ACTIVE-TRANSPORT AND PASSIVE LIQUID MOVEMENT IN ISOLATED-PERFUSED RATLUNGS

The isolated perfused liquid-filled rat lung in a ''pleural bath'' was the model used to study liquid exchange across the lung epithelium. A ctive transport and passive solute movement between the air space, the vascular perfusate, and the bath result in concentration changes of t he three markers (Evans blue-tagged albumin, Na-22+, and [H-3]mannitol ) instilled in the air space. A mathematical model was developed to es timate the active and passive solute transports and to interpret the r esults. Rat lungs were perfused at left atrial and pulmonary arterial pressures of 0 and 8 mmHg, respectively. Six rat lung experiments were conducted at 37-degrees-C and six at 4-degrees-C. The normothermic ex periments demonstrate that active transport accounts for 26% of the Na + movement out of the air space (17.3 +/- 0.7 nm/s) and that passive m echanisms account for the remaining 74% (48.0 +/- 5.7 nm/s). Hypotherm ia inhibits lung liquid clearance but does not affect passive solute m ovement, suggesting that lung liquid clearance is effected by active N a+ transport mechanisms.