Ascorbate uptake by isolated rat alveolar macrophages and type II cells

Abstract

Studies were conducted to measure intracellular ascorbate content and to characterize ascorbate uptake in three fractions of isolated rat pneumocytes (i.e., alveolar macrophages, alveolar type II epithelial cells, and another fraction of small pneumocytes that contains neither macrophages nor type II cells). When cells are incubated in medium containing 0.1 mM ascorbate (i.e., the concentration normally found in plasma), intracellular ascorbate concentrations are 3.2 mM in alveolar macrophages and type II cells and 0.9 mM in other lung cells; ascorbate influx is 1.5 nmol . 10(7) cells-1 . h-1 for alveolar macrophages, 0.24 nmol . 10(7) cells-1 . h-1 for type II cells, and very slow in other pneumocytes. Ascorbate influx displays saturation kinetics in both alveolar macrophages (K1/2 = 2 mM; Vmax = 32.2 nmol . 10(7) cells-1 . h-1) and type II cells (K1/2 = 5 mM; Vmax = 14.2 nmol . 10(7) cells-1 . h-1). After correction for differences in the membrane surface areas of these two types of lung cells, the rates for maximum ascorbate influx (Vmax) are similar in alveolar macrophages and type II cells. In addition, ascorbate uptake by alveolar macrophages and type II cells is dependent on metabolic activity and extracellular sodium. In contrast, ascorbate uptake in other lung cells does not exhibit saturation kinetics and is not dependent on metabolism or sodium. Thus alveolar macrophages and type II cells possess an energy-dependent cotransport system for ascorbate and sodium influx. The high ascorbate content and the existence of a specialized transport mechanism for ascorbate uptake may explain the relative resistance of alveolar macrophages and type II cells to oxidant injury.