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The sensitivity of Chinese hamster ovary cells to ethidium bromide, a deoxyribonucleic acid (DNA)-intercalating cationic dye, was reduced in the presence of caffeine, dimethylsulfoxide (DMSO) or 3-aminobenzamide (3AB). As drug resistance is frequently attained by decreased permeability of the drug, the effects of caffeine, DMSO and 3 AB on the intracellular accumulation of ethidium were examined. These chemicals were found to decrease the intracellular dose of ethidium via their suppressing effects on the dye uptake with no effect on the efflux. The dye uptake was also suppressed by ouabain and K+, suggesting that ethidium was taken up by the K+ transport system. Although the exact mechanism implicated in the inhibitory effects of caffeine. DMSO and 3AB on the dye uptake is not yet clear, these chemicals may interfere with this transport system. The rate of decrease in intracellular dose caused by caffeine, DMSO and 3 AB correlated well with that of the reduction in respective cellular sensitivity. This strongly suggests that the decrease in intracellular dye dose in the presence of these chemicals is mainly responsible for the reduced sensitivity. Caffeine, DMSO and 3 AB caused a decrease in the binding of ethidium to DNA in vitro. The release of ethidium from DNA molecules may lessen DNA damage, thereby contributing to the reduction in cellular sensitivity to ethidium. However, correlation between inhibitory effects of these chemicals on the dye binding and those on cellular sensitivity is not as good as that between the uptake and the sensitivity. This indicates that effects on dye binding to DNA may not be a major reason for the reduction of cellular sensitivity to ethidium.

Decrease in sensitivity to ethidium bromide by caffeine, dimethylsulfoxide or 3-aminobenzamide due to reduced permeability.