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 Ca²⁺-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.

The presence of Ca2+‐activated Cl− channels in the vascular smooth muscle cells (VSMCs) is well established, but their molecular identity is still controversial. Bestrophins and TMEM16 proteins are the most prominent candidates. We have previously characterized a cGMP‐dependent Ca2+‐activated Cl− current (ICl(Ca)) with unique characteristics which is present in VSMCs simultaneously with a classical ICl(Ca). We aimed to study the role of TMEM16A and bestrophin‐3 in the VSMC and downregulated these proteins in 2nd order branches of rat mesenteric arteries in vivo using siRNA. Knockdown was confirmed at mRNA and protein levels 3 days after transfection. The downregulation of bestrophin‐3 induced secondary decrease in bestrophin‐1 and ‐2. The currents were validated by patch clamp and arteries were tested in isometric myograph. In contrast to bestrophin downregulation, which resulted in suppression of the cGMP dependent ICl(Ca), TMEM16A‐siRNA suppressed both the cGMP‐dependent and the classical ICl(Ca) currents. Downregulation of TMEM16A or bestrophins significantly suppressed the amplitude of agonist‐induced rhythmic arterial contractions – vasomotion. The downregulation of bestrophins was without effect on agonist‐induced tonic contraction. In contrast, arteries downregulated for TMEM16A had suppressed contractility to agonist (noradrenaline and vasopressin) and extracellular K+ depolarization. We conclude that both bestrophins and TMEM16A are essential for vasomotion in mesenteric small arteries but only TMEM16A is involved in agonist‐induced tonic contraction. Our results suggest modulatory interactions between these proteins at the expressional and functional levels.