[Hypotensive effect of dopaminergic agonists in the rat]

Dopamine, Bromocriptine and Apomorphine produce arterial hypotension in the rat, but so far remains to be established if the phenomenon takes place at the presynaptic or postsynaptic nerve terminal. Present study deal with the locus of action of these substances. Rats were anesthetized with urethane i.p. and heparinized, maintaining the spontaneous pulmonary ventilation through a tracheal cannula. EKG and intracarotid mean blood pressure were recorded through a polygraph. Dopamine as well as Bromocriptine (3.12 to 25 mu/100 g both) and Apomorphine (0.125 to 4.0 micrograms/100 g) caused a dose-related hypotension, together with moderate bradycardia and in Apomorphine treated rats, atrioventricular block. In 28 animals, chemical sympathectomy was performed with 6-OH-DA (3 mg/100 g i.p.) and 2 hours later the three substances were injected. Under these conditions, Dopamine raised the blood pressure (p less than 0.001) while Apomorphine lessened the hypotensive effect (p less than 0.02) and Bromocriptine caused no changes on this regard. It is suggested that the main effect of Dopamine on blood pressure lies at a prejunctional nerve terminal level whereas Bromocriptine acts at the effector organ post junctional level. In Apomorphine effect it might be mainly involved a prejunctional component.