In-vivo cardiopulmonary recordings

Animals breathed spontaneously through the tracheal cannula. Respiratory flow was measured by a differential pressure transducer (HSE-HA DLP2.5, Harvard Apparatus, Holliston, MA). Respiratory frequency (RF), tidal volume (V_T), expiratory duration (T_E), ABP, and heart rate (HR) were recorded and analyzed using PowerLab and LabChart Pro from ADInstruments (Colorado Springs, CO). Pulmonary chemoreflex responses were measured when intravenous injections of capsaicin (0.75–1.25 μg/kg), a selective transient receptor potential vanilloid receptor 1 (TRPV1) agonist, were administered. At least 15 min was allowed to elapse between two consecutive injections to avoid tachyphylaxis. To determine the intensity of apneic response, the apneic ratio was calculated by dividing the longest T_E occurring within 3 s after the injection by the baseline T_E that was averaged over 10 breaths immediately preceding the injection.