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

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Abstract
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 10(7) cells/rat containing 85% pure type II pneumocytes. Purified type II cells exhibit a high rate of oxygen consumption, 215 nmol O2 . 10(6) cells-1 . h-1, which is unaffected by the addition of succinate. Type II cells contain 2 microliters H2O/10(7) cells and are approximately 325–330 micrometers 3 in volume. These pneumocytes contain 107 nmol 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 and 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 degrees C) and by the addition of ouabain (10(-4) M), Hg (10(-5) M), or Cu (10(-3) M).

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