Identification of β-adrenoceptor subtypes in lower urinary tract of the female pig

Purpose: We investigated the presence and functional role of β-adrenoceptor subtypes in the bladder base and proximal urethra of the female pig. Materials and Methods: Saturation experiments were done with 7 concentrations (0.25 to 16 nM.) of [³H]-dihydroalprenolol (NEN Life Science Products, Boston, Massachusetts). Competition experiments with [³H]-dihydroalprenolol were performed with unlabeled antagonists (β1 selective CGP20712A, β2 selective ICI118551 and β3 selective SR59230A). In functional studies concentration-relaxation curves to the β3-agonist BRL37344 were obtained and antagonist affinities for SR59230A were determined. Results: CGP20712A displaced [³H]-dihydroalprenolol with low affinity, suggesting that β1-adrenoceptors were not present. Displacement with ICI118551 in the bladder base and urethra best fitted a 2-site model with 20% and 28% high affinity sites (β2), respectively. Displacement experiments with SR59230A in the bladder base demonstrated that 59% of binding sites had high affinity (β3). In the urethra displacement with SR59230A best fitted a 1-site model but with a pK_i of 7.2 that was intermediate between that expected for β2 and β3-adrenoceptors. In functional studies BRL37344 induced relaxation with pEC50 values of 5.5 and 8, and a maximum relaxation response relative to 30 βM. isoprenaline of 79% and 90% in the bladder base and urethra, respectively. The affinity value of SR59230A for the response to BRL37344 was 7.87 and 7.71 in the bladder base and urethra, respectively, which were intermediate between those of β2 and β3-adrenoceptors. Conclusions: Apparently β3-adrenoceptors are the predominant β-adrenoceptor subtype present in the lower urinary tract of the pig.