We have examined the effect of prior bronchoconstriction on the bronchial responsiveness to inhaled histamine in nine normal subjects. The airway response to increasing concentrations of histamine aerosol was assessed by measurement of specific airways conductance (sGaw) in a body plethysmograph. The threshold provocative dose of histamine needed to cause a 35% fall in starting sGaw (PD35) and the steepest slope of the response were measured from cumulative log dose response curves. Histamine challenges were performed in duplicate after premedication with 0.9% sodium chloride (control) or methacholine aerosol on separate days. The mean starting sGaw did not change significantly after inhalation of 0.9% sodium chloride but methacholine caused a mean reduction in sGaw of 42%. Mean control PD35 values did not differ significantly from mean PD35 values after methacholine. The mean steepest slope of the response after methacoline was 47% lower than the mean control value. There was a significant linear relationship between starting sGaw and the steepest slope for the control and for the methacholine premedicated challenges. The reduction in slope after methacholine was accounted for by the fall in starting sGaw. Because histamine PD35 was not altered by prior bronchoconstriction, it is concluded that the bronchial hyperresponsiveness of asthmatic subjects to non-specific bronchoconstrictor stimuli is unlikely to be a direct consequence of their low starting airway calibre.
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