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 β-AR subtypes mediating relaxation of rat ileal smooth muscle. 2. In functional studies, (-)-isoprenaline relaxation was unchanged by CGP20712A (β₁-AR antagonist) or ICI118551 (β₂-AR antagonist) but shifted by propranolol (pK(B) = 6.69). (±)-Cyanopindolol, CGP12177 and ICID7114 did not cause relaxation but antagonized (-)-isoprenaline relaxation. 3. BRL37344 (β₃-AR agonist) caused biphasic relaxation. The high affinity component was shifted with low affinity by propranolol, (±)-cyanopindolol, tertatolol and alprenolol. CL316243 (β₃-AR agonist) relaxation was unaffected by CGP20712A or ICI118551 but blocked by SR58894A (β₃-AR antagonist; pA₂ = 7.80). Enhanced relaxation after exposure to forskolin and pertussis toxin showed that β₃-AR relaxation can be altered by manipulation of components of the adenylate cyclase signalling pathway. 4. The β₁-AR agonist RO363 relaxed the ileum (pEC₅₀ = 6.18) and was blocked by CGP20712A. Relaxation by the β₂-AR agonist zinterol (pEC₅₀ = 5.71) was blocked by SR58894A but not by ICI118551. 5. In rat ileum, β₁-, β₂- and β₃-AR mRNA was detected. Comparison of tissues showed that β₃-AR mRNA expression was greatest in WAT > colon = ileum > cerebral cortex > soleus; β₁-AR mRNA was most abundant in cerebral cortex > WAT > ileum = colon > soleus; β₂-AR mRNA was expressed in soleus > WAT > ileum = colon > cerebral cortex. 6. These results show that β₃-ARs are the predominant β-AR subtype mediating rat ileal relaxation while β₁-ARs may produce a small relaxation. The β₂-AR agonist zinterol produces relaxation through β₃-ARs and there was no evidence for the involvement of β₂-ARs in relaxation despite the detection of β₂-AR mRNA.