Characterization of β-adrenoceptor mediated smooth muscle relaxation and the detection of mRNA for β1-, β2- and β3-adrenoceptors in rat ileum

1 Functional and molecular approaches were used to characterize the beta-AR subtypes mediating relaxation of rat ileal smooth muscle. 2 In functional studies, (-)-isoprenaline relaxation was unchanged by CGP20712A (beta(1)-AR antagonist) or ICI118551 (beta(2)-AR antagonist) but shifted by propranolol (pK(B) = 6.69). (+)Cyanopindolol, CGP12177 and ICID7114 did not cause relaxation but antagonized (-)-isoprenaline relaxation. 3 BRL37344 (beta(3)-AR agonist) caused biphasic relaxation. The high affinity component was shifted with low affinity by propranolol, (+/-)-cyanopindolol, tertatolol and alprenolol. CL316243 (beta(3)-AR agonist) relaxation was unaffected by CGP20712A or ICI118551 but blocked by SR58894A (beta(3)-AR antagonist; pA(2) = 7.80). Enhanced relaxation after exposure to forskolin and pertussis toxin showed that beta(3)-AR relaxation can be altered by manipulation of components of the adenylate cyclase signalling pathway. 4 The beta(1)-AR agonist RO363 relaxed the ileum (pEC(50) = 6.18) and was blocked by CGP20712A. Relaxation by the beta(2)-AR agonist zinterol (pEC(50) = 5.71) was blocked by SR58894A but not by ICI118551. 5 In rat ileum, beta(1)-, beta(2)- and beta(3)-AR mRNA was detected. Comparison of tissues showed that beta(3)-AR mRNA expression was greatest in WAT> colon = ileum > cerebral cortex > soleus; PI-AR mRNA was most abundant in cerebral cortex > WAT> ileum = colon > soleus; beta(2)-AR mRNA was expressed in soleus > WAT > ileum = colon > cerebral cortex. 6 These results show that beta(3)-ARs are the predominant beta-AR subtype mediating rat ileal relaxation while beta(1)-ARs may produce a small relaxation. The beta(2)-AR agonist zinterol produces relaxation through beta(3)-ARs and there was no evidence for the involvement of beta(2)-ARs in relaxation despite the detection of beta(2)-AR mRNA.