Uptake of unconjugated bilirubin by isolated rat hepatocytes

Development of a Pharmacokinetic Model That Accounts for the Plasma Concentrations of Conjugated and Unconjugated Bilirubin Observed in a Variety of Disease States

Introduction

For a large variety of liver pathologies, the plasma unconjugated (UB) and conjugated (CB) bilirubin concentrations appear to be coupled. For example, in alcoholic cirrhosis, UB and CB are roughly the same over a large range of total bilirubin, requiring an initial massive increase (about 40-fold) in plasma CB to reach the level of UB and then similar increases in UB and CB as the disease progresses. This coupling has been either unrecognized or ignored and this paper is the first attempt to try to explain it quantitatively in terms of known hepatic cell metabolic and membrane transport properties.

Methods

A simplified pharmacokinetic model is developed and applied to a variety of hyperbilirubinemic pathologies. A central feature of the model is based on the recent observation that double knockout of the rat OATP1A and OATP1B hepatic transporters produces a roughly 400-fold increase in plasma CB, indicating that there is a normal rapid recycling of CB from the cell to the plasma with reuptake via OATP. We use the experimental rat Km of OATP CB transport to show that OATP uptake becomes saturated at relatively low plasma CB concentrations, decreasing uptake, and producing massive (up to 1000-fold) increases in CB in some pathologies. It is assumed that UB and CB are competing for the OATP transporter, producing the increased plasma UB that is observed in "pure" CB pathologies.

Results

The model accurately describes the clinically observed UB and CB for pure UB (Gilbert's, hemolytic anemia) and CB (Dubin-Johnson, Rotor syndrome, biliary atresia) pathologies as well as in cirrhosis.

Conclusion

This model is a preliminary, first attempt to quantitatively describe UB and CB pharmacokinetics. It is hoped that it will stimulate more detailed measurements and analysis.

Evaluation of Hepatic Uptake of OATP1B Substrates by Short Term-Cultured Plated Human Hepatocytes: Comparison With Isolated Suspended Hepatocytes

Hepatic uptake clearance has been measured in suspended human hepatocytes (SHH). Plated human hepatocytes (PHH) after short-term culturing are increasingly employed to study hepatic transport driven mainly by its higher throughput. To know pros/cons of both systems, the hepatic uptake clearances of several organic anion transporting polypeptide 1B substrates were compared between PHH and SHH by determining the initial uptake velocities or through dynamic model-based analyses. For cerivastatin, pitavastatin and rosuvastatin, initial uptake clearances (PS_inf) obtained using PHH were comparable to those using SHH, while cell-to-medium concentration (C/M) ratios were 2.7- to 5.4-fold higher. For pravastatin and dehydropravastatin, hydrophilic compounds with low uptake/cellular binding, their PS_inf and C/M ratio in PHH were 1.8- to 3.2-fold lower than those in SHH. These hydrophilic substrates are more prone to wash-off during the uptake study using PHH, which may explain the apparently lower uptake than SHH. The C/M ratios obtained using PHH were stable over an extended time, making PHH suitable to estimate the C/M ratios and hepatocyte-to-medium unbound concentration ratios (K_p,uu). In conclusion, PHH is useful in evaluating hepatic uptake/efflux clearances and K_p,uu of OATP1B substrates in a high-throughput manner, however, a caution is warranted for hydrophilic drugs with low uptake/cellular binding.

Involvement of Organic Cation Transporters in the Kinetics of Trimethylamine N-oxide

Recent studies suggest that trimethylamine N-oxide (TMAO) is associated with the development of chronic kidney disease and heart failure. In this study, we investigated the importance of organic cation transporters (OCTs) in the clearance and tissue distribution of TMAO. The low-affinity and high-capacity transport of TMAO by mouse and human OCT1 and OCT2 was observed. Uptake and efflux of TMAO by the mouse hepatocytes as well as TMAO uptake into mouse kidney slices were significantly decreased by the addition of tetraethylammonium or Oct1/2 double knockout (dKO). Plasma concentrations of endogenous TMAO and TMAO-d9 given by intravenous infusion was 2-fold higher in Oct1/2 dKO than in wild-type mice due to significant decrease in its renal clearance. These results indicate that OCTs have a crucial role in the kinetics of TMAO in mice. In human, however, the OCT2-mediated tubular secretion in the urinary excretion of TMAO was insignificant because the renal clearance of TMAO was similar to that of creatinine in both young and elderly subjects, suggesting the species difference in the urinary excretion mechanisms of TMAO between mouse and human.

Quantitative assessment of the multiple processes responsible for bilirubin homeostasis in health and disease

Serum bilirubin measurements are commonly obtained for the evaluation of ill patients and to screen for liver disease in routine physical exams. An enormous research effort has identified the multiple mechanisms involved in the production and metabolism of conjugated (CB) and unconjugated bilirubin (UB). While the qualitative effects of these mechanisms are well understood, their expected quantitative influence on serum bilirubin homeostasis has received less attention. In this review, each of the steps involved in bilirubin production, metabolism, hepatic cell uptake, and excretion is quantitatively examined. We then attempt to predict the expected effect of normal and defective function on serum UB and CB levels in health and disease states including hemolysis, extra- and intrahepatic cholestasis, hepatocellular diseases (eg, cirrhosis, hepatitis), and various congenital defects in bilirubin conjugation and secretion (eg, Gilbert's, Dubin-Johnson, Crigler-Najjar, Rotor syndromes). Novel aspects of this review include: 1) quantitative estimates of the free and total UB and CB in the plasma, hepatocyte, and bile; 2) detailed discussion of the important implications of the recently recognized role of the hepatic OATP transporters in the maintenance of CB homeostasis; 3) discussion of the differences between the standard diazo assay versus chromatographic measurement of CB and UB; 4) pharmacokinetic implications of the extremely high-affinity albumin binding of UB; 5) role of the enterohepatic circulation in physiologic jaundice of newborn and fasting hyperbilirubinemia; and 6) insights concerning the clinical interpretation of bilirubin measurements.

