Histamine blocking agents in healthy and asthmatic subjects

We compared the effects of two histamine receptor blocking agents, chlorpheniramine (H₁) and cimetidine (H₂) on the airways of healthy and asthmatic subjects. Eleven healthy subjects and ten asthmatic patients underwent histamine aerosol challenge. A threshold dose (T) for response to histamine was determined for each subject using maximal expiratory flow rates of partial expiratory flow volume curves (MEF40 %[P]). On subsequent study days, the subjects were pre-treated with 8 mg of chlorpheniramine, 300 mg of cimetidine or a lactose placebo. Histamine challenge was performed two hours later with the individual's own T dose and doses one dilution below (T-1) and one dilution above (T+1) that dose. In both asthmatic and healthy subjects chlorpheniramine significantly reduced the bronchoconstrictor responses to histamine (p<0.02 and 0.05, respectively) as measured by MEF40 percent (P) at the T dose. When treated with cimetidine asthmatic patients displayed significantly more bronchospasm at T than with placebo (p<0.035). By contrast, pretreatment with cimetidine did not alter airway responses to histamine in healthy subjects when compared to placebo. We conclude that H₂ receptors mediating bronchodilatation can be demonstrated in asthmatic patients but not in healthy subjects.