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

Purpose: Contraction of coronary smooth muscle is influenced by ion channels controlling membrane potential (Em) and Ca2+ influx. A great deal of attention has been focused on K+ channels, as their opening makes Em more negative, reduces Ca2+, and causes relaxation. We investigated ion channels whose opening would depolarize Em, increase Ca2+, and promote contraction. A candidate for study is TMEM16A, a Ca2+-activated Cl- channel expressed in a variety of smooth muscles. We tested the hypothesis that drugs which influence TMEM16A would alter contraction. We predicted that contraction would be enhanced by a TMEM16A activator (Eact), whereas it would be attenuated by a TMEM16A inhibitor (T16Ainh-A01). Methods: We used isometric tension recording methods on epicardial coronary artery segments from domestic swine. Contractions to K+ were recorded before and after treatment with 5 µM Eact or 5 µM T16Ainh-01. Extracellular K+ was varied by adding K-gluconate, rather than KCl, to keep Cl- constant. Results: K+ contracted rings with an EC50 of 19.1 ± 0.6 mM and a maximum of 11.8 ± 1.4 g. Drug vehicle had no effect on EC50 or maximum. Eact shifted 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 contraction to the right (20.4 ± 0.6 mM; P < 0.05) but did not affect the maximum (90 ± 1% of control). Conclusions: These data suggest that TMEM16A is expressed in porcine coronary arteries and influences electromechanical coupling.