Can nocodazole, an inhibitor of microtubule formation, be used to synchronize mammalian cells? Accumulation of cells in mitosis studied by two parametric flow cytometry using acridine orange and by DNA distribution analysis.

Nocodazole, a temporary inhibitor of microtubule formation, has been used to partly synchronize Ehrlich ascites tumour cells growing in suspension. The gradual entry of cells into mitosis and into the next cell cycle without division during drug treatment has been studied by flow cytometric determination of mitotic cells, analysing red and green fluorescence after low pH treatment and acridine orange staining. Determination of the mitotic index (MI) by this method has been combined with DNA distribution analysis to measure cell-cycle phase durations in asynchronous populations growing in the presence of the drug. With synchronized cells, it was shown that in the concentration range 0.4-4.0 micrograms/l, cells could only be arrested in mitosis for about 7 hr and at 0.04 microgram/ml, for about 5 hr. After these time intervals, the DNA content in nocodazole-blocked cells was found to be increased, and, in parallel, the ratio of red and green fluorescence was found to have changed, showing entry of cells into a next cell cycle without division (polyploidization). It was therefore only possible to partially synchronize an asynchronous population by nocodazole. However, a presynchronized population, e.g. selected G1 cells or metabolically blocked G1/S cells, were readily and without harmful effect resynchronized in M phase by a short treatment (0.4 microgram/ml, 3-4 hr) with nocodazole; after removal of the drug, cells divided and progressed in a highly synchronized fashion through the next cell cycle.