Agonist-dependent desensitization of the β-adrenoceptor-mediated rat urinary bladder relaxation

Wim Vrydag, Martin C. Michel

Acedemic Medical Center, Amsterdam, Netherlands

Overactive bladder syndrome is a prevalent condition in the adult population and markedly reduces the quality of life of the afflicted patients. Multiple β3-adrenoceptor agonists are currently in development for the chronic symptomatic treatment of this syndrome (Colli et al. 2007). The long term clinical efficacy of agonists can be limited by receptor desensitization. Data from cell experiments indicate that the β3-adrenoceptor desensitization is cell specific (Chaudhry & Granneman 1994). Therefore, we have explored the presence and possible mechanisms of such desensitization in the rat urinary bladder.

Male Wistar rat (280-320 g) bladder strips were pre-treated in organ baths for 6 h in Krebs buffer or 24 h in culture medium in the absence or presence of isoprenaline (non selective β-adrenoceptor agonist), fenoterol (β2-adrenoceptor selective agonist) or the β3-adrenoceptor selective agonists YM178 ((R)-2-(2-aminothiazol-4-yl)-4’-{2-[(2-hydroxy-2-phenylethyl)amino]ethyl} acetanilide) and CL 316,243 (disodium 5-[(2R)-2-[[(2R)-2-(3-chlorophenyl)-2-hydroxyethyl]amino]propyl]-1,3-benzodioxole-2,2-dicarboxylate). After wash out, relaxation curves with fresh agonists were generated against a tone induced with 50 mM KCl. Data are means ± S.E.M. of ≥ 3 experiments, and a p<0.05 (ANOVA) was considered significant.

The maximum relaxation by isoprenaline was reduced with 54% after 24 h pre-treatment with isoprenaline, but due to deterioration of contraction and relaxation responses further experiments were performed after 6 h pre-treatment. Isoprenaline-induced relaxation was compared with vehicle (Emax 50 ± 5%) reduced after pre-treatment with isoprenaline (27 ± 4%), fenoterol (27 ± 6%), YM178 (34 ± 3%) or CL 316,243 (33 ± 2% all p<0.05). The fenoterol-induced relaxation was reduced by pre-treatment with isoprenaline (44 ± 4% vs. 23 ± 2%) but not with YM178 or CL 316,243. CL 316,243-induced relaxation was desensitized by pre-treatment with isoprenaline (32 ± 5% vs 13 ± 4%) and to a lesser extent with CL 316,243 (32 ± 5% vs. 22 ± 2%), whereas the YM178-induced relaxation was not affected by pre-treatment with isoprenaline or YM178.

We conclude that relaxation by β-adrenoceptor agonists in rat bladder can desensitize but this mainly involves the β2-component (fenoterol response). Relaxation responses to β3-agonists exhibit compound-specific desensitization with CL 316,243 being sensitive to desensitization and YM178 not. Our data do not provide evidence that long term treatment with YM178 carries a risk of desensitization.

This work was supported in part by Astellas Europe B.V.

Chaudhry & Granneman, J. Pharmacol. Exp. Ther., 1994, 271, 1253-8

Colli et al, Expert Opin. Investig. Drugs, 2007, 16, 999-1006