The effects of EGTA on vascular smooth muscle contractility in calcium-free medium

Abstract

The effect of EGTA, commonly present in Ca²⁺-free physiological saline solution, on the contractile responses induced by Ca²⁺ and phenylephrine was studied in dog mesenteric arteries and aortas of rats and rabbits. EGTA substantially enhanced the contractile responses of these vascular strips or rings to added Ca²⁺ after a prolonged preincubation period in the Ca²⁺-free medium. The maximal level of the enhanced contractile responses was independent of EGTA concentration, but the rate of the maximal responses was faster at higher EGTA concentration, presumably as a result of faster removal of intracellular Ca²⁺. Such a Ca²⁺-induced response was sensitive to the Ca²⁺ antagonist, nifedipine. EGTA present at low concentrations (50 and 400 μM) in Ca²⁺-free medium also inhibited the phenylephrine-induced contractile response more prominently for the longer preincubation periods of the vascular tissues in Ca²⁺-free medium. Our results suggest that EGTA, even when added at low concentrations to the vascular smooth muscle for a sufficiently long period in a Ca²⁺-free medium, may cause destabilization of the cell membranes leading to increased permeability to subsequently added Ca²⁺. EGTA may also remove the superficially bound Ca²⁺ and subsequently reduce the intracellular Ca²⁺ pool via extraction of the intracellular Ca²⁺ at the cell membrane surfaces.