Expression and functional features of NaCT, a sodium-coupled citrate transporter, in human and rat livers and cell lines

In this article, we report on the expression and function of a Na(+)-coupled transporter for citrate, NaCT, in human and rat liver cell lines and in primary hepatocytes from the rat liver. We also describe the polarized expression of this transporter in human and rat livers. Citrate uptake in human liver cell lines HepG2 and Huh-7 was obligatorily dependent on Na+. The uptake system showed a preference for citrate over other intermediates of the citric acid cycle and exhibited a Michaelis constant of approximately 6 mM for citrate. The transport activity was stimulated by Li+, and the activation was associated with a marked increase in substrate affinity. Citrate uptake in rat liver cell line MH1C1 was also Na+ dependent and showed a preference for citrate. The Michaelis constant for citrate was approximately 10 microM. The transport activity was inhibited by Li+. Primary hepatocytes from the rat liver also showed robust activity for Na+-coupled citrate uptake, with functional features similar to those described in the rat liver cell line. Immunolabeling with a specific anti-NaCT antibody showed exclusive expression of the transporter in the sinusoidal membrane of hepatocytes in human and rat livers. This constitutes the first report on the expression and function of NaCT in liver cells.

Kinetic analysis about the bidirectional transport of 1-anilino-8-naphthalene sulfonate (ANS) by isolated rat hepatocytes

The purpose of the present study was to investigate the bidirectional transport of 1-anilino-8-naphthalene sulfonate (ANS) using isolated rat hepatocytes. The initial uptake rate of ANS by isolated hepatocytes was determined. The uptake process of ANS was saturable, with a Km of 29.1 +/- 3.2 microM and Vmax of 2.9 +/- 0.1 mmol/min/mg protein. Subsequently, the initial efflux rate of ANS from isolated hepatocytes was determined by resuspending preloaded cells to 3.0% (w/v) BSA buffer. The efflux process for total ANS revealed a little saturability. The mean value of the efflux clearance was 2.2 +/- 0.1 microL/min/mg protein. The efflux rate of ANS from hepatocytes was markedly decreased at 4 degrees C, indicating that the apparent efflux of ANS might not be attributed to the release of ANS bound to the cell surface, but to the efflux of ANS from intracellular space. The efflux clearance was furthermore corrected for the unbound intracellular ANS concentration on the basis of its binding parameters to cytosol. The relation between efflux rate and unbound ANS concentration was fitted well to the Michaelis-Menten equation with a saturable and a nonsaturable components. The Vmax and Km values were 0.54 mmol/min/mg protein, and 10.0 microM, respectively. Based on the comparison of the ratios of Vmax to Km (Vmax/Km) corresponding to the transport clearance, the influx clearance was two times higher than the efflux clearance. Together with our preliminary studies that ATP suppression in hepatocytes substantially inhibited ANS influx rate, we concluded that the hepatic uptake of ANS is actively taken up into hepatocytes via the carrier mediated transport system.

Mechanism of hepatocellular uptake of albumin-bound bilirubin

We previously demonstrated that unconjugated bilirubin spontaneously diffuses through phospholipid bilayers at a rate which exceeds albumin dissociation, suggesting that solvation from albumin represents the rate-limiting step in hepatic bilirubin clearance. To further examine this hypothesis, we studied the uptake of bovine serum albumin (BSA)-bound bilirubin by cultured hepatoblastoma (HepG2) cells. Uptake of bilirubin was saturable, with a K(m) and V(max) of 4.2+/-0.5 microM (+/-S.E.M.) and 469+/-41 pmol min(-1) mg(-1) at 25 degrees C. Substantial bilirubin uptake also was observed at 4 degrees C (K(m)=7.0+/-0.8 microM, V(max)=282+/-26 pmol min(-1) mg(-1)), supporting a diffusional transport mechanism. Consistent with reported solvation rates, the cellular uptake of bilirubin bound to human serum albumin was more rapid than for BSA-bound bilirubin, indicative of dissociation-limited uptake. Counterintuitively, an inverse correlation between pH and the rate of bilirubin flip-flop was observed, due to pH effects on the rate of dissociation of bilirubin from albumin and from the membrane bilayer. The identification of an inflection point at pH 8.1 is indicative of a pK(a) value for bilirubin in this range. Taken together, our data suggest that hepatocellular uptake of bilirubin is dissociation-limited and occurs principally by a mechanism involving spontaneous transmembrane diffusion.

Uptake of [(3)H]bilirubin in freshly isolated rat hepatocytes: role of free bilirubin concentration

Hepatocytic transport of physiological concentrations of unconjugated bilirubin (UCB) has not been determined in isolated liver cells. Initial uptake of highly purified [(3)H]UCB was measured in rat hepatocytes in the presence of human serum albumin at various free, unbound UCB concentrations, [UCB]. At [UCB]=42 nM (below aqueous solubility of 70 nM), uptake was strictly temperature dependent; this was much less evident at [UCB]=166 nM (supersaturated). At low, physiological UCB concentrations, specific UCB uptake showed saturative kinetics with an apparent K(m) of 41 nM, indicating carrier-mediated transport. With aqueous supersaturation, UCB entered hepatocytes mainly by passive diffusion.

