Influx of MTX in wild-type and resistant cells. Time course of [³H]MTX accumulation in CCRF-CEM (A) and MOLT-4 (B) parental cells (▴) and their MTX-(•) and 7-OHMTX-(○) resistant sublines. Extracellular concentration of [³H]MTX was 1 μM. Bottom panels: Relative linear uptake rates of [³H]MTX determined over 0- to 3-minute incubation time. Unidirectional influx rates of [³H]MTX for each parental cell line are set at 100%. Results are presented as the mean of 3 independent experiments in which SD was less than 17%. Further experimental details are described in “Materials and methods.”