Comparative studies of cardiovascular effects of a new dopamine agonist (SK&F 38393A) and dopamine

SK&F 38393A (DAA) dose-dependently decreased the contractility of isolated frog hearts with an ED50 of 6.60 +/- 0.28 X 10(-5) M, while, dopamine (DA) elicited a dose-dependent increase in the contractility of the preparation with an ED50 of 5.13 +/- 0.36 X 10(-5) M. The results of in situ investigation, using cardiac output as the index of cardiac contractility, were in agreement with the in vitro results. Haloperidol (10(-6) M), a dopaminergic receptor blocker, did not block the cardiovascular effects of DAA and DA. However, propranolol (10(-8) M), a beta-adrenergic receptor blocker, prevented the positive inotropic effects of DA but had no effects on the negative inotropic effects of DAA. The results suggest that DAA might be acting on this preparation through a mechanism which is probably not associated with beta-adrenergic and/or dopaminergic receptors.