Catecholamines relax portal and mesenteric veins from normal and portal hypertensive rats.

The relaxation by catecholamines of superior mesenteric veins and portal veins, precontracted by 65 mM KCl, was studied after a 2-h exposure to phenoxybenzamine. The veins were obtained from portal-hypertensive and sham-operated rats. The beta 2-selective antagonist ICI 118551 attenuated the relaxation caused by epinephrine and isoproterenol but did not change the relaxation caused by norepinephrine in superior mesenteric veins. The beta 1-specific antagonist CGP 20712A reduced the relaxation caused by norepinephrine but only marginally attenuated those caused by epinephrine and isoproterenol in superior mesenteric veins. The combination of ICI 118551 and CGP 20712A abolished the relaxation caused by the catecholamines in superior mesenteric veins. The relaxation caused by norepinephrine, epinephrine, and isoproterenol was markedly reduced by ICI 118551 in portal veins. The pattern of antagonism was similar in veins from portal-hypertensive and sham-operated rats. The results suggest that both beta 1- and beta 2-adrenoceptors mediate relaxation induced by catecholamines in superior mesenteric veins. Relaxation of portal veins appears to be mediated mostly by beta 2-adrenoceptors.