Abstract-The reactivity of the vascular wall to endothelin-1 (ET-1) is influenced by cholesterol, which is of possible importance for the progression of atherosclerosis. To elucidate signaling steps affected, the cholesterol acceptor methyl-␤-cyclodextrin (m␤cd, 10 mmol/L) was used to manipulate membrane cholesterol and disrupt caveolae in intact rat arteries. In endothelium-denuded caudal artery, contractile responsiveness to 10 nmol/L ET-1 (mediated by the ET A receptor) was reduced by m␤cd and increased by cholesterol. Neither ligand binding nor colocalization of ET A and caveolin-1 was affected by m␤cd. Ca 2ϩ inflow via store-operated channels after depletion of intracellular Ca 2ϩ stores was reduced in m␤cd-treated caudal arteries, as shown by Mn 2ϩ quench rate and intracellular [Ca 2ϩ ] response. Expression of TRPC1, 3, and 6 was detected by reverse transcriptase-polymerase chain reaction, and colocalization of TRPC1 with caveolin-1 was reduced by m␤cd, as seen by immunofluorescence. Part of the contractile response to ET-1 was inhibited by Ni 2ϩ (0.5 mmol/L) and by a TRPC1 blocking antibody. In the basilar artery, exhibiting less store-operated channel activity than the caudal artery, ET-1-induced contractions were insensitive to the TRPC1 blocking antibody and to m␤cd. Increased store-operated channel activity in basilar arteries after organ culture correlated with increased sensitivity of ET-1 contraction to m␤cd. These results suggest that cholesterol influences vascular reactivity to ET-1 by affecting the caveolar localization of TRPC1. Key Words: arterial smooth muscle Ⅲ methyl-␤-cyclodextrin Ⅲ caveolae Ⅲ endothelin Ⅲ store-operated Ca 2ϩ channels H ypercholesterolemia increases reactivity to endothelin-1 (ET-1) in experimental animals and humans. [1][2][3][4] This has been pointed out as one possible factor in the progression of atherosclerosis. [5][6][7] The mechanism of action has not been elucidated, although both endothelial dysfunction and altered smooth muscle reactivity have been proposed. 5 Lipoprotein particles may directly influence endothelial membrane-associated endothelial NO synthase activity by interfering with cholesterol-rich domains referred to as caveolae. 8 Although these effects modulate the endothelial influence on vascular tone, less is known regarding direct effects of cholesterol on vascular smooth muscle functions.Caveolae are 50-to 100-nm membrane invaginations that integrate many cellular receptor functions. 9 For instance, ET A receptors expressed in COS cells colocalize with the caveolae-associated protein caveolin. 10,11 The caveolar structure is disrupted after depletion of cholesterol with cyclodextrins, 12 and this correlates with a decreased contractility to ET-1, but not to depolarization or ␣ 1 -receptor stimulation, in endothelium-denuded rat caudal arteries. 13 Cholesterol might thus modulate the strength of caveolae-associated signaling, providing a basis for altered contractility in response to ET-1.Activation of the ET A receptor stimulates Ca 2ϩ inflow ov...

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-␤-cyclodextrin, and the effects on force and Ca 2ϩ handling were evaluated. In cholesterol-depleted preparations, the force responses to ␣ 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 mol/L AlCl 3 were unaffected. In contrast, responses to 5-hydroxytryptamine (5-HT), vasopressin, and endothelin were reduced by Ͼ50%. The rise in global intracellular free Ca 2ϩ concentration in response to 5-HT was attenuated, as was the generation of Ca 2ϩ 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 ␣ 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. Key Words: smooth muscle Ⅲ caveolae Ⅲ 5-hydroxytryptamine Ⅲ endothelin Ⅲ intracellular calcium C ellular cholesterol, of which most (up to 90%) 1 resides in the plasma membrane, is crucial for normal membrane permeability and fluidity and also plays a role in cellular signaling, via several proposed mechanisms that fall into at least 4 categories. First, cellular cholesterol may influence gene transcription in the nucleus through sterol regulatory element binding proteins. 2 Second, the activity of membrane receptors, ion channels, and transporters may depend on the membrane fluidity, per se. 3 Third, membrane protein function may be regulated through specific cholesterol-protein interactions. 3,4 Fourth, cholesterol stabilizes the structure of caveolae and lipid rafts.Caveolae, which are 50-to 100-nm membrane invaginations that are abundant in vascular endothelium and smooth muscle cells, are defined by their characteristic morphology and contents of caveolin and cav-p60. 5,6 No definitive definition of rafts has appeared because they do not exhibit a characteristic structure, but the term is used for planar aggregations of specific lipids and proteins. Caveolae and lipid rafts are envisaged to serve as platforms for a dynamic association of signaling proteins and for the initiation or modulation of signaling. 5,7,8 Some agonists causing contraction of vascular smooth muscle act on receptors that are believed to be located in caveo...

Ca2+ sensitization of smooth muscle contraction involves the small GTPase RhoA, inhibition of myosin light chain phosphatase (MLCP) and enhanced myosin regulatory light chain (LC20) phosphorylation. A potential effector of RhoA is Rho‐associated kinase (ROK). The role of ROK in Ca2+ sensitization was investigated in guinea‐pig ileum. Contraction of permeabilized muscle strips induced by GTPγS at pCa 6.5 was inhibited by the kinase inhibitors Y‐27632, HA1077 and H‐7 with IC50 values that correlated with the known Ki values for inhibition of ROK. GTPγS also increased LC20 phosphorylation and this was prevented by HA1077. Contraction and LC20 phosphorylation elicited at pCa 5.75 were, however, unaffected by HA1077. Pre‐treatment of intact tissue strips with HA1077 abolished the tonic component of carbachol‐induced contraction and the sustained elevation of LC20 phosphorylation, but had no effect on the transient or sustained increase in [Ca2+]i induced by carbachol. LC20 phosphorylation and contraction dynamics suggest that the ROK‐mediated increase in LC20 phosphorylation is due to MLCP inhibition, not myosin light chain kinase activation. In the absence of Ca2+, GTPγS stimulated 35S incorporation from [35S]ATPγS into the myosin targeting subunit of MLCP (MYPT). The enhanced thiophosphorylation was inhibited by HA1077. No thiophosphorylation of LC20 was detected. These results indicate that ROK mediates agonist‐induced increases in myosin phosphorylation and force by inhibiting MLCP activity through phosphorylation of MYPT. Under Ca2+‐free conditions, ROK does not appear to phosphorylate LC20in situ, in contrast to its ability to phosphorylate myosin in vitro. In particular, ROK activation is essential for the tonic phase of agonist‐induced contraction.