An expressive-arts-based life-death education program for the elderly: A qualitative study.

This study endeavors to investigate how healthcare workers, equipped with expressive arts methods, could foster life-death education for the elderly. Forty-nine older adults aged 60 or above joined a 10-session expressive arts-based life-death education program that was led by social workers equipped with expressive arts methods. An ethnographic research approach, with a post-treatment focus group (n = 17), was conducted with the participants. The results showed that expressive arts methods could enhance reorganization of life experiences, promote dealing with ambivalent emotion regarding life-death issues, improve communicating life-death issues with family members, and induce ideas to prepare for death.

Poisoning by toxic plants in Hong Kong: a 15-year review.

INTRODUCTION: Hong Kong has a great diversity of plants, many of which are toxic to humans. The aim of this study was to identify the plant species most commonly involved in cases of plant poisoning in Hong Kong and to provide clinicians with a reference tool for the diagnosis and management of plant poisoning. METHODS: We retrospectively reviewed all plant poisoning cases referred to the Hospital Authority Toxicology Reference Laboratory from 1 January 2003 to 31 December 2017. Demographics, clinical presentation, laboratory findings, treatment and outcomes of patients, as well as morphological identification and analytical testing of the plant specimens, were investigated. RESULTS: A total of 62 cases involving 26 poisonous plant species were identified, among which Alocasia macrorrhizos (Giant Alocasia), Gelsemium elegans (Graceful Jessamine), and Rhododendron (Azalea) species were the three most commonly encountered. Gastrointestinal toxicity (n=30, 48%), neurological toxicity (n=22, 35%), and hepatotoxicity (n=6, 10%) were the three most common clinical problems. Forty-nine (79%) and eight (13%) patients had mild and moderate toxicity, respectively; they all recovered shortly with supportive treatment. The remaining five (8%) patients experienced severe toxicity requiring intensive care support. Most patients (n=61, 98%) used the plants intentionally: as a medicinal herb (n=31), as food (n=29), and for attempting suicide (n=1). Reasons for using the poisonous plants included misidentification (n=34, 55%), unawareness of the toxicity (n=20, 32%), and contamination (n=6, 10%). CONCLUSIONS: Although most plant exposure resulted in a self-limiting disease, severe poisonings were encountered. Epidemiology of plant poisonings is geographically specific. Clinicians should be aware of local poisonous plants and their toxicities.

Budd-Chiari syndrome secondary to toxic pyrrolizidine alkaloid exposure.

In this report, we describe a case of pyrrolizidine alkaloid-related Budd-Chiari syndrome in Hong Kong. A 10-month-old boy presented with ascites, right pleural effusion, and hepatomegaly after consumption of herbal drinks for 3 months. His clinical (including imaging) features were compatible with Budd-Chiari syndrome. Budd-Chiari syndrome is a rare disease entity in paediatric patients. In our case, extensive workup performed to look for the underlying cause of Budd-Chiari syndrome was unrevealing, except for toxic pyrrolizidine alkaloid exposure in his herbal drinks.

ITC recommendations for transporter kinetic parameter estimation and translational modeling of transport-mediated PK and DDIs in humans.

This white paper provides a critical analysis of methods for estimating transporter kinetics and recommendations on proper parameter calculation in various experimental systems. Rational interpretation of transporter-knockout animal findings and application of static and dynamic physiologically based modeling approaches for prediction of human transporter-mediated pharmacokinetics and drug-drug interactions (DDIs) are presented. The objective is to provide appropriate guidance for the use of in vitro, in vivo, and modeling tools in translational transporter science.

Pharmacokinetics of tranexamic acid in patients undergoing cardiac surgery with use of cardiopulmonary bypass.

We conducted a study to assess pharmacokinetics of high-dose tranexamic acid for 24 h after administration of the drug in patients undergoing cardiac surgery with cardiopulmonary bypass. High-dose tranexamic acid involved a bolus of 30 mg.kg(-1) infused over 15 min followed by a 16 mg.kg(-1) .h(-1) infusion until chest closure with a 2 mg.kg(-1) load within the pump prime. Tranexamic acid followed first-order kinetics best described using a two-compartment model, with a total body clearance that approximated the glomerular filtration rate. Mean plasma tranexamic acid concentrations during the intra-operative period and in the first 6 postoperative hours were consistently higher than the suggested threshold to achieve 100% inhibition and 80% inhibition of tissue plasminogen activator. With recent studies implicating high-dose tranexamic acid as a possible aetiology of postoperative seizures following cardiac surgery, the minimum effective yet safe dose of tranexamic acid in high-risk cardiac surgery needs to be refined.

