Lack of tachyphylaxis to histamine in both moderate and mild asthmatic subjects

While some studies have clearly shown that tachyphylaxis occurs in asthmatic subjects when challenged consecutively with inhaled histamine, others were unable to demonstrate this phenomenon. There is reason to believe that these conflicting findings may be related to the different degrees of bronchial reactivity in the subjects studied. We selected 2 groups of 10 asthmatics on the basis of their degree of bronchial reactivity: a group with PC20 less than 1 mg/ml (moderate asthmatics); and a group with PC20 greater than 2.5 mg/ml (mild asthmatics). Each subject underwent 3 successive histamine challenges, allowing recovery of FEV1 after each to within 5% of the baseline value prior to the first challenge. Test results were recorded as the provoking concentration of histamine needed to produce a 20% fall in FEV1 (PC20). No significant change in histamine reactivity occurred in either group. However, one moderate and two mild asthmatics appeared to develop some tachyphylaxis. We conclude that tachyphylaxis to histamine is not a general phenomenon in asthma.