Characterization of β-adrenoceptor mediated smooth muscle relaxation and the detection of mRNA for β(1)-, β(2)- and β(3)-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 (β(1)-AR antagonist) or ICI118551 (β(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 (β(3)-AR agonist) caused biphasic relaxation. The high affinity component was shifted with low affinity by propranolol, (±)-cyanopindolol, tertatolol and alprenolol. CL316243 (β(3)-AR agonist) relaxation was unaffected by CGP20712A or ICI118551 but blocked by SR58894A (β(3)-AR antagonist; pA(2)=7.80). Enhanced relaxation after exposure to forskolin and pertussis toxin showed that β(3)-AR relaxation can be altered by manipulation of components of the adenylate cyclase signalling pathway. 4. The β(1)-AR agonist RO363 relaxed the ileum (pEC(50)=6.18) and was blocked by CGP20712A. Relaxation by the β(2)-AR agonist zinterol (pEC(50)=5.71) was blocked by SR58894A but not by ICI118551. 5. In rat ileum, β(1)-, β(2)- and β(3)-AR mRNA was detected. Comparison of tissues showed that β(3)-AR mRNA expression was greatest in WAT>colon=ileum>cerebral cortex>soleus; β(1)-AR mRNA was most abundant in cerebral cortex>WAT>ileum=colon>soleus; β(2)-AR mRNA was expressed in soleus>WAT>ileum=colon>cerebral cortex. 6. These results show that β(3)-ARs are the predominant β-AR subtype mediating rat ileal relaxation while β(1)-ARs may produce a small relaxation. The β(2)-AR agonist zinterol produces relaxation through β(3)-ARs and there was no evidence for the involvement of β(2)-ARs in relaxation despite the detection of β(2)-AR mRNA