Role of Ca2+- and swelling-activated Cl− channels in α1-adrenoceptor-mediated tone in pressurized rabbit mesenteric arterioles

Background: Ca2+-activated ( I Cl(Ca)) and swelling-induced ( I Cl(swell)) Cl− channels have, respectively, been postulated to participate in the membrane depolarization and contraction mediated by activation of α1-adrenoceptors and vascular wall distension during pressurization. Their respective function in generating active force in pressurized arterioles during α1-adrenoceptor stimulation remains unsettled. Objectives: Experimental protocols were designed to: (1) assess the relative contribution of I Cl(Ca) to the pressure-dependence of lumen diameter of mesenteric arterioles at different states of activation of the α1-adrenoceptor, and (2) investigate the potential role of I Cl(swell) in spontaneous and agonist-mediated myogenic reactivity. Methods: Segments of endothelium-denuded rabbit mesenteric arterioles with a lumen diameter of ∼70 μm were cannulated at both ends and studied under isobaric conditions at 36°C. Steady-state lumen diameter at each pressure step investigated (0–100 mmHg, in 20-mmHg increments) was measured by a video-microscopy edge-detection technique. Results: Under control conditions, 23% of the arterioles developed nifedipine-sensitive spontaneous myogenic tone. In the presence of 1 mM tetraethylammonium chloride (TEA) to inhibit Ca2+-dependent K+ channels, the α1-agonist phenylephrine (PE) contracted the vessels in a concentration-dependent manner (0.1–10 μM) and potentiated myogenic reactivity. The contraction mediated by 1 μM PE/TEA was abolished by 1 μM nifedipine, indicating that Ca2+ entry through voltage-gated Ca2+ channels was a necessary step in the cascade leading to contraction. Niflumic acid (NfA, 100 μM), a relatively selective inhibitor of I Cl(Ca), had no effect on myogenic tone but reversed the PE-induced contraction, varying with the concentration of PE and transmural pressure. For PE concentrations between 0.1 and 1 μM, but not for 10 μM PE, the relaxing efficacy of NfA decreased as applied pressure was raised from 0 to 100 mmHg. At all pressure steps, the NfA-induced relaxation was inversely related to the concentration of PE. DIDS (200 μM), another Cl− channel blocker, inhibited spontaneous myogenic tone, and partially suppressed a component of contraction at elevated transmural pressures in arterioles incubated in 1 μM PE/1 mM TEA/100 μM NfA. Conclusions: Our data indicate that under low to moderate stimulation of the α1-adrenoceptor signaling pathway, I Cl(Ca) channels play an important role in the sustained contraction produced. Their declining contribution to contraction with increasing transmural pressure may be explained, at least in part, by a progressive enhancement of stretch-induced ionic conductances, possibly volume-sensitive Cl− channels.