The influence of amino acids on copper uptake by rat liver slices

Addition of physiological concentrations of 20 l-amino acids to the incubation medium facilitates the uptake of copper by liver slices. This effect can be observed in a wide range of initial concentrations of copper in the medium, including the low physiological concentrations. The uptake of copper was not influenced by the addition of sodium lactate or d-glucose to the incubation media. The facilitation of copper uptake by amino acids is abolished under anaerobic conditions. It is not affected by the presence of 2,4-dinitrophenol. The type of transport and its exact mechanism have not been established, but this type of transport does not fulfill the criteria for classification as simple diffusion.     

Investigations on metabolic modulation of p-aminohippurate accumulation by rabbit renal cortical slices.

Preparation and incubation of renal cortical slices from adult, female, New Zealand white rabbit depleted tissue citrate concentration. Acetate (10.0 mM) in the incubation significantly increased slice citrate concentration and p-aminohippurate (PAH) accumulation. Physiological concentrations of citrate increased PAH accumulation and final medium pH. increasing concentrations of citrate produced a biphasic effect on PAH accumulation. These data suggested that citrate may act as an intracellular modulator of organic anion transport. This hypothesis was tested with other stimulators of PAH accumulation. Physiological concentrations of alpha-ketoglutarate or succinate increased slice accumulation of PAH. Higher concentrations of either substrate significantly inhibited PAH accumulation. Final medium pH increased with increased medium concentration of both substrates. alpha-Ketoglutarate (0.5 mM) increased PAH accumulation but had no effect on slice citrate concentration. Glucose did not alter either PAH accumulation or slice citrate concentration. Slices incubated without substrate were depleted of citrate but not of alpha-ketoglutarate. Acetate (1.0 mM) significantly increased slice concentration of both alpha-ketoglutarate and citrate. These data suggested that organic anion transport could be modulated by several metabolic intermediates acting through similar but separate mechanisms.     

Hepatocellular uptake of taurocholate in the dog.

The purpose of this study was to examine the hepatocellular extraction of taurocholate and to determine the kinetic characteristics of the uptake process. The uptake of taurocholate by the liver of the intact dog was studied by the multiple-indicator dilution method. 51Cr-labeled red blood cells (a vascular indicator), 125I-labeled albumin (an extravascular reference), and [14C]taurocholate were injected into the portal vein. Different doses of unlabeled taurocholate were included in the injection mixture. Hepatic venous dilution curves were obtained. As a consequence of the hepatic uptake, the outflow recovery of [14C]taurocholate was much reduced when compared to that of albumin, but its recovery increased with increasing doses of taurocholate, suggesting a progressive saturation of the uptake process. The analysis of the dilution curves fitted a three-compartment model system well and no return of the extracted taurocholate to the extracellular space could be detected. The initial space of distribution of taurocholate was 1.22 plus or minus 0.12 (SD) times greater than that of albumin. Analysis of the data for uptake was consistent with Michaelis-Menten kinetics. The calculated initial maximal velocity of uptake (Vmax) was 4.53 mumol times s--1 times 100 g of liver--1 and the dose yielding half-maximal velocity (DK) was 7.11 mumol times 100 g of liver--1. These results are consistent with the hypothesis that the uptake of taurocholate is carrier-mediated. The maximal vilocity of uptake was about six times the known maximal capacity of biliary secretion of taurocholate in the dog.     

Species difference in the inhibitory effect of nonsteroidal anti-inflammatory drugs on the uptake of methotrexate by human kidney slices

Simultaneous use of nonsteroidal anti-inflammatory drugs (NSAIDs), probenecid, and other drugs has been reported to delay the plasma elimination of methotrexate in patients. Previously, we have reported that inhibition of the uptake process cannot explain such drug-drug interactions using rats. The present study quantitatively evaluated the possible role of the transporters in such drug-drug interactions using human kidney slices and membrane vesicles expressing human ATP-binding cassette (ABC) transporters. The uptake of methotrexate by human kidney slices was saturable with a K(m) of 45 to 49 microM. Saturable uptake of methotrexate by human kidney slices was markedly inhibited by p-aminohippurate and benzylpenicillin, but only weakly by 5-methyltetrahydrofolate. These transport characteristics are similar to those of a basolateral organic anion transporter (OAT) 3/SLC22A8. NSAIDs and probenecid inhibited the uptake of methotrexate by human kidney slices, and, in particular, salicylate, indomethacin, phenylbutazone, and probenecid were predicted to exhibit significant inhibition at clinically observed plasma concentrations. Among ABC transporters, such as BCRP/ABCG2, multidrug resistance-associated protein (MRP) 2/ABCC2, and MRP4/ABCC4, which are candidates for the luminal efflux of methotrexate, ATP-dependent uptake of methotrexate by MRP4-expressing membrane vesicles was most potently inhibited by NSAIDs. Salicylate and indomethacin were predicted to inhibit MRP4 at clinical plasma concentrations. Diclofenac-glucuronide significantly inhibited MRP2-mediated transport of methotrexate in a concentration-dependent manner, whereas naproxen-glucuronide had no effect. Inhibition of renal uptake (via OAT3) and efflux processes (via MRP2 and MRP4) explains the possible sites of drug-drug interaction for methotrexate with probenecid and some NSAIDs, including their glucuronides.