Transport and function of chloride in vascular smooth muscles.

This review summarizes the current knowledge of Cl(-) transport in vascular smooth muscle cells (VSMCs). VSMCs accumulate Cl(-) intracellularly using two secondary-active transport mechanisms. The Cl(-) equilibrium potential is more positive than the resting membrane potential enabling Cl(-) to be a depolarizing ion upon activation of a Cl(-) conductance. Cl(-) currents are involved in different vascular responses suggesting a number of different Cl(-) channels. All known Cl(-) channel families, with the exception of the GABA-/glycine-receptor family, have been identified in VSMCs. At least one member of the voltage-activated ClC family - ClC-3 - has been suggested to be involved in myogenic constriction, in cell proliferation and to have an anti-apoptotic action. The cystic fibrosis transmembrane conductance regulator is also demonstrated in VSMCs. The molecular identity of the major anion conductance in VSMCs - a Ca(2+)-activated Cl(-) current - is uncertain. Several candidates have been suggested with bestrophin and TMEM16 protein families the current favorites. Specific pharmacological tools are lacking for Cl(-) channels but recent molecular biology developments have made selective gene manipulations possible. A continuing quest within the vascular research field is to explicitly demonstrate the coupling between a putative channel protein and an endogenous Cl(-) current and the importance of these for specific functions.

Bestrophin is important for the rhythmic but not the tonic contraction in rat mesenteric small arteries.

AIMS: We have previously characterized a cGMP-dependent Ca(2+)-activated Cl(-) current in vascular smooth muscle cells (SMCs) and have shown its dependence on bestrophin-3 expression. We hypothesize that this current is important for synchronization of SMCs in the vascular wall. In the present study, we aimed to test this hypothesis by transfecting rat mesenteric small arteries in vivo with siRNA specifically targeting bestrophin-3. METHODS AND RESULTS: The arteries were tested 3 days after transfection in vitro for isometric force development and for intracellular Ca(2+) in SMCs. Bestrophin-3 expression was significantly reduced compared with arteries transfected with mutated siRNA. mRNA levels for bestrophin-1 and -2 were also significantly reduced by bestrophin-3 down-regulation. This is suggested to be secondary to specific bestrophin-3 down-regulation since siRNAs targeting different exons of the bestrophin-3 gene had identical effects on bestrophin-1 and -2 expression. The transfection affected neither the maximal contractile response nor the sensitivity to norepinephrine and arginine-vasopressin. The amplitude of agonist-induced vasomotion was significantly reduced in arteries down-regulated for bestrophins compared with controls, and asynchronous Ca(2+) waves appeared in the SMCs. The average frequency of vasomotion was not different. 8Br-cGMP restored vasomotion in arteries where the endothelium was removed, but oscillation amplitude was still significantly less in bestrophin-down-regulated arteries. Thus, vasomotion properties were consistent with those previously characterized for rat mesenteric small arteries. Data from our mathematical model are consistent with the experimental results. CONCLUSION: This study demonstrates the importance of bestrophins for synchronization of SMCs and strongly supports our hypothesis for generation of vasomotion.