Relaxation by dopaminergic agonists in cerebral and peripheral arteries (in vitro)

The receptors involved in dopamine-induced relaxation were investigated in whole segments of rabbit middle cerebral (M.C.A.) and central ear arteries (C.E.A.) by measuring pressure changes in a constant-flow in vitro set-up. Drug administrations were made in the perfusate. During treatment with high-K+ solution to induce active contraction, and in the presence of 3 x 10(-5) M phenoxybenzamine to prevent alpha-adrenergic contractions, cumulative concentration-response curves were obtained for dopamine, apomorphine, bromocriptine and piribedil. The effects of the dopamine receptor antagonists, sulpiride, haloperidol and droperidol, and a beta-adrenergic antagonist, propranolol, were tested on the concentration-response curves obtained with dopamine. The results can be summarized as follows: the dopaminergic agonists had a relaxant effect in 50% of C.E.A. and in 70% of M.C.A. after alpha-adrenergic receptor blockade. This effect was blocked by the 3 antagonists in the M.C.A., and this antagonism appeared to be competitive in the case of sulpiride. In the C.E.A., haloperidol and droperidol exerted complex effects on the dopamine relaxation, and sulpiride had inconsistent effects. Propranolol reduced the concentration-dependent relaxation in the C.E.A., but did not modify the relaxant effect of dopamine in the M.C.A. The present results are consistent with the hypothesis that dopaminergic agonists relax cerebral arteries by acting on specific receptors. The relaxant action on the C.E.A. seems more complex and may occur partly via beta-adrenergic receptors.