MTX-block during S-phase in BrdU-treated cells depends on species and medium used

Stimulated peripheral blood cells from humans (Homo sapiens), cattle (Bos taurus), river buffaloes (Bubalus bubalis), and sheep (Ovis aries) were cultured in three different mediums, also differing in their respective quantities of folic acid. MTX (0.5 microgram/ml) and BrdU (20 micrograms/ml) were simultaneously added to cell cultures 24 h before harvest and removed 18 h later. The MTX-block in late S-phase was checked by counting the number of cells which replicate once (G + C bands in both chromatids, M1-cells) or twice (one chromatid stained = SCE, M2-cells) in the presence of BrdU. Compared with the control (with BrdU and without MTX), the cellular responses varied within and between species in relation to the medium used.