Ion channels and electrical activity in vascular smooth muscle: Effects of calcium store depletion, cholesterol and cholesterol-lowering drugs

Abstract: Smooth muscle cells (SMCs) in the vessel wall contract to regulate blood flow and pressure, a function essential for normal circulation through tissues. On the other hand, migration and proliferation of SMCs are important factors in vascular disease, such as atherosclerosis and restenosis following surgical dilatation. The conversion of the SMC to this role is marked by a modulation from contractile to synthetic phenotype, with decreased contents of contractile/cytoskeletal proteins, increased synthesis of extracellular matrix proteins and capacity to enter the cell cycle and proliferate in response to mitogenic stimuli. Phenotype regulation involves influences from the extracellular matrix, cell-cell interactions, and intrinsic factors, such as the activity of receptors and ion channels in the cell membrane. Serum-free organ culture of whole blood vessels preserves the tissue environment of the SMCs, and at the same time initiates processes similar to those in vascular lesions, allowing study of early phenotype modulation before proliferation is initiated. In rat cerebral arteries, organ culture causes decreased Ca2+ influx through voltage-dependent L-type channels, while Ca2+ entry upon depletion of intracellular Ca2+ stores, mediated by store-operated channels (SOCs) and encoded by member(s) of the transient receptor potential (TRP) gene family, including TRPC1, increases. Similar plasticity in TRP expression is seen in human internal mammary arteries cultured after balloon-dilatation, suggesting TRP increase to be an early component in phenotype modulation, potentially representing a target for prevention and therapy. Among the vascular actions of hypercholesterolemia is increased sensitivity of arterial smooth muscle to the vasoconstrictor endothelin-1 (ET-1), of possible importance for vascular disease. To elucidate the signaling steps involved, we extracted cholesterol from endothelium-denuded rat arteries using methyl-ƒÒ-cyclodextrin (mƒÒcd). This selectively attenuated one component of the response to ET-1 that was found to be associated with SOC activity, mediated by TRPC1. Conversely, cholesterol loading increased this component, suggesting that association of ET-1 receptors and TRPC1 in cholesterol-rich membrane caveolae is important for the coupling between receptor and channel activation. Cholesterol-lowering statins inhibited vascular contractions by a mechanism not linked to cholesterol or isoprenoid metabolism. Whole-cell patch clamp recordings suggested lipophilicity to be an important factor behind these effects of statins on vascular tone, with L-type Ca2+ channel inhibition as a likely mechanism of action.