Chloride ion currents contribute functionally to norepinephrine-induced vascular contraction

Abstract

Norepinephrine (NE) increases Cl− efflux from vascular smooth muscle (VSM) cells. An increase in Cl− conductance produces membrane depolarization. We hypothesized that if Cl− currents are important for agonist-induced depolarization, then interfering with cellular Cl− handling should alter contractility. Isometric contraction of rat aortic rings was studied in a bicarbonate buffer. Substitution of extracellular Cl− with 130 mM methanesulfonate (MS; 8 mM Cl−) did not cause contraction. NE- and serotonin-induced contractions were potentiated in this low-Cl− buffer, whereas responses to K+, BAY K 8644, or NE in the absence of Ca2+ were unaltered. Substitution of Cl− with I− or Br− suppressed responses to NE. Inhibition of Cl−transport with bumetanide (10−5 M) or bicarbonate-free conditions (10 mM HEPES) inhibited NE- but not KCl-induced contraction. The Cl−-channel blockers DIDS (10−3 M), anthracene-9-carboxylic acid (10−3 M), and niflumic acid (10−5 M) all inhibited NE-induced contraction, whereas tamoxifen (10−5 M) did not. Finally, disruption of sarcoplasmic reticular function with cyclopiazonic acid (10−7 M) or ryanodine (10−5 M) prevented the increase in the peak response to NE produced by low-Cl− buffer. We conclude that a Cl− current with a permeability sequence of I−> Br− > Cl− > MS is critical to agonist-induced contraction of VSM.