Transport of 5,5-diphenylbarbituric acid and its precursors and their effect on P-gp, MRP2 and CYP3A4 in Caco-2 and LS180 cells.

AIM: To examine the transport of 5,5-diphenylbarbituric acid sodium (T2007) and its mono- (MMMDPB) and di- (T2000) methoxymethylated precursors and their inducibility potential in Caco-2 and LS180 cells. METHODS: Transport studies of T2000, MMMDPB and T2007 in Caco-2 cells were performed in Transwells. P-gp and CYP3A4 activities were assayed by [(3)H]digoxin and rhodamine 123 cellular retention and testosterone 6ß-hydroxylation, respectively. Expressions of PXR, VDR and CAR mRNA and CYP3A4, MDR1/P-gp and MRP2 mRNA and protein were determined by qPCR and Western blotting, respectively. PXR siRNA was used to assess the involvement of PXR. RESULTS: The P(app(A→B))s and P(app(B→A))s of T2000, MMMDPB and T2007 were similar (30-35×10(-6)cm/s) in Caco-2 cells. Treatment for 3 days with T2000 (15µM), MMMDPB (70µM) and T2007 (300µM) generally furnished a greater induction in LS180 cells over the Caco-2 cells due to the higher, natural abundance of PXR. Changes in expression were confined mostly to MDR1 and CYP3A4: in LS180 cells, treatment for 3 days increased MDR1 and CYP3A4 but not MRP2 mRNA, and elevated P-gp and CYP3A4 protein expression that led to decreased cellular accumulation of [(3)H]digoxin and rhodamine 123, and enhanced testosterone 6ß-hydroxylase activity towards T2007, respectively. The silencing of PXR by PXR siRNA in LS180 cells significantly attenuated the induction of MDR1 and CYP3A4. CONCLUSIONS: T2000, MMMDPB, and T2007 exhibited high permeabilities but are not P-gp substrates. T2007 and its analogs upregulated CYP3A4 and MDR1 modestly via the PXR.

Implantation metastasis in a 13-year-old girl: a case report.

We report a case of a 13-year-old girl with an osteosarcoma of the right humerus, which had been diagnosed as an aneurysmal bone cyst at our institution. She underwent curettage and bone grafting of the lesion, which resulted in implantation metastasis of the tumour to the ilium. She died 15 months after presentation owing to pulmonary metastases. This report highlights the possibility of metastasis occurring by direct implantation to a graft donor site. We strongly recommend that a biopsy be performed in cases of presumed benign lesions before proceeding with the definitive surgery.

Absorption of benzoic acid in segmental regions of the vascularly perfused rat small intestine preparation.

Oral bioavailability is a consequence of intestinal absorption, exsorption, and metabolism and is further modulated by the difference in activities among segmental regions. The influence of these factors on the net absorption of benzoic acid (BA), a substrate that is metabolized to hippurate and is transported by the monocarboxylic acid transporter 1, was studied in the recirculating, vascularly perfused, rat small intestine preparation. Metabolism of BA was not observed for both systemic and intraluminal injections into segments of varying lengths. But, secretion of BA into lumen was noted. Absorption of BA (0.166-3.68 micromol) introduced at the duodenal end for absorption by the entire intestine was complete (>95% dose at 2 h) and dose-independent, yielding similar absorption rate constants (k(a) of 0.0464 min(-1)). The extent of absorption remained high (92-96% dose) when BA was injected into closed segments of shorter lengths (12 or 20 cm), suggesting a large reserve length of the rat intestine. However, k(a) was higher for the jejunum (0.0519 and 0.0564 min(-1), respectively, for the 12- and 20-cm segments) and exceeded that for the duodenum (12-cm segment, 0.0442 min(-1)) and ileum (20-cm segment, 0.0380 min(-1)) at closed injection sites. The finding paralleled the distribution of monocarboxylic acid transporter isoform 1 detected by Western blotting along the length of the small intestine. Fits of the systemic and oral data (based on duodenal injection for absorption by the whole intestine) to the traditional, physiological model and to the segregated flow model (SFM) that describes partial intestinal flow to the enterocyte region showed a better fit with the SFM even though metabolite data were absent.

