Transient effects of norepinephrine on myocardial oxygen balance

Summary In conscious dogs with experimental atrioventricular block and with ventricles paced at constant rate the effects of norepinephrine (NE) and isoproterenol (ISO) on coronary flow, coronary resistance, and myocardial O₂-balance were investigated. Myocardial O₂-s balance, as estimated from continuous measurement of coronary venous O₂-s saturation, was used for the discrimination of coronary dilation induced either directly by vascular -adrenoreceptor stimulation or indirectly by increased myocardial metabolism.Following bolus injection of NE (0.3 g/kg) or ISO (0.1 g/kg) into the pulmonary artery, coronary venous O₂-s saturation increased from a control of 25±2% O₂-s saturation (mean±S.D.) transiently to 51±5 and 62±5% O₂-s saturation respectively. After₁-adrenoreceptor blockade these increases were reduced to 33±4 and 41±3% O₂-s saturation, respectively. The remaining increase after NE was abolished when atropine was given in addition to₁-b blockade. After₁+2-adrenoreceptor blockade neither NE nor ISO injection had an effect on coronary venous O₂ saturation. After₁-b blockade was superimposed on ganglionic blockade NE injection led to a decrease in coronary venous O₂-s saturation indicating a ratent -a activity of NE.NE seems to act directly via₁-a adrenoreceptors, since no differences were observed in the time courses of changes in coronary venous O₂-s saturation after left atrial injection of NE when compared to adenosine.It is concluded that circulating NE like ISO is able to improve myocardial oxygen balance by a direct vasodilating effect on canine coronary vessels mediated by vascular₁-adrenoreceptors.