Selective and full β1-adrenoceptor agonist action of a catechol derivative of denopamine (T-0509) in the guinea-pig cardiac muscle and trachea: comparison with denopamine, xamoterol and isoprenaline

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Abstract
1 The pharmacological actions of T-0509, a 3-hydroxy derivative of denopamine, were studied in various guinea-pig tissues; these effects were compared with those of isoprenaline, denopamine and xamoterol. 2 The intrinsic activities of the positive inotropic actions of T-0509, denopamine and xamoterol compared with isoprenaline (= 100%) in the papillary muscle were 99%, 83% and 28%, respectively, while their relative potencies (EC50 agonist EC50 isoprenaline) were 0.23, 33 and 1.4, respectively. The intrinsic activities of T-0509, denopamine and xamoterol as positive chronotropic agents in the right atria were 98%, 69% and 48%, respectively, and their equipotent concentrations (isoprenaline = 1) were 0.24, 50 and 4, respectively. 3 The positive chronotropic actions of T-0509 and denopamine were antagonized by bisoprolol (3 × 10−8 m), but not by ICI 118,551 (3 × 10−8 m). 4 The intrinsic activity of T-0509 in histamine-contracted tracheae was similar to that of isoprenaline, but its equipotent concentration was 38; the effects of both agents were antagonized by ICI 118,551 (3 × 10−8 m), but not by bisoprolol (3 × 10−8 m). Denopamine and xamoterol did not show any agonist activity on guinea-pig trachea. 5 Denopamine and xamoterol antagonized the positive chronotropic (pA2, denopamine: 6.98, xamoterol: 7.75) and tracheal relaxant (pA2, denopamine: 5.39, xamoterol: 6.25) effects of isoprenaline. 6 Isoprenaline, T-0509 and denopamine, but not xamoterol, contracted the guinea-pig aorta in a decreasing order in the presence of propranolol (10−6 m). 7 Based on the above studies, T-0509 appears to be a highly selective β1-adrenoceptor agonist with full agonist properties, while denopamine and xamoterol appear to be selective, but partial ft -adrenoceptor agonists.