Guinea pig lung resistance shows circadian rhythmicity not influenced by ozone

Increased circadian variability of airway caliber is a key feature of asthmatic patients, but it has not been addressed in animal models of asthma. Furthermore, animal studies on circadian rhythmicity of airway resistance are very scanty. We used a plethysmographic method for unrestrained guinea pigs to monitor a lung resistance index (iRL) during 24 h. We found circadian variability of iRL values, which were fitted by a sinusoidal curve. Acrophase and bathyphase, characterizing the timing of narrowest and widest airway caliber, respectively, were found at 02:03, and 15:34 h. iRL values at these time-points were statistically different (P < 10(-5)). Moreover, average resistance during the dark period was significantly higher (P < 0.0001) than during the light period. Immediately after an acute ozone exposure (3 ppm for 1 h) an increase in iRL was demonstrated (P < 0.01), which lasted for 2 h, and tended to remain high for the next hour. After guinea pigs recovered from this obstruction, the circadian rhythm and variability of airway caliber were unaffected. Our results show that a circadian rhythm of iRL takes place in guinea pigs, greatly resembling what occurs in humans, and that ozone exposure causes a transient airway obstruction, but fails to reproduce the increased variability of airway caliber observed in asthmatic patients.