Unconjugated bilirubin exhibits spontaneous diffusion through model lipid bilayers and native hepatocyte membranes

The liver is responsible for the clearance and metabolism of unconjugated bilirubin, the hydrophobic end-product of heme catabolism. Although several putative bilirubin transporters have been described, it has been alternatively proposed that bilirubin enters the hepatocyte by passive diffusion through the plasma membrane. In order to elucidate the mechanism of bilirubin uptake, we measured the rate of bilirubin transmembrane diffusion (flip-flop) using stopped-flow fluorescence techniques. Unconjugated bilirubin rapidly diffuses through model phosphatidylcholine vesicles, with a first-order rate constant of 5.3 s-1 (t(1)/(2) = 130 ms). The flip-flop rate is independent of membrane cholesterol content, phospholipid acyl saturation, and lipid packing, consistent with thermodynamic analyses demonstrating minimal steric constraint to bilirubin transmembrane diffusion. The coincident decrease in pH of the entrapped vesicle volume supports a mechanism whereby the bilirubin molecule crosses the lipid bilayer as the uncharged diacid. Transport of bilirubin by native rat hepatocyte membranes exhibits kinetics comparable with that in model vesicles, suggesting that unconjugated bilirubin crosses cellular membranes by passive diffusion through the hydrophobic lipid core. In contrast, there is no demonstrable flip-flop of bilirubin diglucuronide or bilirubin ditaurate in phospholipid vesicles, yet these compounds rapidly traverse isolated rat hepatocyte membranes, confirming the presence of a facilitated uptake system(s) for hydrophilic bilirubin conjugates.

Flow cytometric measurement of intracellular bilirubin in human peripheral blood mononuclear cells exposed to unconjugated bilirubin

Human peripheral blood mononuclear cells were incubated at 37 degrees C with bilirubin in bovine albumin solution. Histological analysis of bilirubin-treated cells demonstrated a prominent brown pigment deposited in the cytoplasm. Homogenates of these cells in chloroform-methanol solution showed an identical absorption spectrum with pure bilirubin dissolved in the same solution. When bilirubin-treated cells were excited at 488 nm, a significant autofluorescence was detected by flow cytometry at 585 nm in a dose-dependent manner, which had a significant correlation with the amount of bilirubin chemically extracted from the cells (r = 0.963, n = 34, p less than 0.001). Intraassay and interassay variability of the autofluorescence by flow cytometric analysis was small (both less than 5%). When bilirubin-treated cells were stained with fluorescein-labeled anti-CD14 antibody, the CD14 positive cell population can be fractionated without interference of autofluorescence derived from intracellular bilirubin. These results suggest that flow cytometric analysis of bilirubin-treated cells can quantitate intracellular bilirubin, and that bilirubin does not interfere with analysis of surface antigens.

Analysis of membrane-bound acceptors. A correction function for non-specific accumulation of poorly water-soluble hydrophobic or amphipathic ligands based on the ligand partition concept

Non-specific ligand accumulation into membrane material, which may contribute considerably to the experimental signal obtained in binding studies with labelled amphipathic and hydrophobic ligands, may be accounted for by linear partition of the ligands into the membrane phase. For application to binding data obtained at a single membrane-lipid concentration, a fitting procedure is proposed which allows one to correct for non-specific ligand partition. If the assumption is met that the amount of acceptor-bound ligand is small compared to the total amount present in the system, one can validly interpret the data in terms of total ligand concentrations. The apparent dissociation constants Kd(app) thus obtained should be corrected for the often large effects of the size of the partition compartment(s), by performing assays at several membrane-lipid concentrations. The importance of the latter correction is stressed and an approach for obtaining the characteristic effective dissociation constants Kd' is indicated. The procedure also yields estimates of the ligand/membrane partition coefficients.

Uptake of organic anions by isolated rat hepatocytes. A classification in terms of ATP-dependency

Uptake of organic anions into isolated rat hepatocytes was studied to examine their ATP dependency. In the presence of rotenone (0.2 microM), the initial velocity of the uptake (Vo) of dibromosulfophthalein (DBSP; 10 microM), 1-anilino-8-naphthalenesulfonate (ANS; 10 microM) and benzylpenicillin (PCG; 0.02 microM) was reduced to 60-70% of the control value, while that of bromosulfophthalein (BSP; 10 microM), rose bengal (RB; 10 microM) and bromophenol blue (BPB; 10 microM) was not affected. Furthermore, we examined the inhibitory effect of rotenone on the uptake at equilibrium of non-metabolizable ligands (DBSP, BPB and RB). The uptake of these ligands reached equilibrium at 30 min with a cel-to-medium concentration ratio (C/M ratio) of 75, 37 and 126, respectively. The C/M ratio at equilibrium of DBSP was reduced by rotenone to approx. 60% of the control value, while that of BPB and RB was not reduced. Other metabolic inhibitors such as sodium azide (10 mM) and carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP; 10 microM) also reduced the Vo of DBSP and PCG, while the uptake of BSP and RB was not reduced by these inhibitors. These results indicate that organic anions can be classified into two groups according to whether they are taken up by hepatocytes in an ATP-dependent manner, i.e., via active transport or in an ATP-independent manner, i.e., via facilitated diffusion. DBSP, PCG and ANS belong to the former group, whereas BSP, BPB and RB belong to the latter.