Hepatic uptake and metabolism of benzoate: a multiple indicator dilution, perfused rat liver study.

Multiple, noneliminated references ((51)Cr-labeled erythrocytes, (125)I-albumin, [(14)C]- or [(3)H]sucrose, and [(2)H](2)O), together with [(3)H]hippurate or [(14)C]benzoate, were injected simultaneously into the portal vein of the perfused rat liver during single-pass delivery of benzoate (5-1,000 microM) and hippurate (5 microM) to investigate hippurate formation kinetics and transport. The outflow dilution data best fit a space-distributed model comprising vascular and cellular pools for benzoate and hippurate; there was further need to segregate the cellular pool of benzoate into shallow (cytosolic) and deep (mitochondrial) pools. Fitted values of the membrane permeability-surface area products for sinusoidal entry of unbound benzoate were fast and concentration independent (0.89 +/- 0.17 ml. s(-1). g(-1)) and greatly exceeded the plasma flow rate (0.0169 +/- 0.0018 ml. s(-1). g(-1)), whereas both the influx of benzoate into the deep pool and the formation of hippurate occurring therein appeared to be saturable. Results of the fit to the dilution data suggest rapid uptake of benzoate, with glycination occurring within the deep and not the shallow pool as the rate-determining step.

Futile cycling of estrone sulfate and estrone in the recirculating perfused rat liver preparation.

The futile cycling of estrone sulfate (E(1)S) and estrone (E1) was investigated in the recirculating, perfused, rat liver preparation. Although E(1)S was not distributed into bovine erythrocytes, the compound was highly bound to albumin [4% bovine serum albumin (BSA), unbound fraction of 0.03 +/- 0.01]. By contrast, E1 was bound and metabolized to estradiol (E2) by bovine erythrocytes, with metabolic clearances of 0.061 to 0.069 ml/min when normalized to the hematocrit. Due to strong binding of E1 to albumin, BSA (4%) greatly reduced the red cell clearance to a minimum (0.0024 to 0.0031 ml/min/unit of hematocrit). Despite the low unbound fractions of E(1)S (0.027 +/- 0.004) and E1 (0.036 +/- 0.006), clearances of the simultaneously delivered tracers [(3)H]E(1)S and [(14)C]E1 in perfusate (4% BSA and 20% erythrocytes) by the recirculating, perfused rat liver (flow rate of 0.91 +/- 0.1 ml/min/g of liver) were high (0.53 +/- 0.08 and 0.85 +/- 0.2 ml/min/g of liver, respectively). Although low levels of [(3)H]E1 were observed following the tracer [(3)H]E(1)S, both parent and metabolite species displayed similar decay half-lives that were characteristic of compounds undergoing futile cycling. The same decay profile was observed for [(14)C]E(1)S but the half-life of administered [(14)C]E1 was shorter in comparison. A series-compartment liver model that incorporated previously noted heterogeneity in estrone sulfation and glucuronidation activities among periportal and perivenous hepatocytes, and homogeneity in sinusoidal transport and desulfation was used to explain the discrepant half-lives. The model described a high partitioning of E1 in the endoplasmic reticulum and the segregation of estrone sulfation activities in the cytosolic space from the desulfation and glucuronidation activities in the endoplasmic reticulum space.

Sulfation is rate limiting in the futile cycling between estrone and estrone sulfate in enriched periportal and perivenous rat hepatocytes.

