The effect of inhaled hexamethonium bromide and atropine sulphate on airway responsiveness to histamine

The degree of protection against inhaled histamine achieved by inhalation of the ganglion blocker hexamethonium bromide plus placebo, hexamethonium plus atropine sulphate, and placebo plus placebo was examined in six atopic subjects, four of whom had current asthma. Hexamethonium was administered until there was systemic evidence of ganglionic blockade with a postural drop in blood pressure of 31 ± 7.5 mm Hg (mean ± SD) (p = 0.01) and an increase in heart rate of 30 ± 3.1 bpm (mean ± SD) (p = 0.01). Atropine was inhaled in a dose (18 mg nebulized during tidal breathing) known to produce systemic inhibition of cardiac and salivary cholinergic (muscarinic) receptors. The airway effects were measured by FEV₁. Hexamethonium caused bronchoconstriction in all four subjects with asthma, which was reversed by atropine. The mean provocation concentration of histamine to provoke a 20% fall in FEV₁ was 2.97 mg/ml after premedication with placebo, it was not different at 2.84 mg/ml after hexamethonium alone, and it increased slightly to 5.31 mg/ml after both hexamethonium and atropine (p = 0.06). The results suggest that the main effect of inhaled histamine is not by reflex bronchoconstriction but rather through stimulation of H₁-receptors on airway smooth muscle. Therefore, histamine hyperresponsiveness in asthma is not primarily caused by a defect in the parasympathetic nervous supply to the airway.