Role of beta2-adrenoceptors (beta-AR), but not beta1-, beta3-AR and endothelial nitric oxide, in beta-AR-mediated relaxation of rat intrapulmonary artery.

The aim of this study was to analyze beta-adrenoceptor (beta-AR)-mediated relaxation in rat intralobar pulmonary artery. The relaxant responses of beta-AR agonists were characterized using beta-AR antagonists in prostaglandin F2alpha (PGF2alpha)-precontracted arteries. The role of nitric oxide (NO) and endothelium in beta-AR-mediated relaxation was also investigated. Isoprenaline (a non-selective beta-AR agonist) and salbutamol (a selective beta2-AR agonist) induced vasorelaxation. ICI 118551 (a selective beta2-AR antagonist) antagonized the effect of both isoprenaline and salbutamol (pA2 values of 9.57 and 9.51 respectively). In contrast, atenolol (1 microM) and CGP 20712A (0.1 microM), two beta1-AR antagonists, did not modify the relaxing effect of isoprenaline. The response to isoprenaline obtained in the presence of nadolol (10 microM, a beta1/beta2-AR antagonist) was not further inhibited by SR 59230A (1 microM, a selective beta3-AR antagonist). The non-beta1/beta2-AR agonists studied (BRL 37344, SR 58611A, and CGP 12177A) did not elicit vasorelaxation. Relaxation to isoprenaline and salbutamol was unaffected by L-N(G)-nitro-arginine methyl ester (100 microM, an inhibitor of NO synthase) or after endothelium removal. These results demonstrate the role of beta2-AR in mediating relaxation in rat intralobar pulmonary artery precontracted with PGF2alpha. They indicate that beta2-AR-mediated relaxation in this artery is NO- and endothelium-independent. Furthermore, they do not provide evidence of a relaxant role of either beta1- or beta3-AR in PGF2alpha-precontracted rat intrapulmonary artery.