The metabolic activities and tissue binding of estrone (E1) and estrone sulfate (E1S) on futile cycling were examined. Desulfation of E1S in the 9000g supernatant fraction (S9) of periportal (PP) and perivenous (PV) rat hepatocytes were of similar V (2.9 +/- 1.0 and 2.4 +/- 0.9 nmol/min/mg of S9 protein), K (30.4 +/- 8.3 and 34.8 +/- 6.6 microM), and desulfation intrinsic clearances (V/K of 77 and 55 microl/min/10(6) cells). The intrinsic clearance towards E1 sulfation (1 microM) in cytosolic preparations of PV hepatocytes was 4 times that of PP hepatocytes (V/K of 26.4 +/- 9.5 versus 6.1 +/- 2.2 microl/min/mg of cytosolic protein or 13 +/- 5 versus 3.1 +/- 1.1 microl/min/10(6) cells). The observation was consistent with the immunolocalization of estrogen sulfotransferase (PV/PP ratio of 3.4 +/- 1.1) but not hydroxysteroid sulfotransferase (PV/PP ratio of 0.29 +/- 0.21) nor phenol sulfotransferase (PV/PP ratio of 1.13 +/- 0.23). Upon incubation of E1S (1-125 microM) with hepatocytes (30 min), higher concentrations of E1S and E1 were observed within PP than in PV cells, and saturation was evident at the higher concentrations. Based on the in vitro metabolic and tissue binding parameters for E1S and E1 and the published zonal uptake clearances of E1S (116 microl/min/10(6) cells), fitting revealed that uptake of E1 (1484 and 1463 microl/min/10(6) cells) by PP and PV cells was rapid and similar, and E1 sulfation was the slowest step in futile cycling. The greater metabolism of E1 in PV region led to higher levels of E1 and E1S in PP hepatocytes, and the nonlinear uptake, binding, and vesicular accumulation of E1S resulted in different t(1/2) values for E1S and E1.

The multiple indicator dilution method and its utility in risk assessment.

The multiple-indicator dilution (MID) technique entails the injection of a mixture of labeled indicators into the blood vessel immediately at the entrance of an organ, e.g., the liver, kidney, heart, or lung, and characterization of outflow dilution profiles from timed venous samples. The mathematical basis of the method encompasses linear systems of partial differential equations that are formulated for flow- or barrier-limited transport combined with intracellular metabolism/excretion. The concept can be generalized to include metabolites. MID experiments are useful for determining tissue partition coefficients as well as kinetic parameters such as membrane permeabilities or metabolic/excretory intrinsic clearances, factors that affect the mean residence times or exposure of solutes to the organ. The main utility of the MID method lies in its role in identifying the basic mechanisms of the interaction of organs with vascular components. The concentration dependence in transport and removal is revealed by the rate coefficients upon varying the input concentrations of unlabeled substances into the organ at steady state. The data obtained with MID experiments can be incorporated into physiologically based pharmacokinetic (PBPK) models such as those used for biological risk assessment. This is especially pertinent in the case where diffusional barriers appear within organs. The insight gained from the MID organ approach may be useful for PBPK models with more realistic representation of organ kinetics.

Uptake of enalapril and expression of organic anion transporting polypeptide 1 in zonal, isolated rat hepatocytes.

Sinusoidal entry is the first obligatory process preceding intracellular drug removal in liver. Transport of the angiotensin converting enzyme inhibitor enalapril (1-750 microM with [(3)H]enalapril), a substrate of Oatp1, the sodium-independent organic anion transporting polypeptide 1 cloned from rat liver, was studied in rat hepatocytes isolated from all zones of the liver (homogeneous) and from enriched periportal (PP) and perivenous (PV) hepatocytes prepared by collagenase perfusion and zone-selective destruction with digitonin, respectively. Uptake was linear over 1 min and was concentration-dependent. Transport by the homogeneous hepatocytes (in the presence and absence of Na(+)) and PP and PV cells was described by single saturable components of similar kinetic constants (K(m) values of 344-461 microM and V(max) values of 9.5-11.6 nmol/min/10(6) cells; P >.05, ANOVA). The K(m) value for enalapril uptake in hepatocytes was of the same order of magnitude compared with that for Oatp1 expressed in HeLa cells transfected with cDNA-Oatp1 and Western blot analysis revealed similar levels of immunoreactive Oatp1 expression in PP and PV hepatocytes. However, enalapril was not taken up by Oatp2 nor by the human OATP expressed in recombinant vaccinia systems.

