We have recently shown that ouabain, an inhibitor of Na+/K(+)-adenosine triphosphatase, causes contraction of bovine and human airways in vitro, and that amiloride causes relaxation and inhibits receptor-operated contraction in bovine trachealis. 2. To determine whether such drugs alter bronchial reactivity in vivo, we have studied the effect of oral digoxin (an inhibitor of Na+/K(+)-adenosine triphosphatase) and oral and inhaled amiloride on bronchial reactivity to histamine in three double-blind, placebo-controlled studies. 3. Histamine reactivity was measured as the provocative dose causing a 20% reduction in the forced expiratory volume in 1 s (PD20FEV1) or, when normal subjects were included, the provocative dose causing a 35% reduction in the specific airways conductance (PD35sGaw); the results are given as geometric mean values. 4. In study 1, 13 atopic asthmatic subjects were given 20 mg of oral amiloride or placebo on separate days. Two hours after the drug, the geometric mean PD20FEV1 for histamine was 0.43 mumol after amiloride and 0.54 mumol after placebo (95% confidence intervals for the difference: 0.9 to -0.2 doubling doses of histamine; P = 0.2). 5. In study 2, six normal and 24 atopic asthmatic men inhaled 10 ml of 10(-2) mol/l amiloride or diluent control in a crossover study. The mean values of PD35sGaw for histamine immediately after inhalation of amiloride and placebo were 3.0 mumol and 4.3 mumol, respectively, in the normal subjects (95% confidence intervals for the difference: -0.53 to 1.52 doubling doses, P = 0.2), and 0.33 mumol and 0.29 mumol in the asthmatic subjects (95% confidence intervals for the difference: -0.95 to 0.57 doubling doses; P = 0.6).(ABSTRACT TRUNCATED AT 250 WORDS)
