Nitric Oxide (NO) Primarily Accounts for Endothelium-Dependent Component of β-Adrenoceptor-Activated Smooth Muscle Relaxation of Mouse Aorta in Response to Isoprenaline

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Isoprenaline is known to produce vascular relaxation through activation of β-adrenoceptors. In recent years, β-adrenoceptor-activated vascular relaxation has been the focus of pharmacological study in terms of both the receptor subtypes and the intracellular signaling mechanisms which trigger smooth muscle mechanical functions. In addition, the possible contribution of the endothelium to β-adrenoceptor-activated relaxation of vascular beds has provoked considerable discussion, with consensus still to be established. In the present study, we examined the effects of isoprenaline on isolated mouse aortic smooth muscles to determine whether the presence of the endothelium plays a substantial role in the relaxation it produces. A possible role for nitric oxide (NO) as a primary endothelium-derived factor released in response to isoprenaline was also elucidated pharmaco-mechanically. In isolated thoracic and abdominal aortae precontracted with phenylephrine (3×10^-7-10^-6M), isoprenaline elicited relaxation in a concentration-dependent fashion (10^-9-10^-5M). In endothelium-denuded preparations, isoprenaline-elicited relaxation was reduced to 40-50% of the response obtained in endothelium-intact preparations. In the preparations treated with N^G-nitro-L-arginine methyl ester (L-NAME, 3×10^-4M; an NO synthase inhibitor) or 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1-one (ODQ, 10^-5M; a soluble guanylyl cyclase inhibitor), isoprenaline-elicited relaxation was attenuated almost to the same degree as the response in endothelium-denuded preparations. The degree of endothelium-dependency in isoprenaline-elicited relaxation was largely diminished when treated with propranolol (3×10^-6M). The present findings indicate that isoprenaline substantially relaxes the mouse aorta with both endothelium-dependent and -independent mechanisms. The endothelium-dependent component seems to correspond to about 50% of the isoprenaline-elicited relaxation, and is almost entirely due to endothelium-derived NO. Activation of propranolol (3×10^-6M)-inhibitable β-adrenoceptors seems to be primarily responsible for the NO-mediated endothelium-dependent pathway in isoprenaline-elicited relaxant response of mouse aorta.
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