The shape of dose-survival curves for mammalian cells and repair of potentially lethal damage analyzed by hypertonic treatment.

During the usual procedure of testing cell survival by colony-forming ability, repair of potentially lethal damage (PLD) takes place. By incubating the cells in hypertonic suspension a certain part of this repair can be inhibited, leading to an exponential dose-survival curve as expected from the Poisson distribution of lethal events in the cells. If such a hypertonic treatment is performed after increasing intervals following irradiation with X rays, curves with increasing shoulder length are obtained. Quantitative analysis of the kinetics of this repair shows that PLD is repaired for about 1 hr after irradiation by a saturated repair system which eliminates about one lesion per 15 min per cell independent of the applied absorbed dose. PLD not eliminated by this 'fast' system is repaired by an unsaturated system with a time constant of several hours. Repair of PLD after X irradiation proceeds quantitatively in this way in plateau-phase cells suspended in a conditioned medium, which seems optimal for such repair. If these cells are suspended after irradiation in normal nutrient medium a certain fraction of the PLD is transformed into irreparable damage. The final survival after repair in nutrient medium is then identical with that obtained by the usual measurement of colony-forming ability on nutrient agar. This indicates that the shoulder in dose-survival curves for plateau-phase cells is partly due to repair of PLD and partly due to manifestation of this damage during repair time

Growth of cells on solid culture medium : II. Cell physiological data of stationary yeast cells and the initiation of cell cycle in nutrient free buffer solution.

For precise experiments with yeast (or other) cells stationary populations are produced by growth on the surface of a solid nutrient medium. The energy supply to these cells is well known from a former publication. The oxygen supply during growth is analysed here in detail. Different types of cell populations can be produced in this way dependent on the thickness of nutrient medium. If such cells are transferred into a liquid buffer solution cell multiplication can be initiated without any nutrient flux into the cell. This new type of initiation of the cell cycle of G1-cells has to be distinguished from the usual initiation by nutrient supply and from the mechanism of meiotic cell division. The dependence of this cell growth on cell volume, pH-value, oxygen concentration and osmotic pressure is analysed and possibilities to avoid this kind of cell multiplication reaction are discussed