Responsiveness of and interactions between airways and tissue in guinea pigs during induced constriction

Mechanical interdependence between airways and tissues can modify the magnitude of induced bronchoconstriction. We questioned whether the guinea pig, an animal with abundant airway smooth muscle, would differ from other species in the relative responsiveness of and interactions between airways and tissues. Therefore we induced constriction with aerosolized methacholine (MCh) and partitioned responses into airway and tissue components. We measured tracheal and alveolar pressures using alveolar capsules in open-chest guinea pigs (n = 9) during mechanical ventilation [frequency = 1 Hz, tidal volume = 6 ml/kg, positive end-expiratory pressure (PEEP) = 5 cmH2O] and calculated the resistance of lung (RL), tissue (Rti), and airway (Raw) before and after administration of aerosols of MCh in progressively doubling concentrations (0.063-16 mg/ml). In separate animals (n = 10), measurements were made at 3-13 cmH2O PEEP. After aerosols of saline and MCh (0.125-32 mg/ml), measurements were repeated at 3, 7, and 11 cmH2O PEEP. At submaximal levels of constriction, the airways and lung tissues demonstrated similar responsiveness. Increasing PEEP increased RL and Rti and decreased Raw under baseline conditions. At low concentrations of MCh, increasing PEEP increased RL but decreased RL at the highest concentration. Increases in PEEP significantly increased Rti at all concentrations of MCh but decreased Raw only at 8 mg/ml of MCh. These observations demonstrate that, in guinea pigs, during submaximal constriction, airways and tissues behave similarly; moreover, airway-parenchymal interdependence is important in determining the level of bronchoconstriction.