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 microM) relaxed ring segments of rabbit aorta partially precontracted with 1 microM phenylephrine (PE) but did not relax those precontracted with 40 mM K+ or 5 microM prostaglandin F2 alpha (PGF2 alpha). The relaxation was not completely inhibited by pretreatment with 10 microM 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 pA2 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.