Influence of endothelium on the membrane-stabilizing effect of calcium

Abstract

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 Ca²⁺ ions. The goal of the present study was to examine the influence of the endothelium on the membrane-stabilizing effect of Ca²⁺ 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% O₂, 5% CO₂ gas mixture at 37^oC and pH 7.4 were examined. The magnitude of the relaxation responses induced by increasing extracellular Ca²⁺ concentration from 5.0 to 25mM in phenylephrine pre-contracted rings was taken as an indirect indicator of the membrane-stabilizing effect of Ca²⁺. The relaxation responses induced by 25mM Ca²⁺ were estimated in endothelium-intact, endothelium-denuded rings as well as following exposure to 10^-6M methylene blue.
Results: In all experiments, an increase in [Ca²⁺]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 Ca²⁺ due to membrane stabilization is endothelium-dependent.

Keywords: Calcium, rabbit aorta, Vascular smooth muscle, Membrane stabilization,
Endothelium, methylene blue, Guanylate cyclase