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This study aimed to elucidate mechanism(s) that mediate dopamine regulation of cardiovascular system (CVS) functions. Forty eight adult male anesthetized Balady rabbits (4 experiments, 8 groups, 6 animals each) were included. Experiment I assessed the effect of intravenous (iv) dopamine infusion (0.1, 1, 4 and 12 μg/kg/min) on diastolic (DBP), systolic (SBP), mean blood pressure (MBP), heart rate (HR), cardiac contractility (CC) and renal sympathetic nerve activity (RSNA). Experiment II assessed the effect of dopamine infusion on ventricular sarcomere length. Experiment III confirmed the contribution of dopamine receptor subtype(s). Experiment IV evaluated adrenergic receptors involved in dopamine's action. Mean BP, CC, HR and RSNA were recorded by physiograph. At low dopamine infusion rate DBP, MBP, CC and RSNA were decreased; while sarcomere length and A:I ratio were increased. At high dopamine infusion rate DBP, SBP, MBP, HR and CC were increased while; sarcomere length and A:I ratio were decreased. D1-like receptor activation decreased MBP; while D2-like receptor activation decreased MBP, CC, and RSNA. Both D1-and D2-like receptors blockade attenuated hypotensive response, whereas CC was abolished by D2 receptor blockade. Mean BP, HR and CC were not changed after D1and D2-like receptors blockade, but decreased after D1and D2 like receptors activation. Low dopamine infusion into animals pre-treated with αadrenoceptor blockade (reserpine) or β-adrenoceptor blockade (propranolol) decreased MBP and CC whereas, with high dopamine infusion, the HR and CC were increased after α-adrenoceptor blockade and MBP was increased after βadrenoceptor blockade. From this study, we can conclude that dopamine elicits biphasic effect on CVS. Low dopamine doses acts via stimulation of D1and D2like receptors. With increasing dose, actions occur via stimulation of αand β-adrenergic receptors. Normal endogenous dopamine may not alter basal cardiovascular functions.

Cardiovascular Functions and Dopamine: Mechanism of Action in Adult Male Anesthetized Balady Rabbits