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Flow cytometric analysis of G1 exit kinetics in asynchronous L1210 cell cultures with the constant transition probability model.

Following addition of mitotic inhibitors to asynchronous cultures of mouse lymphoid cell line L1210, the rate of cell exit from G1 phase accelerates during the first 2–3 hr then declines with apparent first order kinetics. The simplest mathematical model that calls for biphasic G1 exit kinetics is the two-state model with constant transition probability. It posits an exponential distribution of residence times in the first state, which is wholly contained in the G1 phase, and a fixed duration, Tb, for the rest of the cycle. 
 
 
 
We derive the distribution of expected times-to-division predicted by the model for a population in exponential steady state growth, and integrate this to obtain the predicted G1 exit curve for the blocked population as a function of two parameters, Tb and the population doubling time, Td. 
 
 
 
The experimental exit curves were fractions of the population remaining below an early S phase DNA content threshold, as a function of time after adding Colcemid, vincristine or vinblastine to exponentially growing cultures. Doubling times were determined from the rate of accumulation of blocked cells in G2 + M. An iterative method was then used to obtain a least-squares fit of Tb and an observational parameter, Tt, representing the average time-to-division of cells at the chosen DNA threshold. 
 
 
 
The fitted G1 exit curves match the data well, showing that the constant transition probability model is a useful mathematical description of the G1 kinetics of these cells. The method described is a convenient way to estimate the parameters of this model for asynchronous, exponentially growing populations.