Metabolic heterogeneity of the proximal and distal kidney tubules

Application of a physiologically based pharmacokinetic model to assess propofol hepatic and renal glucuronidation in isolation: utility of in vitro and in vivo data

A physiologically based pharmacokinetic (PBPK) modeling approach was used to assess the prediction accuracy of propofol hepatic and extrahepatic metabolic clearance and to address previously reported underprediction of in vivo clearance based on static in vitro-in vivo extrapolation methods. The predictive capacity of propofol intrinsic clearance data (CLint) obtained in human hepatocytes and liver and kidney microsomes was assessed using the PBPK model developed in MATLAB software. Microsomal data obtained by both substrate depletion and metabolite formation methods and in the presence of 2% bovine serum albumin were considered in the analysis. Incorporation of hepatic and renal in vitro metabolic clearance in the PBPK model resulted in underprediction of propofol clearance regardless of the source of in vitro data; the predicted value did not exceed 35% of the observed clearance. Subsequently, propofol clinical data from three dose levels in intact patients and anhepatic subjects were used for the optimization of hepatic and renal CLint in a simultaneous fitting routine. Optimization process highlighted that renal glucuronidation clearance was underpredicted to a greater extent than liver clearance, requiring empirical scaling factors of 17 and 9, respectively. The use of optimized clearance parameters predicted hepatic and renal extraction ratios within 20% of the observed values, reported in an additional independent clinical study. This study highlights the complexity involved in assessing the contribution of extrahepatic clearance mechanisms and illustrates the application of PBPK modeling, in conjunction with clinical data, to assess prediction of clearance from in vitro data for each tissue individually.

Effects of renal diseases on the regulation and expression of renal and hepatic drug-metabolizing enzymes: a review

1. The activity of drug-metabolizing enzymes (DMEs) in extrahepatic organs is highest in the kidneys. Generally, the kidneys contain most, if not all, of the DMEs found in the liver. Surprisingly, some of these DMEs show higher activity in the kidneys than in the liver. 2. Most of the renal DMEs are localized in the cortex of the kidneys, especially in the proximal tubules. DMEs are also found in the distal tubules and collecting ducts. 3. Renal diseases such as acute and chronic renal failure and renal cell carcinoma alter the regulation of both hepatic and extrahepatic phase I and II DMEs. Changes in the expression of these DMEs seem to be tissue and species specific. 4. Generally, there is significant down-regulation of most of the phase I and a few of phase II DMEs at the protein, mRNA and activity levels. Unfortunately, the mechanisms leading to the alteration in DMEs in renal diseases remain unclear, although many theories have been made. 5. The presence of some circulating factors such as cytokines, nitric oxide, parathyroid hormones and increased intracellular calcium play a role in the regulation of DMEs in renal diseases.

Effect of glycerol-induced acute renal failure and di-2-ethylhexyl phthalate on the enzymes involved in biotransformation of xenobiotixs

The effects of di-(2-ethylhexyl)-phthalate (DEPH) on the levels of cytochrome P-450 and b5 monooxygenases were studied in the rat kidney and liver in acute renal failure induced by glycerol. Intramuscular injection of glycerol (50%,10 ml x kg(-1)) to rats produced proximal tubular damage and acute renal failure. The indicators of renal function, serum urea and creatinine significantly increased (480 and 350 percent, respectively). In control and glycerol-treated animals DEPH had no significant effect on the concentrations of serum urea and creatinine. Twenty-four hours after glycerol injection the total amount of cytochrome P-450 and b5 significantly decreased in renal but increased in liver microsomal fractions. Moreover, 48 and 72 hours after glycerol injection the level of cytochrome P-450 and b5 significantly increased in both organs. A single dose of DEPH (2 ml x kg(-1), i.p.) also elevated the total cytochrome P-450 and b5 in control animals. This enhancing effect of DEPH was additive to that of glycerol in glycerol-induced acute renal failure. These results indicate that DEPH and glycerol evoked pathological changes may affect the metabolism of xenobiotics plus endogenous hormones in the liver and in kidney.

