Effect of an inhaled histamine H3‐receptor agonist on airway responses to sodium metabisulphite in asthma.

Histamine H3‐receptor agonists inhibit excitatory neuro‐transmission in human and guinea‐pig airways. Since neural bronchoconstriction may be important in asthma we have studied the effect of a specific H3‐ receptor agonist R‐alpha‐methylhistamine (alpha MeHA) on bronchoconstriction induced by the inhaled irritant sodium metabisulphite (MBS) in six mild asthmatic subjects in a randomised double‐blind crossover study. Subjects received either alpha MeHA, 10 mg (as a chloride salt) or matched placebo (P) and were then challenged with doubling concentrations of MBS (0.3‐80 mg ml‐1) nebulised from a dosimeter at 5 min intervals with measurement of specific airway conductance (sGaw) and FEV1 at 2 and 4 min respectively after each inhalation. There was no effect of alpha MeHA on baseline airway calibre and the log concentrations of MBS required to lower sGaw by 50% (log PC50) and FEV1 by 20% (log PC20) were not significantly different after alpha MeHA when compared with placebo, suggesting that selective stimulation of airway H3‐receptors does not inhibit MBS‐induced bronchoconstriction.