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

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Abstract
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 (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. 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. 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. 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. 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. British Journal of Pharmacology (1999) 127 , 949–961; doi: 10.1038/sj.bjp.0702605

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