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

Functional and molecular approaches were used to characterize the β‐AR subtypes mediating relaxation of rat ileal smooth muscle. In functional studies, (−)‐isoprenaline relaxation was unchanged by CGP20712A (β1‐AR antagonist) or ICI118551 (β2‐AR antagonist) but shifted by propranolol (pKB=6.69). (±)‐Cyanopindolol, CGP12177 and ICID7114 did not cause relaxation but antagonized (−)‐isoprenaline relaxation. 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. 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. 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. 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. British Journal of Pharmacology (1999) 127, 949–961; doi:10.1038/sj.bjp.0702605