Isoproterenol increases Na⁺-K⁺-ATPase activity by membrane insertion of α-subunits in lung alveolar cells

Catecholamines promote lung edema clearance via β-adrenergic-mediated stimulation of active Na⁺ transport across the alveolar epithelium. Because alveolar epithelial type II cell Na⁺-K⁺-ATPase contributes to vectorial Na⁺ flux, the present study was designed to investigate whether Na⁺-K⁺- ATPase undergoes acute changes in its catalytic activity in response to β- adrenergic-receptor stimulation. Na⁺-K⁺-ATPase activity increased threefold in cells incubated with 1 μM isoproterenol for 15 min, which also resulted in a fourfold increase in the cellular levels of cAMP. Forskolin (10 μM) also stimulated Na⁺-K⁺-ATPase activity as well as ouabain binding. The increase in Na⁺-K⁺-ATPase activity was abolished when cells were coincubated with a cAMP-dependent protein kinase inhibitor. This stimulation, however, was not due to protein kinase-dependent phosphorylation of the Na⁺- K⁺-ATPase α-subunit; rather, it was the result of an increased number of α-subunits recruited from the late endosomes into the plasma membrane. The recruitment of α-subunits to the plasma membrane was prevented by stabilizing the cortical actin cytoskeleton with phallacidin or by blocking anterograde transport with brefeldin A but was unaffected by coincubation with amiloride. In conclusion, isoproterenol increases Na⁺-K⁺-ATPase activity in alveolar type II epithelial cells by recruiting α-subunits into the plasma membrane from an intracellular compartment in an Na⁺-independent manner.