Role of Ca2+- and Swelling-Activated Cl− Channels in Α1-Adrenoceptor-mediated Tone in Pressurized Rabbit Mesenteric Arterioles

Background: Ca2+-activated (ICl(Ca)) and swelling-induced (ICl(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 ICl(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 ICl(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 ICl(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, ICl(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.