INHIBITION OF NA-K-ATPASE ACTIVITY AFTER PROLONGED HYPOXIA IN AN ALVEOLAR EPITHELIAL-CELL LINE

Exposure to alveolar hypoxia may induce acute pulmonary edema. Because the vectorial sodium transport by alveolar epithelium represents an i mportant mechanism for alveolar edema clearance, we examined whether h ypoxia affects Na-K-ATPase activity in cultured SV40-transformed rat a lveolar type II cells (SV40 ATII cells). Hypoxic exposures (0 or 5% O- 2 for at least 12 h) induced a time- and O-2 concentration-dependent d ecrease in ouabain-sensitive rubidium (osRb) influx. Neither the sensi tivity of Rb influx to ouabain nor the maximum velocity of the enzyme measured on crude cell homogenates was affected by hypoxia. The osRb i nflux decrease was independent of hypoxia-induced ATP depletion. Na-K- ATPase inhibition was most likely related to impaired calcium homeosta sis, because 1) calcium influx was increased in hypoxic cells, 2) hypo xia-induced osRb in flux decrease was completely prevented by nifedipi ne (10(-5) M), and 3) osRb influx decreased in normoxic cells incubate d with ionomycin (10(-6) M, 15 min). Furthermore, hypoxia-induced Na-K -ATPase impairment might be due, at least in part, to the endogenous r elease by hypoxic cells of a lipidic factor in extracellular medium, b ecause incubation of normoxic cells with hypoxic cells conditioned med ium (CM), or with the lipidic subphase from hypoxic cells CM, also ind uced a partial decrease in osRE, influx. This decrease was associated with increased calcium influx into normoxic cells and was suppressed e ither by the removal of external calcium or by nifedipine, suggesting that the lipidic factor exerted its inhibitory action on Na-K-ATPase v ia an enhancement of calcium entry. These results indicate that prolon ged hypoxic exposure impairs Na-K-ATPase activity in SV40 ATII cells a nd may therefore decrease the vectorial sodium transport by alveolar e pithelium.