Hepatic Uptake Of Organic Cations Research Articles

Functional involvement of organic cation transporter1 (OCT1/Oct1) in the hepatic uptake of organic cations in humans and rats

The contribution of organic cation transporters to the saturable component in the hepatic uptake of 1-methyl-4-phenylpyridinium (MPP), tetraethylammonium (TEA), cimetidine, and metformin was examined by the use of human/rat organic cation transporter (hOCT1/rOct1)-expressing cells and human/rat hepatocytes. Transfection of rOct1 resulted in a considerable increase in the uptake of metformin, whereas that of hOCT1 resulted in only a slight increase. All test compounds (MPP, TEA, cimetidine, and metformin) accumulated in human and rat hepatocytes in a carrier-mediated manner. The Km values for the uptake of MPP, TEA, cimetidine, and metformin into human and rat hepatocytes were comparable with those into hOCT1 and rOct1-expressing cells, respectively. In addition, the relative uptake activities, which were obtained by normalizing the intrinsic uptake clearances of TEA, cimetidine, and metformin against those values of MPP in human and rat hepatocytes, were similar with the uptake activities in hOCT1 and rOct1, respectively. These results suggest that the saturable component in the hepatic uptake of these cationic compounds may be mediated mainly by hOCT1/rOct1; therefore, it is meaningful to evaluate the saturable uptake profile of cationic compounds by the liver using both hOCT1/rOct1-expressing cells and human/rat hepatocytes.

Cloning and functional expression of a mouse liver organic cation transporter.

Hepatic uptake of organic cations is essential for the metabolism and secretion of numerous endobiotics and drugs. Several hepatic organic cation transporters have been kinetically defined, yet have not been isolated or cloned. We have isolated a complementary DNA (cDNA) from both murine liver and kidney cDNA libraries (mOct1/Slc22a1), and have functionally expressed it in Xenopus laevis oocytes. Although mOct1/Slc22a1 is homologous to previously cloned rat and human organic cation transporters, organic cation transport kinetics differed markedly. mOct1/Slc22a1-RNA injection of oocytes resulted in the saturable, time- and temperature-dependent uptake of the quaternary organic cation [14C]-tetraethylammonium ([14C]-TEA), with a Km of 38 micromol/L. TEA uptake was inhibited by several other organic cation drugs, but was not inhibited by the organic cation n-methyl-nicotinamide (NMN), being instead stimulated by it (fourfold). [14C]-TEA uptake was also stimulated by an inside-outside proton gradient. mOct1/Slc22a1-injected oocytes transported the organic cations [3H]-1-methyl-4-phenylpyridium and [3H]-choline chloride, but did not transport other classes of organic compounds. mOct1/Slc22a1 encodes for a hepatic and renal organic cation transporter which may be important for the uptake and secretion of cationic drugs and endobiotics.

POSTNATAL DEVELOPMENT OF ORGANIC CATION TRANSPORT IN THE RAT LIVER

Previous studies have demonstrated that the small permanently charged organic cation 1-methyl-4-phenylpyridinium (MPP+) is avidly taken up by rat hepatocytes. The aim of this study was to characterise the postnatal development of hepatic uptake of organic cations, using the model compound MPP+. Accumulation of [3H]MPP+by liver slices obtained from rats ranging from 1 day to 7 weeks was measured, and the effect of a series of compounds on [3H]MPP+uptake was examined. The accumulation of [3H]MPP+by liver slices was similar in adult (87.5±19.9 pmol g−l;n=7) and neonatal rats (110.6±11.5 pmol g−l;n=15). Verapamil, quinidine (100 μm) and progesterone (200 μm) produced very marked reductions on [3H]MPP+uptake at all ages, and the inhibitory effect of verapamil and quinidine was maximum in livers from 1-day-old rats. Bilirubin (200 μm) significantly reduced [3H]MPP+uptake by liver slices from 1 day, 1 week and 7-week-old rats. However, [3H]MPP+accumulation was reduced by cimetidine, vinblastine and daunomycin (100 μm) in 1-day-old rats, but the effect of these drugs disappeared as the animals age increased. These findings demonstrate that hepatic organic cation uptake capacity is remarkably high shortly after birth and suggest that at least two distinct uptake mechanisms are involved in this process. These uptake systems are the type I hepatic transporter of organic cations, active from birth to adulthood, andP-glycoprotein, active only in very young rats.

Transport of small organic cations in the rat liver. The role of the organic cation transporter OCT1.

