Characterization of β‐adrenoceptors in urinary bladder: comparison between rat and rabbit

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Abstract
1. beta-Adrenoceptor subtypes in rat and rabbit urinary bladder were investigated in functional experiments by use of several agonists and antagonists. 2. All agonists tested produced concentration-dependent relaxation, but the relative potencies varied between both species: BRL 37344 (pD2:8.0) > isoprenaline (7.3) > adrenaline (6.7) = noradrenaline (6.6) in rat bladder, and isoprenaline (8.7) = adrenaline (8.5) > noradrenaline (7.7) = BRL 37344 (7.4) in rabbit bladder. 3. The relaxation response to isoprenaline in rat bladder was relatively resistant to propranolol and ICI 118551, and the slopes of Schild plot for both antagonists were different from unity. The apparent pKB values estimated by single concentrations of propranolol (1, 10 microM) and ICI 118551(10 microM) were 6.6 and 5.4, respectively. 4. On the other hand, the relaxation response to isoprenaline in rabbit bladder was antagonized by lower concentrations (1 nM-100 nM) of propranolol and ICI 118551 in a competitive manner, resulting in pA2 values of 8.7 and 8.6, respectively. 5. These results suggest species-heterogeneity of beta-adrenoceptors in urinary bladder; beta 3 and beta 2 subtypes in rat and beta 2 subtype in rabbit.