Utilization of ATP-depleted cells in the analysis of taurocholate uptake by isolated rat hepatocytes

The usefulness of ATP-depleted rat hepatocytes in transport studies was examined. ATP-depleted hepatocytes were prepared by incubating cell suspensions with 30 microM rotenone. In ATP-depleted hepatocytes, plasma membrane permeability was increased and mitochondrial membrane potential decreased, while both intracellular volume and pH remained normal. Furthermore, in the presence of valinomycin, the initial uptake rates of 3H-tetraphenyl phosphonium (TPP+) with varied medium concentrations of potassium were predicted according to the Goldman-Hodgkin-Katz equation, which demonstrated that a potassium diffusion potential could be produced in this system. Using the thus-characterized ATP-depleted cells, the uptake mechanism of taurocholate was investigated. In the presence of an inwardly directed Na gradient, the taurocholate uptake was markedly stimulated and bile acid was transiently accumulated at a concentration 3-times higher than at equilibrium ('overshoot') in ATP-depleted cells. No overshoot was observed in viable cells, however, which suggests that in ATP-depleted cells the Na gradient, a driving force for taurocholate uptake, decreased with time. In both viable and ATP-depleted cells, the relationship between medium concentrations of Na and the Na-dependent initial uptake rate were sigmoidal, and the Hill coefficients were close to 2. The Na-dependent initial uptake rate of taurocholate was stimulated by a valinomycin-induced inside negative potassium-diffusion potential in ATP-depleted cells, and the movement of a 'one plus' (as a net) charge was revealed by fitting the data to the Goldman-Hodgkin-Katz equation. These results support the hypothesis that sodium-coupled hepatic uptake of taurocholate occuthrough an electrogenic process with the stoichiometry of 2 Na: 1 taurocholate, although this issue is controversial. In the presence of an outwardly directed sodium gradient, efflux of taurocholate from ATP-depleted cells was not stimulated. Consequently, the physiological transport vector of taurocholate from blood to cell is not only due to the direction of the sodium gradient (blood to cell) but also to membraneous orientation of transport carriers. In conclusion, kinetic analysis using ATP-depleted hepatocytes allowed the formulation of a new approach to clarify the as yet unresolved issues concerning transport stoichiometry and the mechanism for vectorial transport of taurocholate.

Uptake of bilirubin glucuronides by isolated rat hepatocytes

The uptake of bilirubin diglucuronide (BDG) into isolated rat hepatocytes was investigated in order to characterize the mechanism by which bile pigments are transported by the liver. The BDG uptake by hepatocytes was saturable. The uptake was inhibited by bilirubin, sulfobromophthalein, and bilirubin monoglucuronide, but not by taurocholate. The uptake was not affected by replacement of sodium with other cations except for choline. Only when sodium was replaced with choline, was significant decrease in uptake observed. When chloride was replaced with nitrate, BDG uptake decreased, but it was not changed by replacement with sulfate. Metabolic inhibitors did not affect BDG uptake significantly. Thus bile pigments share a common sodium-independent and electrogenic potential-dependent transporter in liver cell membranes. A high concentration of albumin interferes with BDG uptake.

Effect of various organic anions on the plasma disappearance of 1-anilino-8-naphthalene sulfonate

The effects of various organic anions on the hepatic transport of an anionic fluorescent dye, 1-anilino-8-naphthalene sulfonate (ANS) were investigated by measuring the plasma disappearance-time profiles in rats. Ten min after the i.v. administration of ANS (3 mumol/kg), various organic anions (60 mumol/kg) were injected in a bolus. Sulfobromophthalein (BSP), bromophenol blue (BPB) and rose bengal (RB) induced a transient increase in the plasma concentration of ANS (the so-called 'counter-transport' phenomena). The effect of rose bengal was somewhat different. After the administration of rose bengal, the plasma concentration of ANS decreased rapidly followed by a gradual increase. On the other hand, after the administration of bilirubin and taurocholate, the transient increases in plasma ANS concentrations were minimal. No effect was observed after the administration of phenolsulfophthalein (PSP) or oleate. The effects of these organic anions on the binding of ANS to rat liver cytosols were examined by equilibrium dialysis. Sulfobromophthalein, bromophenol blue and rose bengal, which yielded an in vivo 'counter-transport' phenomena, markedly inhibited ANS binding to cytosolic proteins. On the other hand, the other organic anions examined had very small, if any, inhibitory effect. The ANS binders in the cytosol were then identified by gel filtration. ANS bound mainly to X and Y (ligandin) fractions in the cytosol. Sulfobromophthalein, which is one of the organic anions exhibiting the in vivo 'counter-transport' phenomenon, remarkably inhibited ANS binding to ligandin fraction. It was thus suggested that the in vivo 'counter-transport' phenomena may be also explained by the enhancement of back diffusion due to the displacement of intracellular binding. In conclusion, one should be more cautious in interpreting data obtained from so-called in vivo 'counter-transport' experiments.

