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In isolated hepatocytes the influence of cellular glutathione (GSH) on initial influx, net uptake and efflux of methotrexate (MTX) was determined. Endogenous glutathione in rat liver cells was depleted by either fasting of rats or by in vivo administration of phorone prior to cell preparation. The initial rate of influx of MTX was found to be higher in hepatocytes of fasted and phorone-treated rats than in those of untreated, fed control rats. The Km values for the methotrexate influx in GSH-deficient hepatocytes were up to 3 times lower than in normal cells, whereas Vmax remained unchanged. These results disclose an increased efficiency of the MTX transport system in cells with diminished cellular GSH levels. On the other hand, titration of external membrane SH groups by 203Hg p-CMBS revealed up to three times higher amounts of free SH groups on cells from starved and phorone-treated rats than on hepatocytes of fed rats. Increased efficiency of the MTX transport system in GSH-deficient cells may, therefore, be interpreted as increased capacity of the MTX transport carrier for which free membrane SH groups are known to be essential. Despite activation of initial transport of MTX here, later net accumulation of MTX became smaller than in cells with normal GSH levels. Efflux of MTX from liver cells was not influenced by fasting or phorone treatment of rats, however, the "nonexchangeable" pool of MTX was found to be decreased, which indicates inhibition of formation of MTX polyglutamates here. This inhibition was most likely responsible for the decreased amounts of MTX finally accumulated in GSH-deficient hepatocytes.

Under certain modulating conditions the liver of the male Syrian golden hamster is a target organ for the carcinogenic effect of the synthetic estrogen diethylstilbestrol (DES). As a basis for mechanistic studies aimed at elucidating the role of metabolic activation in the process of DES-induced neoplasia, the metabolism of 14C-DES was investigated in freshly isolated hamster hepatocytes. These oxidative metabolites of DES, viz. Z,Z-dienestrol,3'-hydroxy-DES and 1-hydroxy-E-DES, were formed in 14.2, 9.1, and 0.3% yield, respectively, when hepatocytes were incubated with 50 nmol DES/mg cellular protein for 60 min. Glucuronides (4.0%) and sulfates (2.8%) of DES and of the oxidative metabolites were also found, and non-extractable binding of radioactivity to cellular protein was observed indicating the formation of reactive intermediates. The capability of hamster hepatocytes to oxidize and conjugate DES should allow the investigation of the effects of modulators on the metabolic activation of DES in this cellular system in order to help clarify the mechanisms of DES-induced hepatocarcinogenesis.

The uptake of methotrexate (MTX) by isolated rat hepatocytes and its changes under the influence of exogenous GSH have been studied under various conditions: GSH concentration, pH of incubation medium, preincubation of cells prior to MTX and GSH addition, ionic composition of the incubation medium (standard saline, Na+-free, Na+ and K+-free, or ion-deficient), after prior treatment of cells by membrane -SH blockers (p-CMBS, 4-CMB and DIP2+) and ATP. It was found that GSH strongly accelerated MTX uptake. This effect depended on GSH concentration and on preincubation of cells. The GSH effect was not dependent on medium pH in spite of an observed close relationship between pH of incubate and MTX transport itself. Activation by GSH of MTX transport was connected to an increase in intracellular K+. It was also noted that while blockers of membrane -SH groups like p-CMBS and 4-CMB inhibited MTX uptake and increased the intracellular Na+/K+ ratio, both effects were partially overcome by GSH. After treatment by DIP2+, Na+/K+ ratio was unaffected, but MTX uptake inhibited. Still GSH abolished inhibition. Added ATP also inhibited MTX uptake and caused loss of cellular K+ and accumulation of Na+. Here neither effect could be reversed by GSH; consequently, high cellular amounts of K+ and MTX accumulated by previous action of GSH were depleted on subsequent ATP addition. MTX uptake was low in sucrose medium. But in this ion-deficient medium, GSH had the greatest stimulatory effect on MTX uptake. It is concluded that binding GSH can affect the redox state of the -S-S-/-SH groups of the cellular plasma membrane and that this effect of GSH might demonstrate involvement of the redox state in the control of MTX permeability.

Diurnal variation of the methotrexate (MTX) initial influx and net uptake in isolated rat liver cells was studied in dependence on diurnal variation of cellular glutathione. It was found that the most significant differences concerning the MTX initial transport and accumulation were observed between hepatocytes prepared at 1200 hr when cellular glutathione reached its maximum, and those isolated at 0000 hr when liver glutathione had its minimal concentration. The initial influx of MTX was the biggest in cells isolated at 0000 hr and the smallest in cells prepared at 1200 hr. The Km values in cells with low cellular glutathione (at 1800 and 0000 hr) were about three times smaller than in cells with high glutathione (at 0700 and at 1200 hr), whereas the Vmax value remained unchanged. Titration of external membrane SH groups by [203Hg]p-CMBS revealed a much larger amount of free SH groups on cells having low glutathione level than on those with high glutathione. Despite big initial influx of MTX found in cells with low cellular glutathione, net accumulation of MTX was significantly smaller in these cells as compared with hepatocytes having high glutathione level. In conclusion, the present studies confirmed a statement of the preceding paper on the important role of cellular glutathione played in methotrexate transport and accumulation in rat liver cells.