Biomedical subjects

M Hanano

Publications and source records attributed to M Hanano.

At least 37 records · Page 2Linked to original sources

PubMed1993

Na(+)-independent multispecific anion transporter mediates active transport of pravastatin into rat liver.

To examine whether the relatively selective inhibition of hepatic cholesterol synthesis by the hydrophilic 3-hydroxyl-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor pravastatin in vivo may be due to the existence of a specific uptake mechanism in the liver, the uptake by isolated rat hepatocytes was investigated. The uptake was composed of a saturable component [Michaelis constant (Km) 29 microM, maximal uptake rate 546 pmol.min-1.mg-1] and nonspecific diffusion (nonspecific uptake clearance 1.6 microliters.min-1.mg-1), inhibited by hypothermia, metabolic inhibitors, sulfhydryl-modifying reagents, and inhibitor of anion exchanger, whereas replacement of Na+ by choline+ or Cl- by gluconate- did not alter the uptake. Competitive inhibition was observed by a more highly lipophilic HMG-CoA reductase inhibitor simvastatin (open acid form), dibromosulfophthalein, cholate, and taurocholate. Pravastatin inhibited Na(+)-independent taurocholate uptake with an inhibition constant comparable with the Km value of pravastatin itself. Furthermore, the hepatic permeability clearance in vivo obtained with intact rats was comparable with that in vitro, indicating that the carrier-mediated active transport system we demonstrated in vitro is responsible for the hepatic uptake in vivo. These findings demonstrated that the hepatic uptake of pravastatin occurs via a carrier-mediated active transport mechanism utilizing the so-called multispecific anion transporter, which is common with the Na(+)-independent bile acid uptake system, and that this is one of the mechanisms for its selective inhibition of hepatic cholesterol synthesis in vivo.

Animals↗

PubMed1993

Different relationships between cellular ATP and hepatic uptake among taurocholate, cholate, and organic anions.

Effects of cellular ATP content on uptake of cholate (CA) and organic anions (OAs; dibromosulfophthalein and benzylpenicillin) by isolated rat hepatocytes were investigated and were compared with that on taurocholate (TCA). Within 5 min of exposure to metabolic inhibitors (MIs), cellular ATP content fell to less than one-fifth of the control value, and the initial velocity of the total uptake of CA, Na(+)-independent uptake of CA and TCA, and the uptake of the OAs dropped in parallel with the decrease in cellular ATP. Whereas for the total uptake of TCA, the initial uptake remained virtually unchanged for a 5-min incubation with the MIs; a significant decrease in uptake was observed only after longer incubation times. Under variously ATP-decreased conditions, the initial velocity of the total uptake of CA and OAs was demonstrated to have a saturable relation to cellular ATP content, irrespective of exposure time to MIs. A difference in the Na(+)-dependent uptake of TCA and CA was also observed in terms of the inhibitory effect of the organic anion pravastatin. That is, the inhibition by pravastatin was partial for TCA uptake but almost complete for CA uptake. These findings suggest the following. 1) The mechanism of Na(+)-dependent CA uptake is different from that of TCA. 2) The Na(+)-independent uptake of bile acids and organic anions may be driven either by ATP hydrolysis (primary active transport) or by an as yet unidentified ion gradient that dissipates more rapidly than the Na+ gradient.

Adenosine Triphosphate↗

PubMed1993

Dose-dependent hepatic handling of l-propranolol determined by multiple indicator dilution method: influence of tissue binding of l-propranolol on its hepatic elimination.

Concentration-dependency in the hepatic elimination of l-propranolol (l-PR) was investigated over a wide range of concentrations from 60 to 2200 microM in an isolated rat liver perfusion system. Under the steady-state condition produced by unlabeled l-PR at various concentrations, 3H-l-PR and 14C-inulin were bolusly injected into the portal vein, and the outflow was collected at 0.5 s intervals over 30 s. Up to 300 microM, the instantaneous hepatic availability of l-PR was approximately 4%, while it abruptly increased when the perfusate concentration exceeded 300 microM. To determine which process (influx or efflux or sequestration process) caused the nonlinearity, we calculated the rate constants k1 (influx), k2 (efflux), and k3 (sequestration) based on the "distributed" model. With increasing l-PR concentration in the perfusate, k2 increased approximately two times, whereas k3 decreased to approximately one-half. In contrast, k1 was independent of the perfusate concentration. The concentration-dependency of k2 was explained by saturation of l-PR tissue binding, since the tissue unbound fraction of l-PR obtained with liver homogenates and isolated hepatocytes increased approximately two times. The efflux and sequestration clearances were then normalized by the unbound fractions in the liver. The efflux clearance for unbound l-PR was constant irrespective of the perfusate concentration, whereas the sequestration clearance for unbound l-PR (CL(int) showed Michaelis-Menten type saturation (Km = 28 microM, Vmax = 2.8 mumol/min/g liver, alpha (nonspecific) = 2- ml/min/g liver).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Construction of a physiologically based pharmacokinetic model to describe the hepatobiliary excretion process of ligands: quantitative estimation of intracellular diffusion.