The kidneys and the liver are the principal organs for the inactivation of circulating organic cations. Recently, an organic cation transporter (OCT1) has been cloned from rat kidney. In order to answer the question whether OCT1 is involved also in hepatic uptake of organic cations, the pharmacological characteristics of organic cation transport in hepatocytes were compared to the characteristics of transiently expressed OCT1. Primary cultures of rat hepatocytes avidly accumulated the small organic cation 3H-1-methyl-4-phenylpyridinium (3H-MPP+). At equilibrium, the hepatocytes accumulated 3H-MPP+ 56-fold. Initial rates of specific 3H-MPP+ transport in hepatocytes were saturable. The half-saturating concentration was 13 mumol/l. 3H-MPP+ transport was sensitive to quinine (Ki = 0.79 mumol/l) and cyanine863 (Ki = 0.097 mumol/l). Quinine and cyanine863 are known inhibitors of type I hepatic transport of cationic drugs and of renal excretion of organic cations, respectively. To compare the functional characteristics of 3H-MPP+ transport in hepatocytes with those of OCT1, OCT1 has been heterologously expressed and characterized in a mammalian cell line (293 cells). Initial rates of 3H-MPP+ transport were saturable, the Km being 13 mumol/l. The rank order of inhibitory potencies of various inhibitors was almost identical in hepatocytes and 293 cells transiently transfected with OCT1. There was a positive correlation between the Ki's for the inhibition of 3H-MPP+ transport in isolated hepatocytes and transfected 293 cells (r = 0.85; P < 0.01; n = 8). The results indicate that OCT1 is functionally expressed not only in the kidney but also in hepatocytes where it is responsible for the transport of small organic cations which, in the past, have been classified as type I substrates.

Carrier-mediated transport in the hepatic distribution and elimination of drugs, with special reference to the category of organic cations

Carrier-mediated transport of drugs occurs in various tissues in the body and may largely affect the rate of distribution and elimination. Saturable translocation mechanisms allowing competitive interactions have been identified in the kidneys (tubular secretion), mucosal cells in the gut (intestinal absorption and secretion), choroid plexus (removal of drug from the cerebrospinal fluid), and liver (hepatobiliary excretion). Drugs with quaternary and tertiary amine groups represent the large category of organic cations that can be transported via such mechanisms. The hepatic and to a lesser extent the intestinal cation carrier systems preferentially recognize relatively large molecular weight amphipathic compounds. In the case of multivalent cationic drugs, efficient transport only occurs if large hydrophobic ring structures provide a sufficient lipophilicity-hydrophilicity balance within the drug molecule. At least two separate carrier systems for hepatic uptake of organic cations have been identified through kinetic and photoaffinity labeling studies. In addition absorptive endocytosis may play a role that along with proton-antiport systems and membrane potential driven transport may lead to intracellular sequestration in lysosomes and mitochondria. Concentration gradients of inorganic ions may represent the driving forces for hepatic uptake and biliary excretion of drugs. Recent studies that aim to the identification of potential membrane carrier proteins indicate multiple carriers for organic anions, cations, and uncharged compounds with molecular weights around 50,000 Da. They may represent a family of closely related proteins exhibiting overlapping substrate specificity or, alternatively, an aspecific transport system that mediates translocation of various forms of drugs coupled with inorganic ions. Consequently, extensive pharmacokinetic interactions can be anticipated at the level of uptake and secretion of drugs regardless of their charge.

Biopharmaceutical study of the hepato-biliary transport of drugs. VIII. Investigation of Hepatic uptake of organic cations by portal infusion.

The liver/plasma (L/P) ratios of acetyl procainamide ethobromide (APAEB), procainamide ethobromide (PAEB) and quinine, which were transported from plasma into liver against a concentration gradient, were greatly decreased by infusing sulfhydryl reagents, p-chloromercuribenzoic acid (PCMB), p-chloromercury phenyl sulfonic acid (PCMBS) and iodoacetamide (IAA), into portal vein on account of the increase in the plasma level and the decrease in the liver level. The metabolism of PAEB and quinine was not affected by the infusion of the reagents. The binding of APAEB, PAEB and quinine was not depressed and that to plasma was not changed in the presence of the reagents. Consequently it was deduced that the hepatic uptake of the organic cations was blocked by the treatment with the sulfhydryl reagents, which acted on the plasma membrane involved in the concentrative uptake of the organic cations. In comparison with the previous results obtained by the intrabiliary retrograde infusion, PCMB and IAA were inhibitory only on the hepatic uptake process and N-ethylmaleimide (NEM) only on the excretory process, whereas PCMBS was effective on both steps. The finding of the specific reagents which are inhibitory only on one process would be of use in the study of the hepato-biliary transport of organic cations.