Assessment of nuclear daunorubicin accumulation after exposure of cells or isolated nuclei to ³[H]-daunorubicin. (A) (⊡) U-937 and (♦) U-A10 cells were exposed to 0 to 500 ng/mL of ³[H]-daunorubicin for 1 hour, and the cells were washed, ruptured, and nuclei purified by pelleting through a sucrose cushion. All isolation steps were performed at 4°C to minimize nuclear drug loss. Nuclear-associated radioactivity was determined by scintillation counting. Shown is a plot of the mean (± the standard error) nuclear daunorubicin accumulation versus extracellular drug concentration for three experiments each performed in duplicate. The differences were statistically significant at each concentration tested (P < .05 for 50 ng/mL; P < .005 for 200 and 500 ng/mL). (B) Nuclei were first isolated by lysing cells in a hypotonic, NP-40–containing buffer and pelleting by centrifugation. They were then exposed to 500 ng/mL of daunorubicin for 15 minutes before sedimenting through silicone oil and radioactivity determined by scintillation counting. Shown are the mean (± the standard error) of three experiments performed in duplicate.