A physiologically based pharmacokinetic model was established to describe the hepatobiliary excretion process for ligands which are excreted into the bile without metabolic conversion. In this model, the following processes were taken into consideration: influx and efflux across the liver sinusoidal membrane, intracellular diffusion, and excretion across the canalicular membrane into the bile. The partial differential equation by which these processes were described was solved to obtain the Laplace transformed solution for the biliary excretion rate of ligands, based on the plasma concentration profiles as an input function. The time profiles for the biliary excretion rate of dibromosulfophthalein (DBSP; 6.80 mg/kg b.w.) or cefodizime (15.0 mg/kg b.w.) after i.v. bolus administration to rats were fitted to the solution using a nonlinear least-squares program based on a fast inverse Laplace transform (MULTI-FILT [Y. Yano et al., Chem. Pharm. Bull., 37, 1035 (1989)] to determine the permeability-surface area product across the plasma membrane as well as the apparent intracellular diffusion coefficient. DBSP and cefodizime were used as model compounds since these two ligands possess different binding characteristics for cytosol protein(s). Although both ligands are present predominantly in the cytosol, DBSP binds to intracellular protein(s) (such as ligandin) to a great extent whereas the protein binding of cefodizime is not so extensive. The fitted lines were superimposed on the experimental results.(ABSTRACT TRUNCATED AT 250 WORDS)

Bile↗

PubMed1993

The influence of glucagon on the hepatic transport of taurocholate in isolated perfused rat liver: kinetic analysis by the multiple indicator dilution technique.

Glucagon has been demonstrated to stimulate the uptake of bile acid in isolated rat hepatocytes (Am. J. Physiol., 249, G427 (1985)). In the present study, we determined the influence of glucagon on the hepatic transport of a bile acid, taurocholate (TCA), in isolated rat livers. A single-pass perfusion and a rapid-injection, multiple indicator dilution method were employed. The hepatic availability at steady-state was 0.04. With the presence of glucagon in the perfusate (from 10(-9) to 10(-7) M), the bile flow rate was stimulated by 30%, while hepatic availability was decreased from 0.04 to 0.02 with a stepwise increase in glucagon concentration. Thirty min after the infusion of glucagon (300 nM), [3H]TCA and [14C]inulin were injected in a bolus state into the portal vein, and the outflow was collected at 1.0 s intervals over 30 s. Glucagon decreased the instantaneous hepatic availability by 50% compared to the control level, and was thus compatible with the steady-state experiments. In the control experiment, the influx clearance (PSinf) was 20 times higher than the efflux clearance (PSeff). Glucagon (300 nM) in the perfusate enhanced PSinf by 50% of the control, whereas sequestration clearance (CLseq) and the biliary excretion rate constant remained unchanged. PSeff was stimulated to 2 times the control, but still remained much smaller than CLseq. Based on the comparison of PSinf, PSeff and CLseq, the rate-determining process of TCA hepatic elimination was the influx process in both the presence and absence of glucagon. Taken together, the enhancement of the influx process was responsible for the decrease in TCA hepatic availability caused by glucagon.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Identification of tissues responsible for the conjugative metabolism of liquiritigenin in rats: an analysis based on metabolite kinetics.

We kinetically examined tissues responsible for the conjugative metabolism (glucuronidation and sulfation) of a component in a crude drug, liquiritigenin (LG; 2,3-dihydro-7-hydroxy-2-(4- hydroxyphenyl)-(S)-4H-1-benzopyran-4-one) in rats in vivo. LG has been found to form five kinds of conjugates (4'-O-glucuronide (M1), 7-O-glucuronide (M2), 4',7-O-disulfate (M3), 4'-O-glucuronide-7-O-sulfate (M4) and 7-O-glucuronide-4'-O-sulfate (M5)). Analysis based on metabolite kinetics [K. S. Pang, J. Pharmacokin. Biopharm., 13, 633 (1985)] of the area under the plasma concentration curves (AUCplasma) and cumulative biliary excretions (Aibile) of the ligands after intravenous or hepatic portal venous administration of LG revealed that the liver has the ability to generate all the metabolites. For M1 and M2, the apparent biliary excretion clearance (CLbile,app) obtained by dividing the biliary excretion rate for the metabolite by the plasma concentration of the metabolite decreased with time, confirming that M1 and M2 were formed in the liver. To further analyze the formation rate constants for metabolites in each tissue, we measured the ligand content in several tissues after intravenous administration of LG. By correcting the content of metabolites that were taken up from the plasma, we found that the formation rates per gram of tissue were largest in the liver, except for M3. The metabolic capability of the kidney for M1 and M2 was 15% and 60%, respectively, to that of the liver whereas for M3, the metabolic ability of the kidney was 2.5-fold greater than that of the liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

