beta-adrenoceptor-mediated responsiveness of human internal mammary artery

the internal mammary artery (ima) is currently the preferred conduit for myocardial revascularization. however, pre-operative vasospasm and a hypoperfusion state during maximal exercise may limit its use as a bypass graft. the mechanism of spasm has not been clearly defined. since β-adrenoceptor activation plays a major role in vasorelaxation, the present study was carried out to investigate the β-adrenoceptor responsiveness of human ima smooth muscle. for this purpose, ima was obtained from patients with atherosclerosis of coronary artery whom have undergone coronary artery bypass. the endothelium-denuded rings of ima were placed on platinum wires (l-shaped) in a krebs solution containing tissue bath, and were connected to an isometric transducer. it was found out that isoproterenol produced a dose-dependent relaxation in endothelium-denuded ma segments, precontracted with phenylephrine (maximal relaxation was 46.33 ± 5.45 %). isoproterenol-induced relaxation was inhibited by atenolol (10-6 m) and proprano1ol (2 x 10-7 m). while atenolol-induced inhibition was partial, propranolol-induced inhibition was complete in a majority of the segments. brl 37344, a selective 3-adrenoceptor agonist, produced a dose-dependent relaxation in phenylephrine pre contracted rings of endothelium-denuded ima (maximal relaxation was 40.35 ± 4.07 %). cyanopindolol, α-adrenoceptor partial agonist, produced a marked relaxation in endothelium-denuded ima rings, precontracted with phenylephrine (maximal relaxation was 58.65 ± 6.2 %). cyanopindolol-induced relaxation was resistant to blockade by propranolol (2 x 10-7 m). in addition, spontaneous contractions of ima rings were observed in some of the cases that were inhibited by isoproterenol and brl 37344. this observation may indicate the important role of β-adrenoceptor activation in prevention of human ima spasm. therefore, it is concluded that human ima smooth muscle possesses both βl- and β2-adrenoceptors. the existence of β3-adrenoceptor in this tissue is also suggested.