Protective effect of methylxanthines against carbachol-induced bronchoconstriction in normal subjects.

The bronchoprotective effect of bioequivalent doses of theophylline (TH; 234 mg) and a combination of TH, proxyphylline (PPH) and diprophylline (DPH) in the proportion 2:3:3 (Neo-Biphyllin, NB; 600 mg) against carbachol-induced bronchoconstriction was studied in 10 healthy non-smokers in a randomised controlled double-blind cross-over trial. The subjects were on a methylxanthine-free diet 4 days prior to and during each study day. Bronchial provocation tests were conducted in the morning and afternoon of the three separate study days--control and two medication days--using a standardised technique. On the control day, the dose of carbachol required to reduce the partial expiratory flow volume at 25% of vital capacity (V25p) by at least 25% was established. A significant protective effect was achieved with both TH (p less than 0.05) and NB (p less than 0.001) as measured by V25p. Bronchoprotection was achieved with low maintenance serum levels of TH.