Effects of capsaicin pretreatment on expiratory laryngeal closure during pulmonary edema in lambs

The present study, performed in nonsedated, conscious lambs, consisted of t wo parts. In the first part, we 1) examined for the first time whether a re spiratory response to pulmonary C-fiber stimulation could be elicited in no nsedated newborns and 2) determined whether this response could be abolishe d by capsaicin pretreatment. Then, by using capsaicin-desensitized lambs, w e studied whether pulmonary C fibers were involved in the sustained, active expiratory upper airway closure previously observed during pulmonary edema . Airflow and thyroarytenoid and inferior pharyngeal constrictor muscle ele ctromyographic activities were recorded. In the first set of experiments, a 5-10 mu g/kg capsaicin bolus intravenous injection in seven intact lambs c onsistently led to a typical pulmonary chemoreflex, showing that C fibers a re functionally mature in newborn lambs. In the second series of experiment s, eight lambs pretreated with 25-50 mg/kg subcutaneous capsaicin did not e xhibit any respiratory response to 10-50 mu g/kg intravenous capsaicin inje ction, implicating C fibers in the response. Finally, in the above capsaici n-desensitized lambs, we observed that halothane-induced high-permeability pulmonary edema did not cause the typical response of sustained expiratory upper airway closure seen in the intact lamb. We conclude that functionally mature C fibers are present and responsible for a pulmonary chemoreflex in response to capsaicin intravenous injection in nonsedated lambs. Capsaicin pretreatment abolishes this reflex. Furthermore, the sustained expiratory upper airway closure observed during halothane-induced pulmonary edema in i ntact nonsedated lambs appears to be related to a reflex involving stimulat ion of pulmonary C fibers.