Alveolar fluid reabsorption is impaired by increased left atrial pressures in rats

Cardiogenic pulmonary edema results from increased hydrostatic pressures across the pulmonary circulation. We studied active Na(+) transport and alveolar fluid reabsorption in isolated perfused rat lungs exposed to increasing levels of left atrial pressure (LAP; 0--20 cmH(2)O) for 60 min. Active Na(+) transport and fluid reabsorption did not change when LAP was increased to 5 and 10 cmH(2)O compared with that in the control group (0 cmH(2)O; 0.50 +/- 0.02 ml/h). However, alveolar fluid reabsorption decreased by approximately 50% in rat lungs in which the LAP was raised to 15 cmH(2)O (0.25 +/- 0.03 ml/h). The passive movement of small solutes ((22)Na(+) and [(3)H]mannitol) and large solutes (FITC-albumin) increased progressively in rats exposed to higher LAP. There was no significant edema in lungs with a LAP of 15 cmH(2)O when all active Na(+) transport was inhibited by hypothermia or amiloride (10(-4) M) and ouabain (5 x 10(-4) M). However, when LAP was increased to 20 cmH(2)O, there was a significant influx of fluid (-0.69 +/- 0.10 ml/h), precluding the ability to assess the rate of fluid reabsorption. In additional studies, LAP was decreased from 15 to 0 cmH(2)O in the second and third hours of the experimental protocol, which resulted in normalization of lung permeability to solutes and alveolar fluid reabsorption. These data suggest that in an increased LAP model, the changes in clearance and permeability are transient, reversible, and directly related to high pulmonary circulation pressures.