Cholesterol Depletion Disrupts Caveolae and Differentially Impairs Agonist-Induced Arterial Contraction

Abstract

Objective— This study assessed the role of cholesterol-rich membrane regions, including caveolae, in the regulation of arterial contractility. Methods and Results— Rat tail artery devoid of endothelium was treated with the cholesterol acceptor methyl-β-cyclodextrin, and the effects on force and Ca ²⁺ handling were evaluated. In cholesterol-depleted preparations, the force responses to α ₁ -adrenergic receptors, membrane depolarization, inhibition of myosin light chain phosphatase, and activation of G proteins with a mixture of 20 mmol/L NaF and 60 μmol/L AlCl ₃ were unaffected. In contrast, responses to 5-hydroxytryptamine (5-HT), vasopressin, and endothelin were reduced by >50%. The rise in global intracellular free Ca ²⁺ concentration in response to 5-HT was attenuated, as was the generation of Ca ²⁺ waves at the cellular level. By electron microscopy, cholesterol depletion was found to disrupt caveolae. The 5-HT response could be restored by exogenous cholesterol, which also restored caveolae. Western blots showed that the levels of 5-HT _2A receptor and of caveolin-1 were unaffected by cholesterol extraction. Sucrose gradient centrifugation showed enrichment of 5-HT _2A receptors, but not α ₁ -adrenergic receptors, in the caveolin-1–containing fractions, suggesting localization of the former to caveolae. Conclusions— These results show that a subset of signaling pathways that regulate smooth muscle contraction depends specifically on cholesterol. Furthermore, the cholesterol-dependent step in serotonergic signaling occurs early in the pathway and depends on the integrity of caveolae.