[Fat-soluble vitamins: Intestinal absorption].

General principals of drug absorption and bioavailability after the oral administration are discussed in each process of drug release and dissolution from the preparation, such as gastric emptying time and flow of drug in intestinal track, transport through intestinal mucosal microvilli, and the first pass effect of drug elimination in the liver. Character of lipid soluble vitamins in these absorption processes are elucidated with the low solubility in water and solubilization in bile acids micelle. Our studies on dl-alpha-tocopherol solubilization into micelles of bile acids and a detergent and the rat intestinal absorption by perfusion of the micellar solutions are introduced. Faster absorption of the tocopherol solubilized in bile acid micelles, than polysorbate 80, was observed in spite of the less solubilizing abilities of bile acids. Relatively high dependency on volume flow through the intestinal wall was observed in the absorption rate of tocopherol. The dependency is larger in bile acid micelles having smaller size than polysorbate 80 having large size.

Animals↗

PubMed1993

Kinetic analysis of hepatobiliary transport of organic anions in Eisai hyperbilirubinemic mutant rats.

The hepatobiliary transport of nonmetabolizable organic anions [cefodizime, dibromosulfophthalein, and indocyanine green (ICG)] was kinetically analyzed in Eisai hyperbilirubinemic rats (EHBR; Sprague-Dawley-derived mutant rats with conjugated hyperbilirubinemia). In EHBR, the biliary excretion of these anions was remarkably impaired, whereas the alteration in initial plasma disappearance was minimal. The kinetic analysis of the disposition of these ligands revealed 1) that the transport rate via bile canalicular membrane was severely impaired in EHBR for cefodizime and dibromosulfophthalein and 2) that the intracellular transport rate of ICG was decreased in EHBR, which contributed more than the decrease in the canalicular membrane transport to the net reduction of the excretion rate of ICG in EHBR. The latter finding was also supported by the in vitro results; the binding of ICG to ligandin(s) in EHBR was less extensive compared with that in normal rats, resulting in the higher distribution of ICG to organelle. Because the ligand molecules bound to organelle diffuse within the cells much more slowly than the molecules in the cytosol, the higher distribution of ICG to organelle in EHBR results in the reduction in the intracellular transport rate. These results indicate that the differential impairment mechanisms can be proposed for the excretion of organic anions: One is the impairment in the transport rate across the canalicular membrane, the other is the impairment in the intracellular transport rate.

Animals↗

PubMed1992

Existence of two pathways for the endocytosis of epidermal growth factor by rat liver: phenylarsine oxide-sensitive and -insensitive pathways.

The effect of phenylarsine oxide (PAO) on the internalization rate of epidermal growth factor (EGF) was investigated using perfused rat liver and isolated rat hepatocytes. In perfused liver, a tracer concentration of 125I-EGF alone or with excess unlabeled EGF (20 nM) was perfused and the internalization rate constants (kint) were measured. In the absence of PAO, kint values did not differ significantly for either dose condition. However, with the addition of PAO to the perfusate, the kint value dropped to 4% of that of the control at the low concentration of EGF, while dropping to only 40% of that of the control at the high concentration of EGF. These results suggest the existence of a PAO-insensitive internalization pathway having a kint value comparable with that of the other pathway. Similar EGF concentration-dependent inhibition of 125I-EGF internalization caused by PAO was ascertained using isolated rat hepatocytes. PAO also decreased the cellular ATP content in isolated hepatocytes. However, when we lowered the cellular ATP content with rotenone, the cell-surface binding and internalization of EGF were comparable with the control levels. We concluded that there exist dual pathways for the internalization of EGF and that excess doses of EGF lead to EGF internalization not only through a PAO-sensitive pathway but also through a PAO-insensitive pathway, whereas at a tracer dose of EGF, the internalization occurs mainly via the PAO-sensitive pathway.

