Non‐specificity of chloride channel blockers in rat cerebral arteries: block of the L‐type calcium channel

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Abstract
The effects of chloride channel blockers on pressure‐induced constriction, K+‐induced force, and whole‐cell calcium channel currents were tested in rat cerebral arteries using isobaric and isometric myography, and patch clamp. Under isobaric conditions at 75 mmHg, NPPB), a chloride channel blocker, reversibly depressed the myogenic constriction with an IC50 of 32.8 ± 0.52 μm (mean ± s.e.m., n= 5). Blockers of Ca2+‐activated chloride channels, flufenamic acid (100 μm) and 9‐AC; 1 mm), and the cystic fibrosis transmembrane conductance regulator (CFTR) Cl− channel blocker, glibenclamide (100 μm), were without effect in this tissue (n= 3). Under isobaric conditions at 20 mmHg, 37 °C, raising [K+]o to 45 mm induced a constriction which was unaffected by 100 μm NPPB (n= 4). In contrast, at 75 mmHg and 18‐21 °C, 100 μm NPPB completely and reversibly blocked a 45 mm K+‐induced constriction (n= 3). Under isometric conditions, NPPB reversibly depressed a 45 mm K+‐induced force with an IC50 of 10.0 ± 0.76 μm (mean ± s.e.m., n= 5). IAA‐94), another chloride channel blocker, depressed the K+‐induced force with an IC50 of 17.0 ± 1.2 μm (mean ± s.e.m., n= 4). Using whole‐cell patch clamp, 100 μm NPPB or 200 μm IAA‐94 blocked calcium channel currents carried by 10 mm Ba2+ by 79.1 ± 1.7 and 39.8 ± 7.0 %, respectively (mean ± s.e.m., n= 6). In summary, chloride channel blockers depress calcium channel currents in rat cerebral arteries, which could contribute to a reduction in myogenic contraction.