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).
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