation. Rapid relaxation of contractions induced by withdrawal of [Mg²⁺]_o 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 > Cd > Mg > Ni. The ability of CaEDTA to potentiate contractions produced by withdrawal of [Mg²⁺]_o, 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⁺]_o or occupy membrane sites which are exchangeable with membrane-bound Ca in certain types of arterial smooth muscle.