β2-Adrenoceptors in Rat Liver Microcirculation

1. Rat liver sinusoids were observed by a microscopic in vivo transillumination method. The diameter of liver sinusoids and intrasinusoid erythrocyte flow velocities were measured quantitatively by a close-circuit television technique. 2. Both isoprenaline and the selective β₂-adrenoceptor agonist terbutaline produced a concentration-dependent dilatation of liver sinusoids and slowed erythrocyte flow velocity. The effects of isoprenaline and terbutaline were antagonized by propranolol but not by the selective β₁-adrenoceptor antagonist atenolol. Propranolol alone produced constriction of the liver sinusoids and increased erythrocyte flow velocity; these effects were not produced by atenolol. 3. The percentage dilatations produced by isoprenaline alone, isoprenaline in the presence of phenoxybenzamine and isoprenaline in the presence of phenylephrine-induced constriction were similar. 4. It is proposed that the β-adrenoceptors in liver sinusoids are of the β₂-type, and their physiological role was to counteract constrictor responses produced by a-adrenoceptor activity.