Functional Evidence for the Presence of β<sub>3</sub>-Adrenoceptors in the Guinea Pig Common Bile Duct and Colon

Abstract

To determine the existence of beta 3-adrenoceptors in functional assays in isolated preparations for which data are lacking, we compared the effects of SR 58611A, a selective beta 3-adrenoceptor agonist, and isoprenaline in the guinea pig common bile duct, distal colon and urinary bladder. SR 58611A and isoprenaline relaxed the common bile duct (EC50: 6.85 and 0.41 mumol/l, respectively). The effect of SR 58611A was resistant to CGP 20712A, ICI 118551, propranolol and tetrodotoxin, but was antagonized by alprenolol (pA2 = 6.86), while the effect of isoprenaline was antagonized by CGP 20712A, ICI 118551, propranolol and alprenolol (pA2 = 7.04, in the presence of propranolol to saturate beta 1- and beta 2-adrenoceptors). In colonic preparations, SR 58611A and isoprenaline relaxed circular muscle strips (EC50: 5.48 and 0.49 mumol/l, respectively). The effect of SR 58611A was resistant to CGP 20712A, ICI 118551, propranolol and tetrodotoxin, but was antagonized by alprenolol (pA2 = 7.01). The effect of isoprenaline was resistant to CGP 20712A, but was antagonized by ICI 118551, propranolol and alprenolol (pA2 = 6.88, in the presence of propranolol). In urinary bladder strips, SR 58611A had no effect, whereas isoprenaline reduced resting tone (EC50:0.87 mumol/l), an effect antagonized by alprenolol (pA2 = 8.14). These data provide functional evidence for the presence of beta 3-adrenoceptors in the guinea pig common bile duct and colon, but not in the urinary bladder. At the concentrations used, the effect of SR 58611A was probably mediated solely by activation of beta 3-adrenoceptors located on smooth muscle cells, whereas the effects of isoprenaline were due to beta 3- and also to beta 1-and/or beta 2-adrenoceptor activation.