Influence of endothelium on the membrane-stabilizing effect of calcium

Background : A decrease in membrane permeability to calcium ions, caused by increased extracellular calcium concentration is referred to as membrane-stabilization. There is a paucity of information on the role of vascular endothelium in the membrane-stabilizing effect of Ca 2+ ions. The goal of the present study was to examine the influence of the endothelium on the membrane-stabilizing effect of Ca 2+ ions in rabbit aortic smooth muscle. Methods : Isometric contractions of 2mm ring segments of rabbit aorta, placed in 20ml organ baths containing physiological salt solution (PSS) and bubbled with 95% O 2 , 5% CO 2 gas mixture at 37 o C and pH 7.4 were examined. The magnitude of the relaxation responses induced by increasing extracellular Ca 2+ concentration from 5.0 to 25mM in phenylephrine pre-contracted rings was taken as an indirect indicator of the membrane-stabilizing effect of Ca 2+ . The relaxation responses induced by 25mM Ca 2+ were estimated in endothelium-intact, endothelium-denuded rings as well as following exposure to 10 -6 M methylene blue. Results : In all experiments, an increase in [Ca 2+ ]o (low bicarbonate PSS) from 5.0 to 25.0mM in rings with intact endothelium resulted in relaxation responses. These relaxation responses were attenuated in endothelium-denuded rings as well as following exposure to methylene blue. Conclusion : The results show that relaxation responses induced by high Ca 2+ due to membrane stabilization is endothelium-dependent. Keywords: Calcium, rabbit aorta, Vascular smooth muscle, Membrane stabilization, Endothelium, methylene blue, Guanylate cyclase