Methotrexate transport in the human intestine. Evidence for heterogeneity.

The transport of methotrexate (MTX) was investigated in organ-cultured endoscopic biopsy specimens of intestinal mucosa from normal subjects. In biopsy specimens from the proximal small intestine incubated with [3H]MTX (0.1 microM) for 2 hr at pH 5.5, [3H]MTX accumulated in the intracellular fluid to a concentration 3.5-fold higher than that of the medium, but at pH 6.5 and 7.5, the concentration was the same as that of the medium. In biopsy specimens from the cecum incubated under similar conditions, no accumulation against a concentration gradient was found. However, the accumulation of MTX was significantly higher at pH 5.5 than at 7.5. At [MTX] = 0.1 microM, the initial rate of MTX transport in the small intestine was significantly affected by medium pH and was optimal at pH 5.5. The relationship between the initial rate of uptake and medium [H+] was hyperbolic, suggestive of saturability with respect to [H+] with a Km of 132.2 nm [H+], corresponding to a medium pH of 6.88. At medium [MTX] = 10 microM, this effect was abolished. At pH 5.5, the relationship between the initial rate of uptake and medium [MTX] was sigmoidal, suggestive of a positive cooperativity, with napp of 1.8. [MTX] at 0.5 Vmax was 20.37 microM. Folic acid inhibited 37% of MTX flux. At pH 7.5, the relationship between the initial rate of uptake of MTX and medium [MTX] was linear. These data indicate the presence of a proton-dependent active transport of MTX in the human proximal small intestine, which is partially shared with folic acid.