Circadian rhythms of specific airway conductance and bronchial reactivity to histamine: the effects of parasympathetic blockade.

In ten healthy, nonsmoking, non-atopic, young volunteers, specific airway conductance and bronchial response to aerosolized histamine were measured plethysmographically at intervals of 4.8 h during two periods of 24 h, i.e. one day without, the other with, a parasympatholytic aerosol (0.20-0.24 mg ipratropium bromide) inhaled 1 h before each measurement, in order to determine the role of the parasympathetic innervation in the circadian rhythms of the airways. Specific airway conductance and bronchial reactivity showed clear circadian variations with corresponding peak times (16.11 and 04.41 h, respectively). Topical vagal blockade markedly increased specific conductance and resulted in a significant reduction of its rhythm amplitude, whereby the strong correlation between specific conductance and heart rate was significantly diminished. On the other hand, bronchial reactivity to histamine was lowered without flattening of its circadian rhythm. It is concluded that central parasympathetic outflow is an essential factor for the circadian rhythm of bronchial tone and, thus, for the increase in bronchial resistance at night.