Increase of peak expiratory flow by atropine is dependent on circadian rhythm

Purpose: To examine whether the bronchodilatory effect of atropine differs in the evening from the morning. Methods: Thirteen adult healthy volunteers with no oral medication intake w ere studied. At 1600, peak expiratory flow (PEF) was measured three times, and the highest value recorded. Subsequently, the volunteer received 0.01 m g.kg(-1) atropine im, and the PEF was measured every 30 min for 180 min. On a different day, at 0400, the effect of atropine on the PEF was measured a gain in the same way. Results: The PEF values before atropine at 1600 and 0400 were 485 +/- 92(35 0-730) and 458 +/- 76 (340-600) 1 min(-1),(P < 0.05). There was no differen ce in PEF values between the 1600 and 0400 time courses after atropine. The PEF value was increased only at 90 min at I 600 (P = 0.0012), but at 30, 6 0, 90 and 120 min at 0400 (P = 0.0001), Conclusion: Atropine administration has a weak bronchodilatory effect in th e evening, but a stronger effect in the morning. Airways are narrower in th e morning than in the evening, and this change is inhibited by atropine suc h that the PEF values are restored to those observed in the evening.