Pharmacological identification of β-adrenoceptor subtypes mediating isoprenaline-induced relaxation of guinea pig colonic longitudinal smooth muscle

Abstract

noradrenaline ≈ adrenaline, whereas the rank order was isoprenaline > noradrenaline > adrenaline in the presence of propranolol (a non-selective β-AR antagonist; 3 × 10-7 M). Atenolol (a selective β1-AR antagonist; 3 × 10-7-10-6 M) acted as a competitive antagonist of isoprenaline-induced relaxation, and the pA2 value was calculated to be 6.49 (95% confidence interval: 6.34-6.83). The relaxation to isoprenaline was not affected by ICI-118,551 (a selective β2-AR antagonist) at 10-9-10-8 M, but was competitively antagonized by 10-7-3 × 10-7 M, with a pA2 value of 7.41 (95% confidence interval: 7.18-8.02). In the presence of propranolol (3 × 10-7 M), the relaxant effect of isoprenaline was competitively antagonized by bupranolol (a non-selective β-AR antagonist), with a pA2 value of 5.90 (95% confidence interval: 5.73-6.35). Conclusion These findings indicated that the β-AR subtypes involved in isoprenaline-induced relaxation of colonic longitudinal guinea pig muscles are β1-AR and β3-AR.