Membrane-limited hepatic transport of the conjugative metabolites of 4-methylumbelliferone in rats

The hepatic transports of 4-methylumbelliferone (4-MU) and its conjugative metabolites, the glucuronide (4-MUG) and sulfate (4-MUS), were investigated in rats with various methods. The extraction ratio (E) was estimated with the multiple indicator dilution (MID) method using isolated perfused rat liver. The values of E for 4-MUG and 4-MUS were much lower (less than 0.2) than that for the parent compound, 4-MU (0.89). In addition, we examined the simulation of the outflow curves of conjugates based on the "distributed" model in which we varied the permeability between the blood and hepatocytes. When the permeability was much smaller relative to the hepatic blood flow, the simulated curve was superimposed on the dilution curve. These results suggest that the influx permeabilities of these conjugates are so low that little extraction occurs during the passage through the liver. Measuring the unidirectional uptake of these conjugates into the liver with the in vitro centrifugal filtration method using isolated hepatocytes, we determined the influx permeabilities (PSinf(total] for the total ligands. The value of PSinf(total) determined with the in vitro method was extrapolated to that per gram of liver, assuming 1 g of liver has 1.3 X 10(8) cells. The values of PSinf(total) for 4-MU, 4-MUG, and 4-MUS were 4.8, 0.06, and 0.11 mL/min/g liver, respectively. Thus, the influx permeabilities for 4-MUG and 4-MUS were much smaller than the hepatic blood flow (1.6 mL/min/g liver), confirming the results of MID method.

Absence of increased electroneutral Na+-H+ exchange in renal cortical brush-border membranes from hyperthyroid rats

The fluorescence quenching of acridine orange was used to compare Na+-H+ exchange and ion conductances in renal cortical brush-border membrane vesicles (BBMV) isolated from euthyroid and hyperthyroid rats. In BBMV from euthyroid animals, Na+-H+ exchange was entirely electroneutral. In BBMV from hyperthyroid rats, the total rates of Na+-H+ exchange were about 30% higher than in BBMV from euthyroid animals. However, the electroneutral exchange in these membranes was similar to that in BBMV from euthyroid rats; the observed increase in exchange was due to electrically coupled Na+ and H+ movements through conductive pathways in the membranes. Ion conductances in isolated BBMV were tested with outwardly directed K+ gradients in the presence of carbonyl cyanide m-chlorophenylhydrazone (K+ conductance) or valinomycin (H+ conductance). The K+ conductance was negligible and similar in BBMV from both groups of rats. A significant H+ conductance was present in both kinds of membrane preparations and was by 37% higher in BBMV from hyperthyroid animals. Therefore, our experiments failed to demonstrate an increased electroneutral Na+-H+ exchange in BBMV from hyperthyroid rats. Instead, a finding of a significant electrically BBMV from hyperthyroid rats. Instead, a finding of a significant electrically coupled Na+-H+ antiport in the presence of increased H+ conductance in BBMV from hyperthyroid rats indicates that these membranes may also have increased Na+ conductance.

Effect of a diffusional barrier to a metabolite across hepatocytes on its kinetics in "enzyme-distributed" models: a computer-aided simulation study

The effect of a diffusional barrier to a metabolite between the blood and hepatocytes on elimination kinetics of formed and preformed metabolites was predicted under various enzymic distributions in the liver by computer-aided simulation. Sequential metabolism by which the primary metabolite (MI) is generated from the parent drug (D) and further metabolized to the terminal metabolite (MII) by enzymes A and B, respectively, was chosen for the simulation. Moreover, four models of enzyme distribution patterns were defined with regard to the hepatic blood flow path. The extraction ratios for the preformed and formed metabolites (designated as Em and Ep----m, respectively) were simulated by varying both the average intrinsic clearance of enzyme B (CLint,B) and the permeability of hepatocytes for MI (Pm), while keeping the average intrinsic clearance of enzyme A (CLint,A) equal to hepatic blood flow (Q). When a rapid equilibrium of MI between the blood and hepatocytes held, i.e., Pm was large relative to Q, Em was equal to or higher than Ep----m for all models, as previously shown by Pang and Stillwell. By contrast, it was found that when a diffusional barrier for MI existed, i.e., Pm was small relative to Q, Em was equal to or lower than Ep----m. Furthermore, it was observed that the smaller Pm became, the larger the difference between Em and Ep----m became. We further simulated the effect of the intrinsic clearance (CLint,C) for a metabolic pathway, which competes for that by enzyme A, on the Ep----m value. In the model assuming even distribution of all the enzymes along the flow path, irrespective of the CLint,C value, a similar effect of Pm on Ep----m was observed when the Pm value was relatively small (Pm less than Q). By contrast, in the case of uneven enzymic distributions of enzymes A and B, the effect of the CLint,C value on the relationship between Pm and Ep----m occurred to some extent. From these simulations, it was concluded that lower membrane permeability (Pm) both diminishes the entry of preformed metabolite into the hepatocytes and accelerates the removal of intracellularly formed metabolite (through sequential metabolism) by diminishing its efflux, yielding lower Em than Ep----m. When Pm becomes small (Pm less than 1/10Q), these mechanisms for lower Em than Ep----m predominate over other mechanisms such as the presence of a competing metabolic route and uneven distribution of enzymes.

Kinetics of triiodothyronine action on Na-K-ATPase in single segments of rabbit nephron

This study was initiated to define the dose- and time-dependence of triiodothyronine (T3) action on Na-K-ATPase in single microdissected nephron segments. For this purpose, the activity and the number of catalytic sites of Na-K-ATPase, as determined by the specific binding of 3H-ouabain, were measured following a single injection of T3 to rabbits thyroidectomized since 8-12 days. Triiodothyronine restored both the activity and the number of catalytic sites of Na-K-ATPase in a dose-dependent manner in all nephron segments where the enzyme was decreased following thyroidectomy, i.e., the proximal and the collecting tubule. At a dose of 50 micrograms/kg bw, T3 restored Na-K-ATPase activity and 3H-ouabain binding with the same kinetics. However, the kinetics depended on the nephron segments: in the proximal tubule, Na-K-ATPase stimulation occurred after a 12 h period of latency and was completed within 24 h whereas in the collecting tubule, the stimulation was biphasic with a first increase within the first 3 h and a second increase concomitantly to that observed in the proximal tubule. These results indicate that thyroid hormones regulate Na-K-ATPase activity by altering the number of catalytic sites of the enzyme. This control depends on two different mechanisms which differ by their time-dependence.

