Regulation of S49 lymphoma cell growth by cyclic adenosine 3':5'-monophosphate. 
S49 lymphoma tissue culture cells arrest in the G1 phase of the cell cycle when treated with agents that elevate endogenous cyclic adenosine 3':5'-monophosphate (cAMP), such as cholera toxin or exogenously added active congeners of cAMP such as N6,O2'-dibutyryl cyclic adenosine 3':5'-monophosphate (Bt2cAMP).  This phenomenon requires that cells contain the appropriate receptors: Mutant cells deficient in adenylyl cyclase fail to arrest in response to cholera toxin, and another mutant that lacks cAMP-dependent protein kinase does not respond to cholera toxin or to Bt2cAMP.  The size distribution of cell populations treated with Bt2cAMP changes in a manner that reflects only the perturbation of cell cycle distribution.  Arrested G1 cells in particular have the same volume as the G1 cells of an exponentially growing population.  When G1 cells that have been arrested by Bt2cAMP are grown in fresh medium free of Bt2cAMP, they begin to reenter S phase after a delay of about 6 hr and do so with pseudo-first-order kinetics, with a half-life of 5 hr.  These and other properties previously described suggest that cAMP regulates S49 cell growth by physiologically significant rather than artifactual mechanisms.