The influence of exogenous thymidine concentration on the incorporation of labelled thymidine and iododeoxyuridine, and on growth of JU56 cells in vitro

Summary The simultaneous incorporation of tracer amounts of initiated thymidine and 125 I‐5‐iodo‐2' deoxyuridine was measured during short term in vitro incubation of the permanent cell line JU56 in the presence of increasing concentrations of unlabelled thymidine. As unlabeled thymidine concentration increased in the range 5 × 10 −8 M‐4 × 10 −5 M, uptake of both tracers decreased in parallel. The extent of the decrease indicated that a rise in the contribution of exogenous thymidine to DNA synthesis had occurred. At higher concentrations of unlabelled thymidine when DNA synthesis began to decline the incorporation of 125 I‐5‐iodo‐2' deoxyuridine decreased more than that of tritiated thymidine. In an experiment using both tracers in the presence of 10 −6 M unlabelled thymidine it was found that the ratio of the fractions of each tracer taken up from the medium was not significantly different from unity, if a correction was made for the inorganic 125 I contaminating the 125, I‐5‐iodo‐2' deoxyuridine preparation used.