Relaxation by beta 3-adrenoceptor agonists of the isolated human internal anal sphincter

Abstract

Aims: In this study, responses of beta(3)-adrenoceptor agonists were examined on human isolated internal anal sphincter (IAS) in order to explore their relaxant effects on hypertonicity of IAS.

Main methods: The relaxant efficacy (E(max)) and potency (-logIC(50)) of BRL37344 and SR58611A, beta(3)-adrenoceptor agonists, were examined in contracted IAS muscle strips. The presence of beta(3)-adrenoceptors, and changes in intracellular calcium and cyclic nucleotide levels in IAS muscle were tested by Western blotting, epifluorescence microscopy and enzyme immunoassay, respectively.

Key findings: BRL37344 and SR58611A relaxed contracted IAS muscle (E(max)=27+/-3% and 35+/-3%; -logIC(50)=6.26+/-0.24 and 4.87+/-0.13; respectively). These relaxant responses were blocked by SR59230A, a selective beta(3)-antagonist but not by beta(1)/beta(2)-selective antagonists, neuronal inhibitor or inhibition of nitric oxide synthase. The E(max) of beta(3)-agonists was similar to that of beta(2)-selective agonists but smaller than that of isoprenaline (nonselective agonist) or beta(1)-selective agonists. BRL37344 (100 microM) increased cAMP (1.5-fold) without cGMP change, and depressed intracellular calcium signal. beta(3)-Adrenoceptor expression was smaller than that of beta(1)- and beta(2)-adrenoceptors.

Significance: This is the first study demonstrating the presence of beta(3)-adrenoceptor in human IAS muscle and beta(3)-mediated relaxation of augmented sphincter tone. However, direct beta(3)-relaxation appears smaller than that obtained for nonselective agonists which may limit their potential use in the treatment of anorectal hypertonicity disorders.