The influence of serum on lymphocyte cultures. II. Cell cycle specificity of serum action in spleen cells.

Serum exerts several effects in lymphocyte cultures, one of them being manifested very early. The presence of fetal calf serum (FCS) results in a markedly higher thymidine uptake within a few minutes, as compared with serum-free cultures. On the other hand, the uptake of uridine and other purine bases seem to be little influences by serum. Experiments comparing the uptake of thymidine into the cytoplasm or into trichloroacetic acid-precipitable material suggested that the intracellular thymidine pool increases in size when FCS is added. Using a Lineweaver--Burk plot for thymidine and uridine uptake over a 4-h period, no changes in uridine uptake were observed in the presence of FCS; on the contrary, serum induced an increased Vmax for thymidine, whereas Km remained constant. Cytofluorographic quantitation of G0 and G1 cells indicated that cells disappear more rapidly from the G1 phase in the presence of FCS. The addition of hydroxyurea to the cultures prevented this disappearance. The results taken together strongly suggest that serum contains a factor promoting the shift of G1 cells into the S phase. When a 4-h culture period was used, all sera from different species tested at low concentrations appeared to contain this activity.