Ionic content and regulation of cellular volume in rat alveolar type II cells

Alveolar type II epithelial cells were isolated by elastase digestion of rat lungs and purified by centrifugal elutriation. This method yields an enriched fraction of 1 x 10⁷ cells/rat containing 85% pure type II pneumocytes. Purified type II cells exhibit a high rate of oxygen consumption, 215 nmol O₂.10⁶ cells^-1.h^-1, which is unaffected by the addition of succinate. Type II cells contain 2 μl H₂O/10⁷ cells and are approximately 325-330 μm³ in volume. These pneumocytes contain 107 mmol K/l cell H₂O and 70 mmol Cl/l cell H₂O. In addition, type II cells have a high Na content, i.e., 156 mmol/l cell H₂O. However, most of this Na is bound with only 33% being exchangeable. Therefore, the cytoplasmic concentration of free Na is 51 mmol/l cell H₂O. Na uptake is very rapid in type II cells, i.e. specific activity equilibrium is reached in 15 min with a half time of 5 min. In addition to this large passive transport of Na, type II cells exhibit a highly active Na-K pump involved in the regulation of cellular volume. Volume regulation is inhibited by a decrease in temperature (2° C) and by the addition of ouabain (10^-4 M), Hg (10^-5 M), or Cu (10^-3 M).