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.