Effects of conditioned medium on nucleoside uptake, cell cycle progression and apparent DNA repair.

Cells in culture can condition the medium they grow in so as to inhibit cellular uptake of nucleosides. Some conditioned media specifically block the uptake of cytosine arabinoside and deoxycytidine, but not other deoxyribonucleosides; some also block thymidine uptake; some have no effect. Detransformation of cells by sodium butyrate can alter their medium-conditioning properties. The inhibition of uptake acts at the level of nucleoside phosphorylation; it is not affected by inhibitors of nucleoside degradation. Cell cycle progression is also blocked by conditioned medium. Inhibition of nucleoside uptake by conditioned medium can cause serious errors in estimations of the incision step of DNA repair, or of DNA replication.