Importance of Tissue Cholesterol and Lipid Rafts in the Sphingosylphosphorylcholine-induced Ca²⁺ sensitization of Vascular Smooth Muscle Contraction

Abstract

Rho-kinase (ROK)-mediated Ca²⁺ sensitization of vascular smooth muscle (VSM) contraction plays a pivotal role in the pathophysiology of abnormal VSM contraction such as vasospasm. As a novel upstream mediator of the ROK-mediated Ca²⁺ sensitization, we previously identified sphingosylphosphorylcholine (SPC). SPC induced sustained contraction without any change of cytosolic Ca²⁺ concentrations, which was blocked by inhibitors of Src-TK and ROK, suggesting the involvement of these kinases. A significant linkage was observed between the extent of SPC-induced contraction and serum total cholesterol level in both human and rabbits. However, possible importance of tissue cholesterol and cholesterol-enriched membrane microdomains, lipid rafts in SPC-induced Ca²⁺ sensitization has been unknown. In this study, putative links between three parameters (tissue cholesterol, lipid rafts, and SPC-induced Ca²⁺ sensitization) were explored in the same samples of porcine coronary VSM strips. Sucrose density gradient fractionation analysis showed the presence of two peaks of caveolin-1 content in the fractions, only lighter one of which depended on cholesterol and Ca²⁺ sensitization. Our findings in this report are the first to demonstrate the strong correlations between the three parameters. The data raise the possibility that tissue cholesterol and lipid rafts play important roles in the SPC-induced Ca²⁺ sensitization of VSM.