Adenosine Triphosphate↗

PubMed1992

Circulating thrombomodulin as a novel endothelial cell marker: comparison of its behavior with von Willebrand factor and tissue-type plasminogen activator.

Circulating thrombomodulin is a novel endothelial cell marker, which may reflect the endothelial injury. Plasma levels of thrombomodulin were quantitated by an enzyme-linked immunosorbent assay (ELISA) in patients with hematological malignancies, liver disease, diabetes mellitus, collagen disease, thrombotic disease, and disseminated intravascular coagulation (DIC), and the thrombomodulin values were compared with those of von Willebrand factor antigen (vWf:Ag) and tissue-type plasminogen activator (t-PA) which are released from stimulated or damaged endothelial cells. The mean plasma concentrations of thrombomodulin in these disease states were elevated as compared with healthy subjects. A relatively high mean thrombomodulin level was observed in DIC, liver disease, and collagen disease. Abnormally high thrombomodulin values (greater than normal mean value + 3 SD) were found in 32.3% of patients with hematological malignancies, 57.7% of patients with liver disease, 39.3% of patients with diabetes mellitus, 30.0% of patients with collagen disease, 23.1% of patients with thrombotic disease, and 69.0% of patients with DIC. Plasma concentrations of both vWf:Ag and t-PA were also elevated in these patients. On the whole, the plasma thrombomodulin concentration was positively correlated with vWf:Ag (r = 0.441, P less than 0.001) and t-PA (r = 0.398, P less than 0.001). These findings indicate that the elevation of plasma thrombomodulin is frequently seen in a variety of diseases and circulating thrombomodulin is possibly useful for evaluating the endothelial damage in selected disease states.

Biomarkers↗

PubMed1992

Coexistence of congenital afibrinogenemia and protein C deficiency in a patient.

A rare association of congenital afibrinogenemia and hereditary protein C deficiency is described in a 37-year-old female who suffered from ischemic necrosis in the left first toe. The diagnosis of afibrinogenemia was assessed by the absence of fibrinogen in clotting and immunological assays. The diagnosis of hereditary heterozygous type I protein C deficiency was based on the evidence of proportional decreases of activity and antigen of plasma protein C in the propositus, her mother, and two maternal aunts.

Adult↗

PubMed1992

Effect of basic drugs on the hepatic uptake of ouabain by sinusoidal plasma membrane vesicles isolated from rat liver.

Although antiarrhythmic drugs are used to treat digitalis-induced cardiac disorders, some of these drugs have been reported to increase the serum digoxin concentration in patients, causing the severe side-effects. We have previously shown that many basic drugs including antiarrhythmic drugs inhibited the hepatic uptake of cardiac glycosides into isolated rat hepatocytes, which could be a cause for the increased serum digoxin concentration. The present study was designed to examine the mechanism of this inhibition using isolated rat sinusoidal plasma membrane vesicles. The effect of nine basic drugs (dipyridamole, nifedipine, verapamil, chlorpromazine, lidocaine, quinidine, ajmaline, disopyramide, and propranolol) on the uptake of ouabain was studied. Quinidine reduced the initial uptake rate of ouabain (30 s) while it did not change the uptake of ouabain in an equilibrium condition (30 min). Other basic drugs, such as verapamil, dipyridamole, and nifedipine also significantly reduced the initial uptake rate of ouabain. These basic drugs had no effect on the membrane fluidity. The inhibitory effects on the vesicular uptake were significantly correlated with the inhibitory effects on ouabain uptake by the isolated rat hepatocytes. These findings may suggest that the mechanism of the inhibition involves the inhibition of the transport process via the sinusoidal plasma membrane.

Animals↗

PubMed1992

Nonlinear pharmacokinetics of hepatobiliary transport of rose bengal in rats after i.v. bolus administration with varying doses.

To investigate the nonlinear kinetics in the hepatobiliary transport of rose bengal (RB), the time profiles of plasma concentration and biliary excretion rate after its i.v. administration at various doses were measured in rats. The total body clearance decreased remarkably with increased dose. The hepatic uptake clearance also showed a similar dose dependency, and saturation of hepatic uptake at least partly accounts for the dose-dependent change in total body clearance. The peak biliary excretion rate approached the transport maximum (approximately 150 nmol min-1 kg-1) with increased dose. To further clarify which process in RB hepatobiliary transport has nonlinearity, we analysed thus obtained data based on a three-compartment model. The hepatic uptake and sequestration rate constants decreased remarkably with increased dose. The initial hepatic uptake rates assessed from the plasma disappearance rate during the early phase fit well to the Michaelis-Menten equation with a saturable and a nonsaturable component. The maximum uptake velocity and Michaelis constant were 4.7 mumol min-1 kg-1 and 360 microM, respectively. That hepatic uptake has a much higher capacity (about 30 fold) than biliary excretion suggests that biliary excretion can be a rate-determining process in the overall hepatobiliary transport of RB. We conclude that the saturation of both hepatic uptake and biliary excretion could be the main causes for the nonlinear pharmacokinetics of hepatobiliary transport of RB.

