Uptake from the Extracellular Fluid

FATE OF ASCORBIC ACID IN THE LUNG 4.1. Uptake from the Extracellular Fluid 

Ascorbic acid is transported in the plasma in association with the protein albumin (see Chapter 4). The reduced form of the vitamin normally predominates 

Ascorbic acid influx displays saturation kinetics in both PAMs (Kj/2 = 2.2 mmol 32.2 nmol/10 cells/hr) and type II pneumocytes (K,/2 = 4.5 mmol max 14.2 nmol/10 cells/hr). After correction for differences in membrane surface areas of these two cell types, the rates of maximum ascorbic acid influx (Vmax) type ff cells is equal to or exceeds that in alveolar macrophages (Castranova et al., 1983b). This transport system is specific for the reduced form of ascorbic acid (Combs, 1992). 

Ascorbic acid uptake by these two cells is dependent on their metabolic activity and extracellular sodium concentration. In contrast, ascorbic acid uptake by the other lung cells does not exhibit saturation kinetics and is not dependent on metabolism or extracellular sodium (Castranova et al, 1983). This implies that uptake of ascorbic acid by other lung cells is a passive process and that the ascorbic acid content of these cells is dependent on the plasma level of ascorbic acid. Exposure of the lung to high concentrations of oxidants (for example, during smoking), which reduce blood ascorbic acid, will inevitably render these cells more susceptible to oxidant injury. 

Studies with metabolic inhibitors indicate that ascorbate influx in type II pneumocytes is more sensitive to inhibition of the electron transport chain and less 

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