Effects of inhibition of RBC HCO3-/Cl- exchange on CO2 excretion and downstream pH disequilibrium in isolated rat lungs.

To determine the effects of the speed of the erythrocyte membrane chloride shift on pulmonary gas transfer, CO2 exchange and the kinetics of pH equilibration were measured with isolated rat lungs perfused with 20% suspensions of human erythrocytes. The lungs were ventilated with room air, and the inflowing perfusate blood gases were similar to those in mixed venous blood in vivo. All experiments were performed at 37 degrees C. Rates of CO2 excretion were determined by measuring the fraction of CO2 in mixed expired gas in a steady state. The time-course of extracellular pH equilibration in the effluent perfusate was measured in a downstream stopflow pH electrode apparatus. CO2 excretion was reduced by approximately 16% when the lungs were perfused with suspensions containing erythrocytes whose HCO-3/Cl- exchange rates was inhibited, compared with CO2 excretion when the lungs were perfused with normal erythrocyte suspensions. A fall of 0.06 in effluent perfusate extracellular pH was noted during perfusion with inhibited erythrocyte suspensions, in contrast to no observable downstream pH change during perfusion with normal erythrocyte suspensions. These results are in close agreement with the predictions of a theoretical model. Our observations suggest that CO2 transfer in capillary beds will be adversely affected in vivo when the rate of the erythrocyte HCO-3/Cl- exchange is abnormally low. Since a number of commonly used drugs are known to inhibit the chloride shift in human erythrocytes, these findings may have important clinical implications, especially in patients with impaired lung function.

Evidence for active Na+ transport by cultured monolayers of pulmonary alveolar epithelial cells.

Primary cultured type II alveolar epithelial cells grown to confluence on nonporous surfaces form many small fluid-filled hemicysts or domes. These domes are generally thought to result from active transport of solutes from the medium above the cell monolayer to the substratum, with water following passively. We have investigated the characteristics of active transport by primary cultured monolayers of type II alveolar epithelial cells from rat lungs. Changes in dome density were measured after exposure to metabolic inhibitors, Na+ or Cl- transport inhibitors, and low-Na+ or low-Cl- culture media. Metabolic and Na+ transport inhibitors, and low-Na+ medium, lead to disappearance of domes, whereas Cl- transport inhibitors and low-Cl- medium seem to have no effect on dome density. These results suggest the presence of a Na+-dependent active transepithelial transport process across the monolayer, which is responsible for the formation of domes. This finding implies that absorption of fluid by mammalian alveolar epithelium in vivo may be important in the maintenance of normal lung fluid balance.