Vascular smooth muscle relaxation by alpha-adrenoceptor blocking action of dobutamine in isolated rabbit aorta

We investigated the mechanism of vascular relaxation produced by dobutamine, a positive inotropic agent with beta(1)-adrenergic action. Dobutamine concentration-dependently (10 nM-10 mu M) relaxed ring segments of rabbit aorta partially precontracted with 1 mu M phenylephrine (PE) but did not relax those precontracted with 40 mM K+ or 5 mu M prostaglandin F-2 alpha (PGF(2 alpha)). The relaxation was not completely inhibited by pretreatment with 10 mu M propranolol. Dobutamine did not significantly increase tissue cyclic AMP levels concomitantly with relaxation as does isoproterenol (ISO) in rabbit aorta, Dobutamine produced a parallel rightward shift in concentration-response curves to PE. The Schild plot analysis resulted in a linear regression of a slope of 1.077 +/- 0.077, which was not significantly different from unity. The pA(2) value of dobutamine as compared with PE in rabbit aorta was 6.81 +/- 0.03. Dobutamine causes arterial dilatation mediated not only through a beta-adrenergic action but also through an alpha-adrenergic blocking action in rabbit aorta.