In vitro methods of assessing renal damage

Freshly isolated and primary cultures of rat kidney cells derived from specific nephron segments can be useful in vitro models for studying processes such as drug metabolism, membrane transport, and biochemical mechanisms of chemically induced toxicity. Proximal tubular (PT) and distal tubular (DT) cells were isolated from rat renal cortex by collagenase perfusion and Percoll density-gradient centrifugation. Oxidants produced glutathione (GSH) oxidation and lipid peroxidation and were markedly more cytotoxic to DT cells than to PT cells. Similarly, alkylating agents that target soft nucleophiles such as GSH and protein sulfhydryls were more toxic to DT cells than to PT cells, whereas an alkylating agent that targets hard nucleophiles was equally cytotoxic in the 2 cell types. DT cells were also more sensitive to brief periods of oxygen deprivation and were markedly more susceptible to ATP depletion by treatment with iodoacetate and cyanide than were PT cells. Serum-free, hormonally defined conditions have been optimized for primary culture of rat renal PT and DT cells to maintain differentiated function for up to 9 days. Primary cultures exhibited similar susceptibilities as freshly isolated cells to acute injury from chemical toxicants and the cultures express several isoforms of cytochrome P-450. These studies show that freshly isolated and primary cultures of rat renal PT and DT cells can be used to study both short-term and long-term responses to toxic chemicals.

Renal cell type specificity of cephalosporin-induced cytotoxicity in suspensions of isolated proximal tubular and distal tubular cells

We have developed an in vitro model for investigation of nephron heterogeneity and cell type-specific patterns of renal injury. To further validate our model and to study biochemical mechanisms of cephalosporin-induced injury, cytotoxicity of three cephalosporins was studied in freshly isolated proximal tubular (PT) and distal tubular (DT) cells from rat kidney. The three cephalosporins [cephaloridine (CPH), cephalexin (CXN), cephalothin (CTN)] were chosen because they exhibit varying degrees of nephrotoxicity in vivo and contain different functional groups. CPH produced greater amounts of lactate dehydrogenase release from PT cells than either CXN or CTN, indicating greater toxicity of CPH, which agrees with in vivo observations. DT cells were not affected by any of the cephalosporins. Thus, the cephem ring is sufficient to produce PT cell injury but the presence of other functional groups modifies toxicity. SKF-525A and alpha-tocopherol protected PT cells from both CPH and CTN, suggesting involvement of cytochrome P-450 metabolism and oxidative stress. Both PT and DT cells exhibited transport of CPH or CXN and transport of CPH into PT cells was inhibitable by probenecid, consistent with action of a specific carrier. Transport alone, therefore, cannot account for the cell type specificity pattern in vitro. Effects on intracellular glutathione status, malondaldehyde formation, and uncoupler-stimulated respiration were also investigated, and these generally correlated with cell type specificity patterns but not always with degree of cytotoxicity. These results validate further the isolated PT and DT cells as in vitro models to study cell type-specific renal injury and show a role for oxidative stress, cytochrome P-450 bioactivation, and mitochondrial dysfunction in cephalosporin-induced PT cell injury.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.

Isolation of two distinct populations of cells from rat kidney cortex and their use in the study of chemical-induced toxicity

