Reabsorption of Solutes and Water from Fluid-Filled Rabbit Lungs

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Abstract
Uncertainty persists concerning the mechanisms responsible for fluid clearance from the lungs after the air spaces become flooded in severe pulmonary edema. In this study, solute and water fluxes were investigated in an isolated, fluid-filled, perfused rabbit lung preparation. These lungs were perfused with physiologic 1.0 or 5.0 g/dl albumin solutions, and the air spaces were flushed and filled with the same solutions. Samples were obtained at intervals from the perfusate, and at the end of 1 or 2 h, fluid was pumped from the trachea into collection tubes. Concentrations of albumin (labeled with Evans blue) in the air space increased by 2.4 .+-. 0.7% (SEM) at 1 h and by 7.0 .+-. 0.8% at 2 h. Approximately half of the increase at 2 h could be attributed to dehydration (as judged by increases in perfusate and air-space Na+ concentration). Because previous studies have indicated that the movement of labeled protein between these compartments is very slow in this preparation, it can be concluded that fluid is being reabsorbed from the air spaces. However, reabsorption appears to be slower in rabbits than in rats and it is not stimulated by terbutaline, an effect observed in other species. Under control conditions, potassium concentrations in the air-space fluid fell from 4.01 .+-. 0.05 (SEM) mEq/L to 3.37 .+-. 0.14 mEq/L at 1 h. Concentrations of K+ in the perfusate rose during this interval from 3.95 .+-. 0.05 mEq/L to 4.39 .+-. 0.08 mEq/L. A decrease in air-space K+ concentrations and an increase in perfusate K+ were also observed during a 2-h period when the lungs were perfused with the 5 g/dl albumin solution. Addition of 10-5 M terbutaline to the airway and perfusion solutions resulted in an increase in the concentration of K+ in the air space from 3.97 .+-. 0.05 mEq/L to 4.49 .+-. 0.13 mEq/L. This action of terbutaline was largely blocked by 10-5 M propranolol. These studies suggest that fluid may be transported out of the air spaces of rabbit lungs by focus other than hydrostatic pressure or protein concentration differences. In addition, electrolyte transport across the pulmonary epithelium may differ between species.

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