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We studied airway reactivity (AR) to aerosolized histamine, carbachol, and citric acid in lambs 1 mo of age to adulthood. Awake lambs were intubated and studied in a plethysmograph that measured dynamic compliance (Cdyn), resistance of the lung (RL), and functional residual capacity (FRC). Pleural pressure was measured using a Silastic balloon in the pleural space, and airway opening pressure (Pao) was measured using a catheter placed 1-2 cm distal to the nasotracheal tube. At the ages of 1, 3, 5, and 7 mo and adulthood, measurements of Cdyn, RL, and FRC were obtained in 46 sheep (22 males, 24 females). AR to carbachol, histamine, and citric acid was measured in each sheep in randomized order on three separate days by giving increasing concentrations of the drug in a noncumulative fashion. The dose that would have caused a 35% reduction in Cdyn (ED65Cdyn), a doubling of RL (ED200RL), or a 50% increase in FRC (ED150FRC) was calculated. In both males and females, base-line Cdyn increased (r = 0.81, P less than 0.01) with age, as did FRC (r = 0.87, P less than 0.01). There was no significant change in RL in either sex with age or in the group as a whole. There was a significant increase in AR to both histamine and carbachol with increasing age as measured by a decrease in ED65Cdyn (P less than 0.01 and P less than 0.05, respectively) with age. There was no significant change in AR with age as measured by RL or FRC for any of the three bronchoconstrictors tested.(ABSTRACT TRUNCATED AT 250 WORDS)

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