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The previously reported in vivo inotropic selectivity of dobutamine and dopamine compared with isoprenaline has been demonstrated in anaesthetised bivagotamised open chest dogs. Responses of heart rate, right ventricular tension, left ventricular dP/dtmax, and blood pressure were measured. Compared with isoprenaline, dobutamine and dopamine were 1.8 and 2.4 times more active in increasing cardiac contractility than rate. The inotropic selectivity of dopamine but not that of dobutamine was abolished by pretreatment of dogs with syrosingopine. In guinea-pig isolated atria, isoprenaline, dobutamine, and dopamine were all slightly rate selective although this was less for dopamine. In atria incubated with phenoxybenzamine from guinea-pigs pretreated with reserpine only the dopamine dose-response curves were displaced to the right indicating a considerable indirect sympathomimetic component. The in vivo inotropic selectivity of dopamine could therefore be explained on the basis of this indirect activity being manifested at lower doses in the myocardium where the stores of catecholamines are more abundant than at the sinoatrial node. However, it is concluded that the inotropic selectivity of dobutamine seen in vivo is not due to indirect activity, reflex effects, or to a difference in the beta-adrenoceptors mediating the rate and tension responses of the heart. Possible alternative explanations are discussed.
Lumley et al. (Sat,) studied this question.