Thymidine secretion by cultured chicken embryo fibroblasts and NIH/3T3 cells: quantification and time course

Cultured chicken embryo fibroblasts (CEF) as well as mouse NIH/3T3 fibroblasts secrete substantial amounts of thymidine into the medium. The rate of secretion is maximal when the cells become confluent. In the medium of confluent primary cultures of CEF up to 10 microM thymidine was measured. Lower concentrations (1 to 2 microM) were found in media of NIH/3T3 cultures. In media conditioned by NIH/3T3 cells, the degradation product thymine was also found, which was not detectable in the sera and the media of CEF. Moderate inhibitory effects on the growth of CEF and NIH/3T3 cells (at least 10% with a relatively large range of variation) indicated that thymidine (2 to 5 microM) might contribute to cellular homeostasis. The secretion of high amounts of endogenous thymidine into cell culture media may falsify the assessment of DNA synthesis with labeled thymidine; thus a change of the medium is recommended when performing a thymidine pulse.