Changes in growth characteristics and macromolecular synthesis on recovery from severe hypoxia.

Chinese hamster ovary cells subjected to severe hypoxia stop growing. When oxygen was reintroduced growth resumed, but at a slower rate. The longer the hypoxic stress, the slower the recovery growth rate. Six hours of hypoxia caused very little decrease in growth rate while a 24 h period almost halved the rate. Short hypoxic periods resulted in almost no growth lag, while longer periods caused significant lag. Clonogenic survival was 60% after 12 h of hypoxia and rose slowly during recovery, reaching control levels after 60 h. Following 24 h of hypoxia, survival remained around 60% throughout recovery. The cell cycle distribution after hypoxia was similar to that of aerobic cultures. After 4-6 h of recovery, a subpopulation of cells entered S phase, and reached G2 by 12 h. During this time few G2-M cells divided. With longer recovery, cells much larger than aerobic cells emerged, containing greater than 4C DNA content and enhanced amounts of RNA. When these cells were isolated, they exhibited slightly slower growth kinetics, greatly lengthened lag time and decreased survival when compared to aerobic cells or the smaller cells. Most of the extra DNA and RNA was lost within one cell cycle.