Insulin stimulates renal glomerular sodium-potassium adenosine triphosphatase activity

The effect of insulin on total and ouabain-inhibited membrane-bound adenosine triphosphatase (ATPase) activity in renal glomeruli isolated from adult white rats was examined. In concentrations of 1-10 micrograms/ml, insulin significantly stimulated the ouabain-inhibited (Na+ + K+)-ATPase activity, without affecting total (composite) ATPase activity. These results, coupled with previous findings demonstrating that glomerular (Na+ + K+)-ATPase activity is reduced in acute streptozotocin diabetes, suggest that the renal glomerulus is a target tissue with respect to this biologic effect of insulin.

Sodium-dependent modulation of the renal Na-K-ATPase: influence of mineralocorticoids on the cortical collecting duct

Mineralocorticoids play a major role in the regulation of sodium transport in a variety of tissues, including the cortical collecting duct (CCD) of the mammalian nephron. To assess, in part, the underlying mechanism(s) of this control, the present studies were designed to evaluate, first, the influence of mineralocorticoids on the Na-K-ATPase activity in the rabbit CCD and, secondly, a possible role of sodium entry into the cell at the luminal border on the regulation of the Na-K-ATPase. In the first series of studies, rabbits were maintained on a low sodium diet which raised serum aldosterone levels from 16 to 70 ng/dl after 3-4 days, with further elevations being expressed with treatment for two weeks or more. In CCDs isolated from these animals, the Na-K-ATPase increased from 13 to 40 pmol ADP min-1 mm-1 after 3-4 days on the low sodium regimen, but then declined, returning to control values after approximately 2 weeks. This decline in activity was preceded by a decrease in the Na+ concentration of the urine to low levels and hence, likely coincided with a decreased delivery of sodium to, and sodium entry into the cells of, the CCD. If dietary manipulations were used to maintain a high delivery of sodium to the CCD in the animal, elevation of plasma mineralocorticoid levels by treatment with deoxycorticosterone acetate (DOCA) caused a similar elevation in the Na-K-ATPase activity after 3-4 days, which did not decline with continued treatment for up to 2 weeks. Furthermore, it was observed that mineralocorticoids only exerted their effect on the Na-K-ATPase after a latent period of 1 day, well after sodium excretion had fallen, indicating that sodium entry into the CCD cells was already stimulated. If animals were simultaneously treated with DOCA and the sodium channel blocker amiloride for 3-4 days, the effects on the Na-K-ATPase were markedly reduced, whereas amiloride treatment alone had no effect on the enzyme activity. Since others have shown that mineralocorticoids induce synthesis of the Na-K-ATPase subunits in toad bladder cells in an amiloride-insensitive manner, sodium must be exerting its effect on a process after translation. It is concluded that the initial effect of mineralocorticoids in the CCD is on sodium entry with a delayed induction of the Na-K-ATPase, which is regulated by Na-dependent modulation of a posttranslational process.

Short term effect of low doses of tri-iodothyronine on proximal tubular membrane Na-K-ATPase and potassium permeability in thyroidectomized rats

Tri-iodothyronine (T3), even when administered for short time and at low doses, induces a large increase in the isotonic fluid reabsorption (Jv) in proximal tubules of thyroidectomized rats (TX). In order to investigate the role of the Na-K-ATPase in this process, we measured the Na-K-ATPase activity in early proximal convoluted tubules (S1) and proximal straight tubules (S2) microdissected from TX rats and rats treated with low doses of T3 (10 micrograms/kg body wt), either for 3 days (TX + 3T3) or for 7 days (TX + 7T3). In both segments no changes in Na-K-ATPase activity were found in TX + 3T3 rats versus TX rats, while an increase was registered in TX + 7T3 rats. Using micropuncture techniques, Jv measured on the same tubular segments increased by 68% in TX + 3T3 rats versus TX. Thus, no correlation between Jv and Na-K-ATPase activity measured in vitro could be detected after short term treatment of TX rats with T3. Na-K-ATPase activity in vivo is also regulated by the potassium permeability of the membrane, which might be altered by tri-iodothyronine. This hypothesis was tested by perfusing intraluminally and peritubularly proximal tubules of TX rats with the K ionophore, valinomycin (1 microgram/ml). In the dual perfusion experiments valinomycin elicited 40% of the action induced on Jv by 3 days treatment with T3. On the other hand, no further increase in Jv was recorded when valinomycin was applied in TX rats pretreated with T3.(ABSTRACT TRUNCATED AT 250 WORDS)

Difference in hepatic uptake kinetics of aspirin and salicylamide in rats

Immediately after intraportal administration to rats, the ratio of liver to plasma concentrations for total aspirin was close to unity, whereas that for total salicylamide ranged from about 3 to 7. The hepatic accumulation of salicylamide appeared to be capacity-limited because the ratio decreased with increases in the dose. In vitro experiments with isolated hepatocytes indicated that aspirin was slowly transported into the hepatocytes by an apparently linear process only, while salicylamide was taken up very rapidly by both saturable and apparently linear transport processes. The cell to medium concentration ratio estimated for the initially net transported component of the unchanged drug was significantly larger with salicylamide, which give ratios from 3.5 to 19, than with aspirin which gave an almost constant value lower than 2 despite wide variations in the initial concentration. For the capacity-limited uptake process of salicylamide, the kinetic parameters were estimated as Vmax = 0.325 nmole X (mg cellular protein)-1 X sec-1 and Km = 201 microM. Among various metabolic inhibitors, 2,4-dinitrophenol (50 microM) inhibited the uptake of salicylamide most extensively. The present comparison of the in vivo and in vitro data for aspirin with those for salicylamide confirmed the previously reported difference in the hepatic first-pass effect of these two drugs.

