Selective dopamine D
receptor agonist induced airway smooth muscle relaxation after an acetylcholine EC
contraction in intact guinea pig trachea. The dopamine D1-like receptor agonist, A68930, in a concentration-dependent manner, relaxes Ach-induced contractions in guinea pig tracheal rings. (A) Representative tension tracing in guinea pig tracheal ring illustrating relaxation of Ach (EC50) contraction by A68930 (1 - 100 μM) in the presence or absence of dopamine D2 receptor selective antagonist L-741626 (10 μM). (B) The effect of cumulative concentration of A68930 (1 - 100 μM) on Ach (EC50)-induced guinea pig tracheal ring contraction in the presence or absence of L-741626 (10 μM). Open circles: A68930 alone. Filled circles: L-741626 + A68930. Filled triangle: time control. Means ± SEM. *P < 0.05, **P < 0.01 and ***P < 0.001 compared with time control. #P < 0.05 compared with A68930 alone. N = 6.