AREGU January 47/1

Stephenson, Richard, Ravi M. Mohan, James Duffin, and Tim M. Jarsky. Circadian rhythms in the chemoreflex control of breathing. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 278: R282–R286, 2000.—Mechanisms underlying the circadian rhythm in lung ventilation were investigated. Ten healthy male subjects were studied for 36 h using a constant routine protocol to minimize potentially confounding variables. Laboratory light, humidity, and temperature remained constant, subjects did not sleep, and their meals and activities were held to a strict schedule. Respiratory chemoreflex responses were measured every 3 h using an iso-oxic rebreathing technique incorporating prior hyperventilation. Subjects exhibited circadian rhythms in oral temperature and respiratory chemoreflex responses, but not in metabolic rate. Basal ventilation [i.e., at subthreshold end-tidal carbon dioxide partial pressure (PETCO2)] did not vary with time of day, but the ventilatory response to suprathreshold PETCO2 exhibited a rhythm amplitude of ,25%, mediated mainly by circadian variations in the CO2 threshold for tidal volume. We conclude that the circadian rhythm in lung ventilation is not a simple consequence of circadian variations in arousal state and metabolic rate. By raising the chemoreflex threshold, the circadian timing system may increase the propensity for respiratory instability at night.