Uptake of N-nitrosobis(2-oxopropyl)amine by isolated rat and hamster hepatocytes: species differences and evidence for an active carrier-mediated transport process.

Abstract

The rates of uptake of the carcinogen N-nitrosobis(2-oxopropyl)amine (BOP) by hepatocytes isolated from Fischer rats and Syrian hamsters were determined in order to investigate species differences in cellular transport of the carcinogen. Initial rates of uptake of [1-14C]BOP by hepatocytes were measured using a rapid centrifugation technique. At cell densities from 1.5 to 6 x 10(6) cells/ml, initial rates of uptake were as much as 4-fold more rapid in hamster hepatocytes than in those of the rat. The cell/medium distribution ratio for hamster hepatocytes reached a value of 9.0 after a 20-min incubation with an extracellular BOP concentration of 20 microM. Under the same conditions, the cell/medium distribution ratio for rat hepatocytes was only 2.4. These results indicated that BOP uptake proceeded against a concentration gradient and was more rapid in hamster hepatocytes. In both species, the rates of uptake were saturable with increasing concentration (2-685 microM) and displayed biphasic kinetics characteristic of high-affinity (Km less than 20 microM) and low-affinity (Km greater than 30 microM) processes for the uptake of BOP. Evidence for the involvement of an ATP-dependent active carrier-mediated transport process was obtained from experiments in which hepatocytes were preincubated with metabolic inhibitors. Significant inhibition of uptake was observed in the presence of KCN, carbonyl cyanide-3-chlorophenylhydrazone, antimycin A, oligomycin and other agents which interfere with electron transport or ATP generation. Based on the reduction in uptake rates, rat hepatocytes were more sensitive to the effects of these inhibitors. These results suggest that the entry of BOP into hepatocytes is under cellular regulation and that the more rapid rate of uptake in liver cells of the hamster may be one factor responsible for the observation that BOP is a more potent hepatotoxin and carcinogen in this species.