SELECTION OF N-(PHOSPHONACETYL)-L-ASPARTATE RESISTANT CHINESE-HAMSTERMUTANTS IN THE PRESENCE OF THE URIDINE UPTAKE INHIBITOR DIPYRIDAMOLE

In mammalian cells selected in culture for resistance to PALA the CAD gene is amplified and these cells are a widely used model system to st udy gene amplification. Selection of resistant mutants is routinely pe rformed in medium supplemented with dialyzed serum, because the cytoto xic effect of PALA is reversed by uridine, which is contained in serum . We have shown that in Chinese hamster cells dipyridamole reduced uri dine uptake to less than 5% with limited effect on cell survival. More over, in medium supplemented with complete serum and 10 mu M dipyridam ole the toxicity of PALA was similar to that obtained in medium contai ning dialyzed serum. We then used 10 mu M dipyridamole to inhibit urid ine uptake during selection of PALA resistant colonies and found that both the frequency and the type of mutants were as those obtained in t he presence of dialyzed serum. In particular, in the five mutants test ed, the mechanism of resistance of PALA was amplification of the CAD g ene.