Effect of serotonin on expiratory pulmonary resistance in cats.

In five anesthetized paralyzed cats, mechanically ventilated with tidal volumes of 36-48 ml, the isovolume pressure-flow (IVPF) relationships of the lung were studied under control conditions and during serotonin-induced bronchoconstriction. At the end of a tidal inspiration, airway opening pressure was set between +3 and -15 cmH2O for single tidal expirations. After control measurements, animals were treated with progressively increasing doses of intravenous serotonin (10, 20, 50, and 100 micrograms.kg-1.min-1) and all measurements were repeated at each dose. No animal became flow limited during passive expiration against atmospheric pressure. Disregarding flow-limited segments, IVPF plots for three lung volumes showed that the resistive pressure-flow relationships were curvilinear under all conditions, thus fitting Rohrer's equation. Under control conditions and during the lowest dose of serotonin, the volume dependence of pulmonary resistance (RL) tended to balance its flow dependence so that during lung deflation against atmospheric pressure RL remained nearly constant. However, as bronchoconstriction became more pronounced, RL often increased disproportionately at the lower lung volumes. Changes in expiratory RL with serotonin relative to control values varied according to the flow rates used to make comparisons. The technique used to determine RL will partly determine the results obtained.