The kinetics of serum-induced initiation of DNA synthesis in BHK 21/C13 cells, and the influence of exogenous adenosine.

After the re-addition of serum in the presence of adenosine (25 muM), the entry of quiescent, serum starved BHK 21 cells into DNA synthesis follows first order kinetics after a well defined lag period of eight hours, and with a rate constant dependent on serum concentration. Initiation of DNA synthesis under these conditions can therefore be considered to be a random event occurring with a "Transition Probability" determined by the serum concentration. In the presence of adenosine, the change of Transition Probability following the addition of serum occurs abruptly. In the absence of exogenous adenosine, however, the change of Transition Probability after serum addition appears to be both gradual and bi-phasic. The initial changes in the absence of adenosine, though smaller in magnitude, display a similar dependence on serum concentration to the changes occurring in the presence of the nucleoside. In contrast, the secondary gradual increase of Transition Probability in the absence of added purines exhibits a higher serum requirement. It is suggested that the regulation of Transition Probability by serum involves some purine-dependent process, and that in the absence of an exogenous supply this becomes limited by endogenous synthesis which in turn may be dependent on serum concentration.