Evidence that dopamine receptors identified by [3H]dopamine in the ventricles of guinea-pig heart are of DA2 type

Apomorphine-induced myocardial protection is due to antioxidant and not adrenergic/dopaminergic effects

Apomorphine (Apo), a dopaminergic agonist used for treatment of Parkinson disease, is a potent antioxidant. In addition to its antioxidative effects, the dopaminergic and adrenergic effects of Apo were studied. Isolated perfused rat hearts were exposed to 25 min of no-flow global ischemia (37 degrees C) and 60 min of reperfusion (I/R, control). Drugs were introduced for the first 20 min of reperfusion. The LVDP of the control group recovered to 54.6 +/- 3.3%. Apo-treated hearts had significantly improved recovery (61.6 +/- 5%, p < 0.05). The recovery of the work index LVDP x HR was even bigger: 67.8 +/- 3.7% (Apo treatment) vs 41.7 +/- 4.6% (control, p < 0.001). Haloperidol, a dopaminergic antagonist, did not affect the recovery with Apo. Propranolol, a beta-adrenergic blocker, initially inhibited the effect of Apo. However, the recovery of the combined group (Apo + propranolol) increased and reached significance (LVDP, p < 0.05 vs control group) after cessation of propranolol perfusion. At 60 min of reperfusion this group was superior to Apo-treated hearts (LVDP, p < 0.05). Propranolol (without Apo) did not improve the hemodynamic recovery. The same pattern of recovery applies also to the recovery of the +dP/dt during the reperfusion. L-DOPA was less effective than Apo. I/R caused significant increase in carbonylation of proteins. Apomorphine inhibited the increase in carbonylation. Haloperidol did not affect this beneficial effect of Apo. L-DOPA significantly decreased the carbonylation of proteins. We conclude that the antioxidative effect of Apo is its main mechanism of cardioprotection.

Electrophysiologic and hemodynamic effects of apomorphine in dogs

Apomorphine is a dopamine receptor agonist used as an emetic, for Parkinson's disease, and for treating erectile dysfunction. This study was conducted to monitor cardiovascular function in dogs given the standard emetic dose (0.05 mg/kg) or 10 times that. Measurements were made during baseline and at 1, 5, 15, 30, 45, and 60 min after iv administration. There were no changes produced by the 0.05 mg/kg dose of apomorphine except for a decrease in mean systemic arterial pressure (AoPm) at the 1 through 15 min recordings. For the 0.5 mg/kg dose, there were reductions in systemic vascular resistance at the 1 and 5 min recordings and in AoPm at the 1 through 60 min recordings. Although not significant, when AoPm fell, heart rate, stroke volume, and cardiac output tended to increase. Action potentials were recorded from superfused Purkinje and endocardial ventricular fibers while exposed to 10(-9) to 10(-5) M apomorphine (10(-10) M is considered therapeutic and 10(-7) M is considered lethal). There were no changes in action potential characteristics of Purkinje fibers, but action potential duration at 90% repolarization prolonged approximately 10-12% in endocardium at concentrations of 10(-6) M and greater. At the usual emetic dose (0.05 mg/kg) apomorphine resulted in no signs of cardiovascular toxicity and, at 0.5 mg/kg, cardiovascular changes were minimal. The emetic dose is higher than that for Parkinson's disease or erectile dysfunction; thus apomorphine appears to be a safe compound for clinical use in dogs and by extrapolation to man.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.