Kinetic analysis of entry into S phase in resting rat 3Y1 cells stimulated by serum. Effects of serum concentration and temperature.

The kinetics of entry into S phase after stimulation of resting 3Y1 cells by serum was examined in relation to serum concentration, temperature and the time at which the serum was withdrawn or at which the temperature was shifted. The kinetics of entry into S phase could be represented not only by a lag phase followed by a negative exponential curve (fit 1), but also by a normal distribution of the reciprocals of the time required for cells to enter S phase (velocities) (fit 2). As the temperature was lowered below 37 degrees C, the exponential slope decreased and the lag period increased (fit 1), and both the mean velocity and its standard deviation decreased (fit 2). As the serum concentration decreased below 10%, the exponential slope decreased without change in the lag period (fit 1), and the mean velocity decreased with increase in the standard deviation (fit 2). The cells which did not enter S phase within 8 h on removal of serum, stopped or delayed entry into S phase. In this case the lag phase was not changed (11 h). When serum was removed just before the end of the lag phase, no effect was seen on the kinetic curve. When the temperature was shifted at any time, including after the lag phase, the characteristics of the kinetic curve (lag phase, synchrony) changed. These facts indicate that there is a serum-non-requiring, but temperature-dependent period before S phase. Most of the asynchrony in entry into S phase under conditions of low serum seems to be generated during the serum-requiring period presumably by the random transition to the state in which cells are committed to enter S phase or by the variability of reaction rates at unpredictable times due to undeterministic effects.