1. Magnesium is a dietary cation with a wide range of actions of potential relevance to asthma. 2. To determine the dose-response relationship and time-course of the effect of inhaled magnesium sulphate on the airway, we have studied the effect of 0, 90, 135, 180 and 360 mg of magnesium sulphate given by nebulizer on specific airways conductance (sGaw) in 20 normal subjects, and forced expiratory volume in one second (FEV1), forced vital capacity (FVC), flow at 25% forced vital capacity (Vmax25) and peak expiratory flow (PEF) in 19 asthmatic subjects. 3. On five occasions after baseline measurements of airway calibre, one of the five doses of magnesium sulphate in 3 ml normal saline was administered by nebulizer in a randomized, double-blind design. Measurements of sGaw or FEV1, FVC, Vmax25 and PEF were made at 5 and 10 min after nebulization and at 10 min intervals thereafter up to 90 min. 4. There was no significant difference in the mean area under the curve (AUC) for change from baseline in sGaw or maximum increase from baseline between doses in normal subjects. 5. In asthmatic subjects there was no significant difference in the mean AUC for change from baseline in FEV1, FVC or Vmax25 when compared between doses by analysis of variance. There was a difference in the mean AUC for change from baseline in PEF between doses (ANOVA P for all groups 0.052) but this can be explained by a detrimental effect of the maximum dose of magnesium sulphate. 6. It would appear that inhaled magnesium does not act as a bronchodilator in normal or asthmatic subjects.
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