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The structural requirements for aromatic amino acid reabsorption and secretion in the dog were investigated using a series of tyrosine analogs. In clearance experiments, l-tyrosine and l-phenylalanine underwent net reabsorption at rates 5 to 6 times greater than those of the corresponding d-enantiomers and about 1.5 time those of o- and m-dl-tyrosine. 3-(p-hydroxyphenyl)Propionic acid underwent net secretion under conditions of polyuria and high urinary pH and net reabsorption under conditions of oliguria and low pH. In stop-flow experiments, (U/Pl-amino acid)/(U/Pln) ratios, where U/P = concentration in urine and plasma and In = inulin, of proximal tubular samples were 1.5 to 5-fold greater than the control (free-flow) values of 0.1 to 0.3, indicating tubular secretion. Secretion was inhibited, and net reabsorption enhanced, by probenecid, p-aminohippuric acid and 2,4-dinitrophenol. (U/Pd-amino acid)/(U/PIN) ratios of proximal tubular samples were below the control values of 0.6 to 0.8, indicating tubular reabsorption. Probenecid had no effect on the overall pattern but caused a slight decrease in the relative clearance values. The stop-flow pattern of 3-(p-hydroxyphenyl)propionic acid excretion was similar to that of the I-amino acids. It was concluded that the I-amino acids undergo bidirectional transport and that the I-configuration and amino group are required for optimal active reabsorption, whereas ring hydroxylation has little effect on reabsorption transport. No absolute structural requirements for active secretion were elucidated.

Thin cortical slices of cynamolgus and rhesus monkey kidney were used to study the renal transport of methotrexate (MTX). In experiments with renal cortical slices, MTX uptake at 25 degrees C was linear over the initial 30 min and was temperature-dependent. The Km was 0.094 mM for MTX uptake at 25 degrees C and Vmax was 0.098 mumol/g of tissue/30 min. In the presence of either 1 mM 2,4-dinitrophenol, p-aminohippurate or acetylsalicylate, MTX uptake was competitively inhibited. 2,4-Dinitrophenol had the greatest and acetylsalicylate had the least inhibitory effect. Folinic acid, folic acid and ouabain produced little or no effect on MTX uptake. MTX efflux from preloaded slices (preincubated with 0.5 mM MTX for 45 min) was a first-order process with T 1/2 of 7.13 +/- 0.86 min. In the presence of vincristine or p-aminohippurate the half-lives for MTX were 15.25 +/- 0.91 and 4.59 +/- 0.47 min, respectively. Thus vincristine, an organic base, was found to augment MTX uptake, due to a reduction in the rate of efflux of MTX from the cortical tissues, whereas p-aminohippurate, an organic acid, was found to decrease MTX intracellular concentration by blocking influx and stimulating efflux. It was concluded that the renal transport of MTX in monkey kidney is mediated predominantly by an organic anion secretory process and that there is probably little or no reabsorptive transport.