Differential paraquat uptake and redox kinetics of rat granular pneumocytes and alveolar macrophages

Possible causes for differences in cellular susceptibility to paraquat (PQ) toxicity were investigated with isolated lung cells and perfused lungs. Uptake of radiolabeled PQ was measured as a function of both time and concentration (1 x 10^-6-1 x 10^-3 M). With alveolar macrophages, uptake was linear with concentration but with time reached a plateau as internal approached external concentration. With granular pneumocytes, uptake was linear with time but nonlinear with external concentration. Accumulation above external concentration occurred and was prevented by inhibitors of ATP synthesis. Apparently, PQ can enter both cell types by diffusion, but granular pneumocytes can also accumulate PQ by an energy-dependent process. With lungs, uptake was nonlinear with time; in the 1st hr a plateau occurred when the intracellular approached extracellular (1 x 10^-5 M) concentration, but a slower second process resulted in a 6-fold accumulation by 4 hr. The effect of PQ on NADPH oxidation was measured with the supernatant fraction (12000 x g, 10 min) from cells and lungs. The maximal rate was about 2 times higher (6.8 ± 0.8 nmol/mg of protein^. min) in granular pneumocytes than in alveolar macrophages, but the estimated kinetic constants were similar (K(m-NADPH) ~ 5 x 10^-6 M; K(m-PQ) ~ 2 x 10^-4 M). The results suggest that PQ-dependent NADPH oxidation rates could differ markedly between cell types at low PQ concentration because of energy-dependent transport into granular pneumocytes but would be fairly close at high PQ concentration where diffusion predominates. These conclusions suggest a mechanistic explanation for how differences in the toxic effects of PQ exposure might occur over a range of PQ concentrations.