GUINEA-PIG LUNG RESISTANCE SHOWS CIRCADIAN RHYTHMICITY NOT INFLUENCEDBY 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 unr estrained 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 ti ming of narrowest and widest airway caliber, respectively, were found at 02:03, and 15:34 h. iRL values at these time-points were statistica lly different (P < 10(-5)). Moreover, average resistance during the da rk period was significantly higher (P < 0.0001) than during the light period. Immediately after an acute ozone exposure (3 ppm for 1 h) an i ncrease 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 f rom this obstruction, the circadian rhythm and variability of airway c aliber were unaffected. Our results show that a circadian rhythm of iR L takes place in guinea pigs, greatly resembling what occurs in humans , and that ozone exposure causes a transient airway obstruction, but f ails to reproduce the increased variability of airway caliber observed in asthmatic patients. (C) 1998 Elsevier Science B.V. All rights rese rved.