Effect of zonal transport and metabolism on hepatic removal: enalapril hydrolysis in zonal, isolated rat hepatocytes in vitro and correlation with perfusion data.

Previous studies showed that the transport of enalapril occurred homogeneously among zonal rat hepatocytes. However, the metabolism of hepatic arterially delivered enalapril, swept into the rat liver by the portal or hepatic venous flow (HAPV and HAHV perfusion), was more abundant in the perivenous (PV) than the periportal (PP) region. Hence, metabolic activities toward enalapril in 9000g supernatant (S9) fractions of enriched rat PP and PV hepatocytes were examined. Although Michaelis-Menten kinetics were invariably observed, the metabolic activity toward enalapril (intrinsic clearance or V(max)(met)/K(m)(met) of 0.008 ml/min/mg of S9 protein, V(max)(met) of 21 +/- 6 nmol/min/mg of S9 protein, and K(m)(met) of 2612 +/- 236 microM) was greater in PV than in PP (V(max)(met) of 5.5 +/- 3.1 nmol/min/mg of S9 protein and K(m)(met) of 1049 +/- 335 microM; intrinsic clearance of 0.0052 ml/min/mg of S9 protein) hepatocytes. These metabolic intrinsic clearances were much lower than the sinusoidal influx clearances observed from previous transport studies, revealing metabolism as the rate-limiting step. Substitution of the scaled-up transport and metabolic intrinsic clearances into the "well stirred", "parallel-tube", and "dispersion" models predicted higher steady-state extraction ratios for HAHV perfusion. By contrast, integration of the scaled-up in vitro parameters on zonal metabolism and homogeneous transport into a "zonal-compartment" model of three zonal subcompartments (1, 2, and 3) provided an improved description of the extraction ratios during HAPV and HAHV. Zonal factors are important for the scale-up of data in vitro to the whole organ.

Route-dependent metabolism of morphine in the vascularly perfused rat small intestine preparation.

PURPOSE: 1. To compare the disposition of tracer morphine ([3H]M) following systemic and intraduodenal administration in the recirculating, rat small intestine preparation in absence or presence of verapamil (V), an inhibitor of P-glycoprotein. 2. To develop a physiological model to explain the observations. METHODS: A bolus dose of [3H]M was added to the reservoir or injected into the duodenum of the rat small intestine preparation. V (200 microM in reservoir) was either absent (control studies) or present. Intestinal microsomal, incubation studies were performed to evaluate the effect of V on morphine glucuronidation. RESULTS: After systemic administration, [3H]M was not metabolized but was exsorbed into lumen. By contrast, both [3H]M and the 3beta-glucuronide metabolite, [3H]M3G, appeared in reservoir and lumen after intraduodenal administration. A physiologically-based model that encompassed absorption, metabolism and secretion was able to describe the route-dependent glucuronidation of M. The presence of V resulted in diminished levels of M3G in perfusate and lumen and mirrored the observation of decreased glucuronidation in microsomal incubations. Verapamil appeared to be an inhibitor of glucuronidation and not secretion of M. CONCLUSIONS: M was secreted and absorbed by the rat small intestine. Route-dependent glucuronidation of M was explained by physiological modeling when M was poorly partitioned in intestinal tissue, with a low influx clearance from blood and a even poorer efflux clearance from tissue. The poor efflux rendered a much greater metabolism of M that was initially absorbed from the lumen. V increased the extent of M absorption through inhibition of M glucuronidation.

A new physiologically based, segregated-flow model to explain route-dependent intestinal metabolism.

