EFFECTS OF LUNG-VOLUME ON AIRWAY-RESISTANCE DURING INDUCED CONSTRICTION IN PAPAIN-TREATED RABBITS

It has been reported that both the elasticity of the cartilage and air way-parenchymal interdependence can modify shortening of the airway sm ooth muscle and airway narrowing during induced constriction. We hypot hesized that induced softening of the cartilage could alter airway com pliance and/or the forces of mechanical interdependence, resulting in an increased degree of airway narrowing in response to a contractile s timulus. To test this hypothesis, we compared the effects of changing lung volume on airway resistance (Raw) under baseline conditions and d uring methacholine (MCh)-induced constriction in papain-treated (n = 6 ) and control rabbits (n = 6). With use of the alveolar capsule techni que, Ram was directly measured under baseline conditions at different levels of end-expiratory transpulmonary pressure (Ptp = 4-12 cmH(2)O). Then, aerosolized MCh was delivered (0.2-25 mg/ml) and measurements w ere performed at different levels of Ptp (4 and 12 cmH(2)O). From meas ured tracheal flow and tracheal and alveolar pressure in open-chest an imals during mechanical ventilation (tidal volume = 6 ml/kg, breathing frequency = 1 Hz), we calculated Raw by subtracting tissue resistance from lung resistance. Papain treatment significantly increased Raw bo th under baseline conditions and after induced constriction. We found that increasing Ptp decreased Raw before and after MCh in both groups; however, the effects of changing Ptp on Raw were less in papain-treat ed animals. These observations suggest that both cartilage elasticity and mechanical interdependence are important-determinants of airway sm ooth muscle shortening. The observation that volume dependence of Raw was less in papain-treated animals is consistent with the hypothesis t hat papain effects significant-changes in the parenchymal attachments.