BACKGROUND--Inhaled frusemide but not bumetanide, another loop diuretic, reduces bronchial responsiveness to sodium metabisulphite (MBS). To investigate whether the effect of frusemide could be mediated through mechanisms other than Na+/K+/Cl- cotransporter inhibition, the effects of amiloride--an inhibitor of sodium channels in the airway epithelium--and of acetazolamide--a specific inhibitor of carbonic anhydrase--against MBS challenge were studied. METHODS--In two separate randomised double blind placebo controlled studies, 10 subjects with mild asthma attended on four separate occasions to inhale 7.5 mg amiloride or matched placebo, and 500 mg acetazolamide or placebo, immediately before MBS challenge. The concentration of MBS required to cause a 20% fall in baseline FEV1 (PC20) was measured. RESULTS--Amiloride and acetazolamide had no effect on baseline FEV1. Amiloride had no effect against MBS challenge, but acetazolamide increased -log PC20 from a mean (SE) of 0.75 (0.09) to 0.98 (0.06) representing a 0.77 (0.24) doubling dose increase. CONCLUSIONS--These results suggest that carbonic anhydrase activity in the airways, but not sodium flux, modulates bronchial responsiveness to MBS challenge. The action of frusemide is not likely to involve inhibition of carbonic anhydrase activity.
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