Processes of intestinal absorption, metabolism, and secretion must be considered simultaneously in viewing oral drug bioavailability. Existing models often fail to predict route-dependent intestinal metabolism, namely, little metabolism occurs after systemic dosing but notable metabolism exists after oral dosing. A physiologically based, Segregated-Flow Model (SFM) was developed to examine the influence of intestinal transport (absorption and exsorption), metabolism, flow, tissue-partitioning characteristics, and elimination in other organs on intestinal clearance, intestinal availability, and systemic bioavailability. For the SFM, blood flow to intestine was effectively segregated for the perfusion of two regions, with 10% reaching an absorptive layer-the enterocytes at the villus tips of the mucosa where metabolic enzymes and the P-glycoprotein reside, and the remaining 90% supplying the rest of the intestine (serosa and submucosa), a nonabsorptive layer. The traditional, physiologically-based model, which regards the intestine as a single, homogeneous compartment with all of the intestinal blood flow perfusing the tissue, was also examined for comparison. The analytical solutions under first order conditions were essentially identical for the SFM and traditional model, differing only in the flow rate to the absorptive/removal region. The presence of other elimination organs did not affect the intestinal clearance and bioavailability estimates, but reduced the percentage of dose metabolized by the intestine. For both models, intestinal availability was inversely related to the intrinsic clearances for intestinal metabolism and exsorption, and was additionally affected by both the rate constant for absorption and that denoting luminal loss when drug was exsorbed. However, the effect of secretion by P-glycoprotein became attenuated with rapid absorption. The difference in flow between models imparted a substantial influence on the intestinal clearance of flow-limited substrates, and the SFM predicted markedly higher extents of intestinal metabolism for oral over i.v. dosing. Thus, the SFM provides a physiological view of the intestine and explains the observation of route-dependent, intestinal metabolism.

Uptake and glutathione conjugation of ethacrynic acid and efflux of the glutathione adduct by periportal and perivenous rat hepatocytes.

We assessed the impact of zonal factors on the hepatic reduced glutathione (GSH) conjugation of ethacrynic acid (EA). Uptake of EA by enriched periportal (PP) and perivenous (PV) rat hepatocytes was characterized by both saturable (V(max)(uptake) = 3.4 +/- 1.7 and 3. 2 +/- 0.8 nmol/min/mg protein and K(m)(uptake) = 51 +/- 13 and 44 +/- 15 microM) and nonsaturable (12 +/- 5 and 12 +/- 3 microl/min/mg protein) components. Values for the overall GSH conjugation rates of EA (200 microM) were similar among the zonal hepatocytes and resembled those for the influx transport rates. In the absence of the hepatocyte membrane, GSH conjugation in PV and PP hepatocyte cytosol was similar, but a higher perivenous GSH conjugation activity toward EA (PV/PP of 2.4) that mirrored the higher PV/PP ratios of immunodetectable GSTs Ya (1.7) and Yb2 (2.5) was found in cell lysates obtained by the dual-digitonin-pulse perfusion technique. The GSH conjugation rates in the subcellular fragments were, however, much greater than those observed for intact hepatocytes. Efflux rates of the glutathione conjugate EA-SG from zonal hepatocytes were similar, as were levels of the immunodetectable multidrug-resistance protein 2/canalicular multispecific organic anion transporter (Mrp2/cMoat) in the 100,000g pellets. The composite results suggest that the GSTs responsible for EA metabolism are more abundant in the PV region, albeit that the gradient of enzymatic activities is shallow. Despite the existence of zonal metabolic activity, the overall GSH conjugation rate of EA is homogeneous among cells because the reaction is rate limited by uptake, which occurs evenly. Results on EA-SG efflux suggest the acinar homogeneity in Mrp2/cMoat function for canalicular transport.

Bimolecular glutathione conjugation kinetics of ethacrynic acid in rat liver: in vitro and perfusion studies.

The conjugation kinetics of glutathione (GSH) and ethacrynic acid (EA) were studied in rat liver perfusion studies, where efficient removal occurred (steady-state extraction ratio E(ss), approximately 0.8-0.4 at concentrations ranging from 10-200 microM) despite the appreciable plasma protein binding. The declining E(ss) paralleled the saturation in GSH conjugate (EA-SG) formation; EA-SG primarily appeared in bile as the unchanged glutathionyl adduct (90%) and minimally as cleavage products. The GSH conjugation of EA in perfused liver was described by the constants K(m)(overall) of 67 microM and V(max)(overall) of 0.23 micromol/min/g liver. These differed from those observed for the bimolecular nonenzymatic (constant of 126 microM(-1) min(-1)) and enzymatic (K(m) for GSH and EA were 1.2 mM and 94 microM, respectively; V(max) of 533 nmol/min/mg liver cytosolic protein or 32 micromol/min/g liver) GSH conjugation of EA in vitro. But they were similar to those estimated for EA uptake in isolated rat hepatocytes by saturable (K(m)(uptake) = 57 microM, and V(max)(uptake) = 0.55 micromol/min/g liver) and nonsaturable (0.015 ml/min/mg) processes. At increasing EA concentrations (>25 microM), time-dependent changes were observed for E(ss) and EA-SG formation, which rapidly decreased with time after the attainment of steady state due to the rapid loss of cellular GSH. The composite data were described adequately by a physiological model that accounted for transport and the GSH-dependent conjugation of EA. The results suggest that the rate-limiting process for hepatic EA GSH conjugation is cellular uptake, but cosubstrate availability controls the rate of metabolism when GSH becomes depleted.

