METHACHOLINE-INDUCED BRONCHOCONSTRICTION AND PHRENIC MOTONEURON ACTIVITY IN DOGS

To evaluate the effect of changes in bronchomotor tone on the firing pattern of single phrenic motoneurons, efferent activity of both the C5 branch phrenic nerve and a single fiber from the contralateral phrenic nerve were studied in dogs anesthetized (chloralose/urethane) and breathing spontaneously (65% O2). Bronchoconstriction was induced by inhalation of methacholine (MCh) aerosol, which increased total lung resistance, decreased dynamic lung compliance and tidal volume. The rate of rise of phrenic activity was significantly increased by the changes in pulmonary mechanics induced by MCh. Single phrenic motoneurons could be separated into early (EOM) and late (LOM) onset types based on the onset time of firing relative to the beginning of whole phrenic nerve activity. The increased phrenic nerve activity was due primarily to the recruitment of LOMs. The recruitment of LOMs consisted of a shift in the time of onset to an earlier portion in inspiration, a longer firing duration, and an increase in the number of spikes per inspiratory cycle. After bilateral cervical vagotomy, all the responses of phrenic nerve to changes in pulmonary mechanics induced by aerosolized MCh were attenuated. These results suggest that phrenic motoneurons activity is modulated by alterations in pulmonary mechanics via a reflex mediated by the vagus nerve and that LOM play an important part in this response.