Animals↗

PubMed1992

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.

Adenosine Triphosphate↗

PubMed1992

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.

Adenosine Triphosphate↗

PubMed1992

'Albumin-mediated transport phenomenon' observed for ligands with high membrane permeability. Effect of the unstirred water layer in the Disse's space of rat liver.

In this paper, we offer experimental evidence of the rate-limiting diffusion of ligands through the unstirred water layer (UWL) as an explanation for the so-called albumin-mediated transport phenomenon. The relative membrane permeability of various ligands was first evaluated using isolated rat hepatocytes. Then, the effect of albumin on the uptake of ligands of a wide range of membrane permeabilities was examined using the perfused rat liver. The results were similar to those expected from the UWL model: ligands with high membrane permeability (warfarin, diazepam and taurocholate) clearly exhibited albumin-mediated transport, those with medium membrane permeability (tolbutamide and salicylate) showed less albumin-mediated transport, and ligand with low membrane permeability (cefodizime) did not show albumin-mediated transport. These results were explained by simulation studies of two separate cases based on the UWL model; one assuming the rapid equilibrium of ligand binding with albumin, and the other considering the slow dissociation of ligands from albumin. In light of these findings, we suggest that the rate-limiting diffusion through the UWL plays an important role in the so-called albumin-mediated transport phenomenon.

Animals↗

PubMed1992

Kinetic analysis of hepatobiliary transport of vincristine in perfused rat liver. Possible roles of P-glycoprotein in biliary excretion of vincristine.

Recent studies using bile canalicular membrane vesicles have suggested that P-glycoprotein may play a role in excreting some anticancer drugs from the liver to the bile. At steady state after a continuous single-pass perfusion of a tracer concentration of [3H]vincristine in the rat liver, the extraction ratio was approximately 0.6, and 70% of the extracted drug was excreted into the bile mostly in unchanged form. The liver/perfusate and bile/liver unbound concentration ratios obtained after correction for intracellular binding and the inside-negative membrane potentials and/or pH difference between the inside and outside of the cells, were approximately 2-3 and 160-280, respectively, suggesting a highly concentrated biliary excretion process. We also examined the effects of verapamil, a P-glycoprotein-related transport inhibitor in cancer cells, on the hepatobiliary transport of [3H]vincristine. Verapamil 50 microM in the perfusate caused a decrease in the biliary excretion rate of [3H]vincristine, whereas [14C]taurocholate (reference compound) remained constant. In contrast, the hepatic uptake rate of [3H]vincristine exhibited minimum reduction, suggesting that verapamil selectively inhibited the biliary excretion of [3H]vincristine at the canalicular membrane. The fact that verapamil had little effect on the initial velocity of [3H]vincristine uptake by isolated hepatocytes also supports the above findings. Since the effect of 150 microM verapamil in the perfusate was not selective for vincristine, the biliary excretion rates of both compounds ([3H]vincristine, [14C]taurocholate) were reduced by this concentration of verapamil. In conclusion, the concentrative excretion of vincristine into the bile and its selective inhibition by a moderate concentration of verapamil provide indirect evidence for the contribution of P-glycoprotein to the biliary excretion of vincristine in a perfused rat liver system.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1992

Kinetic modeling of ouabain tissue distribution based on slow and saturable binding to Na,K-ATPase.

The significance of the binding to Na,K-ATPase in the tissue distribution of ouabain was previously documented (Harashima et al., Pharm. Res. 9:474-479, 1992). The purpose of this study was to obtain a kinetic model of ouabain tissue distribution. In most tissues, the ouabain concentration continued to rise after the termination of infusion (5 min), with the peak tissue concentration at approximately 20 min. This delay could not be explained by the rapid equilibrium model (RE model), nor could the kinetics of ouabain be explained by an RE model modified for saturable binding. Since ouabain binding to Na,K-ATPase is slow, the association and dissociation processes were incorporated into a model that can accurately fit the observed time courses of ouabain. The obtained binding parameters corresponded well with the observed values in the in vitro binding experiments, except for muscle. These results quantitatively support the role of the slow and saturable binding of ouabain to Na,K-ATPase in its tissue distribution.

Animals↗