Inhibition of esterolysis of enalapril by paraoxon increases the urinary clearance in isolated perfused rat kidney.

The effect of competing elimination pathways on the metabolic and excretory clearance estimates was examined with tracer concentrations of [(3)H]enalapril, which was both metabolized and excreted by the rat kidney. Perturbation was achieved with use of the carboxylesterase inhibitor paraoxon, which inhibited [(3)H]enalapril metabolism to [(3)H]enalaprilat in rat renal S9 fraction. At 0.1, 0.5, 1, and 10 microM paraoxon, esterolysis of enalapril was inhibited by 76 +/- 7, 93 +/- 5, 96 +/- 5, and 93 +/- 6%, respectively. The lowest concentration (0.1 microM) of paraoxon was chosen for single-pass isolated perfused kidney (IPK) studies because viability was least compromised, and the sodium and glucose reabsorptive functions of the IPK remained constant. After an equilibration period (15-20 min at constant pressure, 90-100 mm Hg), perfusion of the rat kidney with [(3)H]enalapril was carried out under constant flow (8 ml/min) for 30 min in the absence and presence of paraoxon (0.1 microM). The metabolic (from 1.83 +/- 0.52 to 1.48 +/- 0.47 ml/min/g) and total renal (from 1.87 +/- 0.46 to 1. 57 +/- 0.41 ml/min/g) clearances of [(3)H]enalapril in the IPKs were decreased significantly (p <.05) in the presence of paraoxon when compared with controls. Concomitantly, the urinary clearance (from 0. 04 +/- 0.07 to 0.09 +/- 0.09 ml/min/g) and the fractional excretion (from 0.23 +/- 0.18 to 0.52 +/- 0.25) of [(3)H]enalapril doubled (p <.05). The study illustrates that a reduction in cellular metabolism of the kidney brings forth a rise in the estimate of clearance of its complimentary pathway, estimate of the excretory (urinary) clearance.

The multiple indicator-dilution method for the study of enzyme heterogeneity in liver: theoretical basis.

The theoretical basis of the use of the multiple indicator dilution technique to account for the heterogeneous distribution (or zonation) of enzymes in the liver was explored. The microcirculation was assumed to consist of identical capillaries perfused in parallel, with enzymatic activities for drug metabolism being distributed uniformly over the upstream half (periportal or pp) or the downstream half (perivenous or pv) of the flow path, whereas all other transport/removal processes were assumed to be homogeneously distributed. Outflow dilution profiles for parent drug and metabolite were estimated by inversion of Laplace transforms or by a finite difference method. The areas under the curves for parent and metabolite, the mean transit times of parent (MTT) and metabolite (MTTM, mean time from injection of parent to exit of metabolite from organ), and their relative dispersions (CV2 or CVM2) were estimated from analytical expressions. When the influx-efflux ratio (or cellular-sinusoidal distribution ratio) for metabolite was equal to or smaller than that of the parent, the MTTM ranking was: pp < homogeneous < pv. The ranking was reversed when the influx-efflux ratio for metabolite greatly exceeded that for the parent. The presence of elimination pathways for the metabolite reduced its MTTM and CVM2, more for pp than for homogeneous and pv cases. The theory can be applied to determine enzyme zonation in multiple indicator dilution studies with use of the area under the curve for the metabolite and MTTM during prograde (from portal vein to hepatic vein) and retrograde (from hepatic vein to portal vein) perfusion.