Smooth Muscle Contraction Is Regulated by Chloride Channels: Functional Evidence for TMEM16A in Porcine Coronary Arteries

Abstract

Contraction of coronary smooth muscle is influenced by ion channels that control membrane potential and, thus, Ca2+ influx. A great deal of attention has been focused on K+ channels, as their opening makes membrane potential more negative, reduces Ca2+ influx, and causes relaxation. Our objective was to investigate ion channels in coronary smooth muscle whose opening would depolarize membrane potential, increase Ca2+ influx, and promote contraction. One potential candidate for study is the Ca2+-activated Cl− channel TMEM16A, which is expressed in a variety of smooth muscle types. We tested the hypothesis that drugs which influence TMEM16A activity would alter contraction. Specifically, we tested the idea that coronary smooth muscle contraction would be enhanced by a TMEM16A activator (Eact), whereas coronary smooth muscle contraction would be attenuated by a TMEM16A inhibitor (T16Ainh-A01). We used isometric tension recording methods on epicardial coronary artery segments (3 mm length) from domestic swine to test the predictions. Contractions in response to elevated extracellular K+ (5-30 mM in 5 mM increments) were recorded before and after treatment with 5 μM Eact or 5 μM T16Ainh-01. Extracellular K+ was varied by adding potassium gluconate, rather than potassium chloride, in order to keep the Cl− concentration constant. Concentration-response curves were fit by nonlinear regression and compared with two-way repeated measures ANOVA. K+ contracted coronary artery rings with an EC50 of 19.1 ± 0.6 mM and a maximum of 11.8 ± 1.4 g. Drug vehicle (DMSO; 0.05%) had no effect on contraction (EC50 or maximum). EACT shifted K+-induced contraction to the left (17.8 ± 0.9 mM; P < 0.05), but did not affect the maximum (105 ± 3% of control). T16Ainh-A01 shifted K+-induced contraction to the right (20.4 ± 0.6 mM; P < 0.05), but did not affect the maximum (90 ± 1% of control). These data suggest that TMEM16A is expressed in the porcine coronary artery and can influence function through electromechanical coupling.