No evidence of vascular dopamine receptors in the rat portal vein.

Dopamine (5.3 X 10(-6) to 5.3 X 10(-5) M) decreased the force of spontaneous myogenic contractions of the rat portal vein in a concentration-dependent manner in the presence of the alpha-adrenoreceptor antagonist BE-2254. The DA1-selective agonist fenoldopam and the DA2-preferential agonist (-)-EOE were both inactive. Dopamine-induced relaxation was not inhibited by (+)-butaclamol (10(-8) M), but was reduced by a potent beta 2-selective adrenoreceptor antagonist ICI 118,551 (2 X 10(-8) M). Our results demonstrate that the rat portal vein is devoid of postsynaptic DA1 and DA2 dopamine receptors.