Procedures for the isolation and enrichment of cell populations from suspensions of rat kidney cortical cells were developed. Using Percoll density-gradient centrifugation, two populations of cells were obtained; marker enzymes [alkaline phosphatase and gamma-glutamyltransferase for proximal tubular (PT) cells and hexokinase for distal tubular (DT) cells] and functional responses (stimulation of PT cell oxygen consumption by succinate and inhibition of DT cell oxygen consumption by amiloride) were then employed to identify and assess the purity of the two fractions. The PT cell fraction was estimated to contain 97% PT cells and the DT cell fraction was estimated to contain 88% DT cells. Staining with toluidine blue and light microscopy showed that PT cells contained a brush border, were larger than DT cells, and had more intensely staining nuclei than DT cells. To demonstrate the usefulness of these cell preparations in the study of biochemical mechanisms of renal cell injury, time- and concentration-dependent effects of the PT cell-specific nephrotoxin cephaloridine (CPH) on PT and DT cell trypan blue exclusion were examined. CPH was toxic in PT cells but not in DT cells; viability of PT cells incubated with 0.1 or 1 mM CPH for 2 h was 57 or 34%, respectively, compared to 81% for control cells; viability of DT cells incubated with 0.1 or 1 mM CPH for 2 h was 74 or 71%, respectively, compared to 74% for control cells. This method thus provides highly enriched preparations of freshly isolated PT and DT cells that retain their unique properties and are suitable for studies of biochemical mechanisms of chemical toxicity and nephron heterogeneity.

Beta-adrenoceptors in kidney tubules of spontaneously hypertensive and normotensive rats

Beta-adrenoceptor binding characteristics were determined in different fractions of rat kidney tubules using a [125Iodo]-(-)-cyanopindolol (ICYP) binding assay. The highest amount of binding sites was found in a fraction containing predominantly distal tubular fragments. In a separate series of experiments the ICYP binding characteristics were compared in whole tubular fractions from spontaneously hypertensive (SHR) and normotensive Wistar Kyoto rats (WKY) of different ages. The maximum number of binding sites was significantly higher both in young (3 weeks) and adult (14 weeks) SHR when compared to age-matched WKY. These studies showed the presence of beta-adrenoceptor binding sites in rat kidney tubules and support the potential importance of tubular beta-adrenoceptors in the development of spontaneous hypertension and in the mechanism of antihypertensive action of beta-blockers.

Measurement of unscheduled DNA synthesis in rat kidney cells following in vivo treatment with genotoxic agents

The kidney is a key target tissue in animal and human carcinogenesis, yet there are no established short-term tests for studying the genotoxicity of chemicals in the kidney. We have developed an assay for the measurement of chemically induced DNA repair as unscheduled DNA synthesis (UDS) in isolated rat kidney cells following in vivo treatment. Male Fischer-344 rats were injected intraperitoneally with chemicals dissolved in saline or corn oil. After various treatment times, the kidneys were perfused with a collagenase/trypsin solution (CTS), minced into small pieces, and stirred in CTS at 37 degrees C for 1 hr to dissociate cells. Cultures contain a high proportion of epithelial cells from the proximal and distal tubules. Cultures were incubated for 16-18 hr with 3H-thymidine in Williams' Medium E supplemented with 20% fetal bovine serum. UDS was measured by quantitative autoradiography as net grains/nucleus (NG). The percentage of cells in repair (% IR) was defined as the percentage of cells with greater than or equal to 3 NG. Saline- or corn oil-injected controls consistently produced -3 to -5 NG with less than 1% IR. The time course of DNA repair following treatment with the direct-acting mutagen methylmethane sulfonate (MMS) or the renal carcinogen azaserine showed a peak response at 2 hr after treatment. Azaserine showed a rapid decline in UDS at 12 and 24 hr, whereas MMS exhibited a relatively high UDS level at 24 hr. The renal carcinogens methylazoxymethanol acetate, N-methyl-N-nitrosourea, and streptozotocin all yielded strong positive UDS responses. The liver and intestinal carcinogen 1, 1-dimethylhydrazine at doses up to 50 mg/kg was cytotoxic to kidney cells, but induced less than 0 NG. Treatment with 1,2-dimethylhydrazine, which induces kidney tumors in mice but not rats, also induced less than 0 NG. Treatment with o-anisidine, a weak renal carcinogen, did not induce UDS in the kidney, suggesting that it may be acting as a tumor promoter. These results demonstrate the usefulness of this assay for the detection and study of a variety of genotoxic kidney carcinogens.