Cyclosporin A protects liver cells against phalloidin

Cyclosporin A at concentrations of more than 10 nM protects isolated hepatocytes against the action of phalloidin. Cyclosporin A at 100 nM inhibits the uptake of demethyl[3H]phalloin by 50%, and at 5 microM also that of [14C]cholate. This inhibition is independent of the preincubation period and is not reversed by washing the cells. With a 30-60-fold excess of cyclosporin A, affinity labeling of plasma membrane proteins using 12 microM [3H]isothiocyanatobenzamido cholate was reduced to 40-60% of the control. These findings indicate that transport inhibition by cyclosporin A in liver cells cannot be explained by simple competition on the level of the membrane protein(s) involved.

Stimulation of rat renal Na+, K+-ATPase activity by thyroid hormones

The effect of thyroid hormones (T4, T3 and reverse T3) on rat renal Na+,K+-ATPase activity was investigated by a cytochemical technique. T3 caused stimulation of Na+,K+-ATPase activity in the renal medulla but not in the renal cortex. There was a peak in enzyme activity after cultured renal segments had been exposed to T3 for 11 min and this time of maximal stimulation did not vary with the concentration of T3. A rectilinear response in Na+,K+-ATPase activity was observed over T3 concentration range 10 pmol l-1 to 100 nmol l-1; at higher T3 concentrations, Na+,K+-ATPase activity was inhibited. The enzyme response was totally blocked by specific T3 antiserum. Addition of T4 and reverse T3 (100 fmol l-1 -1 mmol l-1) failed to stimulate Na+,K+-ATPase activity in any part of the kidney. Plasma (neat and diluted 1:10) stimulated the enzyme in parallel with the dose response curve and the stimulatory effect was abolished by prior addition of specific T3 antiserum.

Effects of organic anions on the uptake of 1-anilino-8-naphthalenesulfonate by isolated liver cells

Uptake of the fluorescent probe, 1-anilino-8-naphthalene-sulfonate (I) into isolated rat liver cells was studied using both fluorescence and filtration methods. The time course of the fluorescence enhancement of I after addition to the isolated liver cells was analyzed in terms of rapid, medium, and slow phases. The slow phase (half-time approximately 7 min) was characteristic of viable cells. The fluorescence enhancement was proportional to the amount of I taken into the cells, as measured by the filtration method. The uptake of I followed Michaelis-Menten kinetics with an apparent Km of 39 microM and Vmax of 1.4 nmole/10(6) cells/min. The temperature coefficient (Q10) of the uptake of I was found to be approximately 1.9. No pH optimum was observed, and various metabolic inhibitors did not affect the uptake of I. Among the amino acid reagents used, only 2,4-dinitrofluorobenzene decreased the uptake of I (by approximately 45%). The effects of various organic anions on the uptake of I were measured. The inhibition of the uptake of I by sulfobromophthalein could be analyzed in terms of competitive inhibition; the slight inhibition by sodium taurocholate could not. It is concluded that the uptake of I is a carrier-mediated facilitated process, and that the carrier is common to both I and sulfobromophthalein.

Lack of stimulation of renal (Na+ +K+)-ATPase by thyroid hormones in the rabbit

The effect of 1-3,5,3'-triiodothyronine (T3) and thyroxine (T4) on (Na+ +K+)-ATPase activities was examined in rabbit kidneys because in this tissue almost 80% of the metabolism is connected to active sodium transport. T3-receptor concentrations were estimated as 0.62 and 0.80 pmol/mg per DNA in the cortex and outer medulla, respectively. A dose of 0.5 mg T3/kg body weight for 3 days increased basal metabolic rate by almost 60%, and the mitochondrial 1-alpha-glycerophosphate dehydrogenase activity was increased by 50% in both the cortex and medulla. (Na+ +K+)-ATPase activity in the liver was raised by almost 50%. However, no changes in (Na+ +K+)-ATPase activities or binding sites for [3H]ouabain in either the kidney cortex or medulla could be observed. T4 at 16 mg/kg daily for 14 days was also without effect on renal (Na+ +K+)-ATPase activities. Furthermore, the response to T3 was absent at high sodium excretion rates induced by unilateral nephrectomy and extracellular volume expansion. Thus, despite stimulation of basal metabolic rate and renal 1-alpha-glycerophosphate dehydrogenase activity by T3 and T4, the (Na+ +K+)-ATPase activity in the rabbit kidney is identical in euthyroid and hyperthyroid states. However, thyroid hormones prevent the normal natriuretic response to extracellular volume expansion.

