Human diploid cells (WI-38) were serially subcultivated at partial pressures of oxygen (Po2) ranging from 5.6 mm Hg to 608 mm Hg. At a Po2 of 5.6 mm Hg, the number of doublings to phase out was less than that of control cells at a Po2 of 137 mm Hg. Cultures grown at Po2's of 24, 49, or 137 mm Hg grew at the same rate and phased out after a similar number of population doublings. Population lifespan was markedly shortened by chronic exposure to elevated Po2's, a phenomenon that was, in part, reversible. d-1-alpha-Tocopherol (10 microgram/ml or 100 microgram/ml) homogenized into the medium at each weekly subcultivation did not extend the lifespan of cells at reduced, ambient, or elevated oxygen tensions. These results indicate that neither oxygen toxicity nor free radical reactions play a significant role in limiting the lifespan of WI-38 cells grown in vitro under ambient oxygen tensions (Po2 137 mm Hg).

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