Altered expression of unidirectional extrusion routes for methotrexate and cholate in an efflux variant of L1210 cells.

The specificity and function of two unidirectional anion-efflux pumps in mouse L1210 cells were evaluated using a variant cell line selected for growth in the presence of cholate and bromosulfophthalein. Transport analysis revealed that cholate efflux in the variant L1210/C7 cell line had declined 8-fold, due to the loss of a bromosulfophthalein-sensitive efflux system, the major extrusion route for cholate in parental cells. Efflux measurements showed further that a bromosulfophthalein-sensitive efflux system for methotrexate was also absent in L1210/C7 cells. Total unidirectional efflux of methotrexate, however, was similar in the variant and parental cells, since the loss in the bromosulfophthalein-sensitive system was compensated by a rise in a second probenecid-sensitive route. The latter was identified from inhibitor studies to be the same system which acts as a minor efflux route for methotrexate in parental cells. These results support the hypothesis that L1210 cells contain a bromosulfophthalein-sensitive efflux system which mediates the unidirectional extrusion of either methotrexate or cholate, and a second probenecid-sensitive route which differs from the bromosulfophthalein-sensitive system in inhibitor specificity and also in its ability to transport methotrexate but not cholate.