ENHANCEMENT OF MITOMYCIN C UPTAKE BY ISOPROTERENOL IN RAT ASCITES HEPATOMA

Abstract

The amounts of mitomycin C (MMC) taken up into rat ascites hepatoma cells were determined by measuring the decrease in the absorbance of the incubation medium at 363 nm. The extracellular concentration of MMC decreased progressively in AH130 cell suspension and no peaks other than MMC were detected by analytical high-performance liquid chromatography (HPLC). Isoproterenol (IPN) enhanced the uptake of MMC into AH44 and AH130 cells but not AH13 cells in which the uptake increased by N⁶, O^2'-dibutyryl cyclic adenosine 3' : 5'-monophosphate (dibutyryl cyclic AMP). The increase of uptake of MMC by IPN was inhibited by propranolol and the uptake of MMC increased in a dose-dependent manner by theophylline in AH130 cells. The maximum combined cytotoxicity was observed when AH130 cells were treated with IPN at 10^-7 M for 30 min before the exposure to MMC and, in this pretreatment condition, the uptake of MMC was enhanced in parallel with the increase of cyclic adenosine 3' : 5'-monophosphate (cyclic AMP) level in the cells. On the other hand, MMC, which had no stimulating effect on the intracellular cyclic AMP level, nevertheless maintained a high level of intracellular cyclic AMP elevated by IPN for a longer period than in the treatment with IPN alone and prolonged the period of acceleration of the uptake of MMC. In the in vivo combined treatment with IPN and MMC, the life span of AH44-bearing rats was prolonged and 2 out of 6 rats were cured, while the mean survival time of the rats treated with MMC alone was 11.3 d. These results indicated that the intracellular cyclic AMP, which is maintained at a high concentration for a long period by the combined action of IPN and MMC, enhanced the uptake and the antitumor effect of MMC.