Endothelium‐dependent relaxation to acetylcholine in the rabbit basilar artery: importance of membrane hyperpolarization

1 Muscarinic stimulation of isolated, preconstricted segments of the basilar artery, with either acetylcholine or carbachol, was followed by endothelium‐dependent smooth muscle relaxation and membrane hyperpolarization. 2 Smooth muscle relaxation to acetylcholine was stimulated in the presence of lower concentrations than the associated hyperpolarization (EC50 values 3.2 μm and 31.6 μm, respectively), and was sustained during agonist application, while the hyperpolarization was relatively transient. 3 Repeated exposure to acetylcholine was associated with loss of membrane hyperpolarization, while smooth muscle relaxation was unaltered. Following a second exposure to 100 μm acetylcholine, mean hyperpolarization was markedly depressed from 8.5 to 2 mV, and subsequent exposures failed to induce any hyperpolarization. Relaxations with a similar amplitude and rate of development, were recorded with each subsequent addition of acetylcholine. 4 The competitive substrate inhibitors for nitric oxide synthase, l‐NG‐monomethyl arginine (100 μm l‐NMMA) or l‐NG‐nitro arginine methyl ester (100 μm l‐NAME), modified the form and amplitude of both the relaxation and the hyperpolarization to acetylcholine. In the majority of experiments, both the hyperpolarization and the relaxation were almost totally abolished. 5 Neither nitric oxide, applied directly in physiological salt solution, nor sodium nitroprusside, produced smooth muscle hyperpolarization except in high concentrations. Reproducible, small amplitude (around 2 mV) hyperpolarization followed the application of either NO gas (15 μm) or sodium nitroprusside (100 μm), both of which induced almost maximal smooth muscle relaxation. 6 These data show that muscarinic stimulation of endothelial cells in the rabbit basilar artery is followed by both smooth muscle hyperpolarization and relaxation. They indicate that nitric oxide is involved in both of these responses, but that the smooth muscle hyperpolarization is not an essential component of the relaxation.