BRONCHOMOTOR RESPONSES TO HYPOXIA AND HYPERCAPNIA IN DECEREBRATE CATS

Decerebrate animals are often used in investigations of the control of breathing because anesthesia-induced depression of respiratory reflex es is absent. We therefore investigated the level of tone and responsi veness of airway smooth muscle in seven decerebrate, paralyzed, and ve ntilated cats. Specifically, we measured the changes in pulmonary resi stance (RL) and dynamic pulmonary compliance (CLdyn) in response to hy poxia and hypercapnia. All cats responded to hypoxia (similar to 10% O -2 in N-2) with significant increases (mean 49%, range 5-156%) in RL f rom a mean control value of 0.0197 +/- 0.0081 (SD) cmH(2)O .ml(-1).s. During inhalation of 5% CO2 in O-2, RL increased significantly (mean 5 9%, range 16-135%) from a mean control value of 0.0190 +/- 0.0056 cmH( 2)O.ml(-1).s. Decreases in CLdyn during hypoxia and hypercapnia were m uch smaller, averaging -9 and -11%, respectively. After atropine was a dministered, average control RL fell 50%, from 0.0269 to 0.0134 cmH(2) O.ml(-1).s (P < 0.05; n = 4). Hypoxic and hypercapnic gas mixtures did not affect pulmonary mechanics after atropine was administered. In th ree cats, oscillations of RL were synchronized to phrenic activity but only at low respiratory frequencies (similar to 12 cycles/min), indic ating that airway smooth muscle responded slowly to vagal input. Pento barbital sodium, like atropine, reduced control RL in three cats. Thes e cats lost their bronchoconstrictor response to hypercapnia but had a ugmented responses to hypoxia compared with preanesthetic responses. W e conclude that decerebrate cats possess resting bronchomotor tone and retain their responsiveness to hypoxia and hypercapnia. Thus the dece rebrate cat is a useful model for studying the control of tracheobronc hial smooth muscle.