Abstract

The characteristics of the uptake of 3H-Actinomycin D (3H-AMD) by mammalian cells was studied in vitro. Chang liver (CH) cells accululated 3H-AMD over 100 times above extracellular levels. CH cells accumulated and released 3H-AMD at a slow rate. Treatment of cells with ethanol-acetone or Tween 80 significantly increased the rates of drug uptake and release by cells, indicating that the cell membrane effectively slows down passage of 3H-AMD in and out of cells. Cellular 3H-AMD uptake is temperature dependent and not energy dependent. The rates of 3H-AMD uptake after short and prolonged incubation suggest that AMD entry into cells consists of two phases, an initial phase of rate-limiting diffusion process followed by a second phase of binding to deoxyribonucleic acid. Although lymphocytes take up less 3H-AMD than CH cells, the differential inhibitory effect of AMD on nucleic acid synthesis in the two cell types is small. The time required for cells to be fully saturated with 3H-AMD varies with the cell type and is based on permeability differences of the cell membrane for 3H-AMD. The time required for lymphocytes and CH cells to be fully saturated with 3H-AMD is reflected in the differential effect of the drug on nucleic acid synthesis. The physiological basis of AMD resistance can be explained on the basis of impaired diffusion of the drug into cells.