Effect of spironolactone on bilirubin metabolism in rat liver and small intestinal mucosa

In vitro and in vivo experimental models were designed for the study of the effect of spironolactone (SP) on bilirubin metabolism in rat liver and small intestinal mucosa. In vitro studies included uptake of bilirubin by liver slices and intestinal sheets, determination of glucuronyltransferase activity in mucosal homogenates, and the handling of bilirubin by the isolated perfused liver after bilirubin overload. In vitro studies were carried out to measure the plasma disappearance rate of bilirubin and to determine the extent of bilirubin conjugation and biliary excretion of the pigment infused intravenously. The results obtained suggested that the mechanisms involved in the uptake of bilirubin by tissues were not influenced by SP pretreatment. Glucuronyltransferase activity in the small intestinal mucosa was significantly induced by SP, as previously observed in rat liver. Isolated perfused livers from SP-treated rats, as well as treated living rats, exhibited a greater than normal capacity for bilirubin excretion into bile at the expense of bilirubin diglucuronide. Conjugated bilirubin in the small intestinal mucosa of rats infused with unconjugated pigment was also increased after SP pretreatment. The results favoured the conclusion that SP is an inducer of bilirubin conjugation in the livers as well as in extrahepatic tissues, such as the small intestinal mucosa.

Uptake of bile acids by isolated rat hepatocytes

Hepatic uptake of five common bile acids was examined in isolated rat hepatocytes. Taurocholic acid (TCA), glycocholic acid (GCA), cholic acid (CA), deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA) were studied. Uptake was extremely rapid and was linear for at least 45 sec for all bile acids tested at substrate concentrations from 1 to 400 micrometers. Both nonsaturable binding to the cell surface membrane and the initial rate of uptake (V0) of the dihydroxy bile acids (DCA and CDCA) were about ten times greater than those of the trihydroxy bile acids (TCA, GCA and CA) which correlates with the higher lipophilicity of the dihydroxy bile acids. Kinetic analysis demonstrated that uptake of these bile acids was due both to a saturable process and a linear process. The apparent diffusion constant (Dapp) of the unsaturable process for the dihydroxy bile acids was also ten times greater than that for the trihydroxy bile acids. After correction for the nonsaturable binding to the cell membrane and linear entry, the Km and Vmax for the uptake was determined. Conjugation with taurine decreased the Km of CA but not the Vmax, while glycine conjugation did not alter either parameter, suggesting that conjugation with taurine may increase its affinity for the transport system. The trihydroxy bile acids have a higher affinity but a lower transporting capacity for the saturable process than the dihydroxy bile acids. In vivo hepatic extraction appears to be more dependent on the affinity of the bile acid for the transport system than the capacity at which it can be transported.

Effect of triiodothyronine on (Na+ G K+)-adenosine triphosphatase and (Na+ + Mg2+)-dependent phosphorylated intermediate in rat salivary glands

The present studies concern the effect of triiodothyronine (T3) on rat salivary (Na+ + K+)-adenosine triphosphatase (NaK-ATPase). The results indicate that T3 selectively increased submandibular and parotid NaK-ATPase units with a single, large dose of T3, different dosage of T3, three successive doses of T3 given on alternate days, and "physiological" doses of T3 with daily injection for 2 weeks. Sublingual NaK-ATPase was insensitive to T3 in the above experiments. The effect of T3 on submandibular and parotid NaK-ATPase is selective since Mg-ATPase and 5'-nucleotidase showed no significant differences under different thyroid status. The lack of response of NaK-ATPase to reverse T3, further substantiates the specificity of T3 on salivary NaK-ATPase.

Effect of triiodothyronine on renal growth and renal sodium reabsorption in hypothyroid rats

Experiments were carried out to compare temporal changes in the paraaminohippuric acid clearance (CPAH), renal sodium reabsorption (RNa+), ribonucleic acid (RNA), and deoxyribonucleic acid (DNA) content in hypothyroid rats after a single injection of triiodothyronine (T3) (50 micrograms/100 g body wt). CPAH and RNa+ showed no changes at 24 and 48 h. At 72 h, however, significant increases of 41% and 42% (per g kidney wet wt) were observed in CPAH and RNa+, respectively. The cortex in T3-treated hypothyroid rats showed a significant increase in the protein/DNA and RNA/DNA ratios at 24 h and progressive increases to a level of 24%, and 37%, respectively, at 48 h. No changes in DNA content were observed at either time-points. The results show that the increases in RNA/DNA and protein/DNA ratios upon T3 treatment preceded the increases in CPAH and RNa+, suggesting a direct effect of T3 on renal cortical growth, rather than a secondary response to a primary increase in renal functions.

Uptake of sulfobromophthalein-glutathione conjugate by isolated hepatocytes

Uptake of sulfobromophthalein-glutathione conjugate (BSP-GS) was studied using isolated hepatocytes and was compared with previous results with the parent BSP (Schwenk et al., Eur. J. Biochem. 64: 189-197, 1976). Adsorption of BSP-GS to the liver cell membrane has two sites with different affinities and binding capacities. Uptake of the conjugate is decreased by two metabolic inhibitors and appears to be saturable up to 12.5 microM with an apparent Km of 4 microM (7 for BSP) and a Vmax of 0.16 nmol x mg protein-1 x min-1 (2.6 for BSP). Similar to the parent BSP at higher concentrations a second mechanism of uptake is observed indicated by the nonlinear Hanes plot. SP-GS transport is competitively inhibited by BSP with a Ki of about 1 microM, and, similar to BSP, replacement of sodium ions in the medium had little effect on the rates of uptake. These results suggest a common transport mechanism for BSP and its glutathione conjugate. Furthermore, the effect of organic anions, bases, and steroids on uptake of BSP-GS has been tested.