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

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 D1- and D2-like receptors blockade, but decreased after D1- and D2 like
receptors activation. Low dopamine infusion into animals pre-treated with α-
adrenoceptor 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 β-
adrenoceptor blockade. From this study, we can conclude that dopamine elicits
biphasic effect on CVS. Low dopamine doses acts via stimulation of D1- and D2- like
receptors. With increasing dose, actions occur via stimulation of α- and β-adrenergic
receptors. Normal endogenous dopamine may not alter basal cardiovascular
functions.