The achilles heel in melatonin: asthma.

Asthma  is a clinical syndrome  characterized by chronic  airway inflammation, airway responsiveness, and expiratory airflow limitation. Nocturnal symptoms and decreases in lung function are common aspects of the asthma clinical syndrome. Nocturnal symptoms also appear to be associated with asthma-related mortality. In addition to its importance to the regulation of human circadian rhythms, an accumulating body of evidence also suggests that melatonin is also involved in the regulation of  smooth  muscle tone.  For  this  reason,  this  study aimed to  evaluate contraction  and relaxation responses  in  tracheal  smooth  muscle  rings  obtained  from  rats  treated  with melatonin. Following administration of melatonin (50mg/kg/day) at the same time every day for 6 weeks,  in vitro organ bath experiments were performed with rat tracheal preparations exposed to contractile (acetycholine and serotonin) and relaxant (theophylline and papaverine) agents. Melatonin treatment strengthened contraction responses, but did not affect relaxation responses in rat tracheal preparations. We think that melatonin might play a role in the pathogenesis of nocturnal asthma. Therefore,  clinicians should  be  aware of  the  importance  of  melatonin  to  nocturnal exacerbation of asthma symptoms and alert asthmatic patients that use exogenous melatonin supplementation of its potential negative effects.