Analytical cell fractionation of isolated rabbit renal proximal tubules

Defining the Intravital Renal Disposition of Fluorescence-Quenched Exenatide

Despite the understanding that renal clearance is pivotal for driving the pharmacokinetics of numerous therapeutic proteins and peptides, the specific processes that occur following glomerular filtration remain poorly defined. For instance, sites of catabolism within the proximal tubule can occur at the brush border, within lysosomes following endocytosis, or even within the tubule lumen itself. The objective of the current study was to address these limitations and develop methodology to study the kidney disposition of a model therapeutic protein. Exenatide is a peptide used to treat type 2 diabetes mellitus. Glomerular filtration and ensuing renal catabolism have been shown to be its principal clearance pathway. Here, we designed and validated a Förster resonance energy transfer-quenched exenatide derivative to provide critical information on the renal handling of exenatide. A combination of in vitro techniques was used to confirm substantial fluorescence quenching of intact peptide that was released upon proteolytic cleavage. This evaluation was then followed by an assessment of the in vivo disposition of quenched exenatide directly within kidneys of living rats via intravital two-photon microscopy. Live imaging demonstrated rapid glomerular filtration and identified exenatide metabolism occurred within the subapical regions of the proximal tubule epithelia, with subsequent intracellular trafficking of cleaved fragments. These results provide a novel examination into the real-time, intravital disposition of a protein therapeutic within the kidney and offer a platform to build upon for future work.

Choline Incorporation into Phospholipids in Mesothelial Cells in Vitro

Objective

To determine the effect of extracellular choline concentration on phospholipid production and handling by peritoneal mesothelial cells in vitro.

Design and Measurements

Radiolabeled choline was used to monitor the formation of phosphatidylcholine {PC), sphingomyelin (SPH), and Iysophosphatidylcholine (LPC) by rat and rabbit mesothelial cells as a function of concentration and time of exposure to choline. The subcellular location of the newly formed phospholipids was examined by ultracentrifugation in Percoll-sucrose gradients using analytical cell fractionation techniques. The fatty acid composition of the PC formed was determined by thin-layer chromatography (TLC) and gas chromatography.

Results

Choline incorporation into PC, SPH, and LPC increased with extracellular choline levels up to 640 μmol/L, which is 100 times greater than physiological levels of choline in plasma and 20 times higher than choline levels measured in peritoneal dialysis effluent. The newly formed, radiolabeled phospholipids were primarily found in a single subcellular compartment that exhibited a buoyant density of 1.05 g/mL in Percollsucrose gradients. Analysis of the fatty acyl groups of PC obtained from the mesothelial cells showed enrichment in palmitic [16:0], oleic [18:1], and linoleic [18:2] acids.

Conclusion

The rate of phospholipid formation by mesothelial cells in vitro can be manipulated, in part, by choline concentration.

Primary cultures of rabbit renal proximal tubule cells: II. Selected phase I and phase II metabolic capacities

Specific characteristics of cells vary as a function of time in culture. We have determined the stability of selected Phase I and Phase II biotransformation capacities in rabbit renal proximal tubule cells in primary culture. When grown in hormonally-defined medium, proximal tubule cells lost Phase I metabolic capacity. Cytochrome P-450 content and associated mixed-function oxidase activities present in kidney cortex microsomes were not detectable after 14 days in culture. Phase II glutathione-dependent metabolic functions were well retained in cultured cells compared with freshly isolated proximal tubules (FIPT). Cellular total glutathione content was 2.8 mug/mg protein in FIPT compared with approximately 10 mug/mg protein in stable confluent cultures. A higher total glutathione content of 20.6 mug/mg was noted in preconfluent cultures. The glutathione redox state was initially perturbed in FIPT with 37% of the total glutathione present found in its oxidized form. Tubule cells recovered to a normal ratio (6-13% of total glutathione in the oxidized form) while in culture. The glutathione S-transferase activity in 4-day-old cells in culture was reduced to 50% of the 4 U/mg protein level found in FIPT. No appreciable further decline in glutathione S-transferase activity was detected during 15 days in culture. The level of gamma-glutamyl-transpeptidase (a brush-border enzyme necessary for glutathione uptake into proximal tubule cells) declined from 1499 mU/mg protein in homogenates of FIPT to 636 mU/mg in homogenates of 8-day-old cultured cells. A further decline in activity occurred during the next 7 days in culture. In conclusion, although Phase I metabolic functions were diminished in primary cultured rabbit proximal tubule cells, Phase II metabolic functions were retained at levels comparable with FIPT and well above those found in several established kidney cell lines.

Estrogen modulates ClC-2 chloride channel gene expression in rat kidney

ClC-2 is a CLC family member of chloride channels sensitive to changes in cell volume, pH and voltage. The ClC-2 is widely distributed along the nephron although in the kidney its role still not well understood. Aldosterone studies suggest that ClC-2 expression in the kidney may be hormonally regulated. To explore the possibility that estrogen control ClC-2 expression, we investigated whether its expression changed in the kidney of female Wistar rats subjected to ovariectomy with or without near-physiological or high doses of 17beta-estradiol benzoate treatment for 10 days. Total RNA isolated from rat kidney and dissected nephron segments was analyzed by ribonuclease protection assay and/or a semi-quantitative RT-PCR. The renal ClC-2 protein expression was analyzed by Western blot. The decreased renal expression of ClC-2 mRNA and protein observed in ovariectomized rats was restored to control levels after treatment with low doses of estradiol. Higher dose estradiol lead to an even greater increase in ClC-2 mRNA and protein expression. This change in overall expression was shown to be caused by the modulation of ClC-2 mRNA expression in the proximal tubule. These results suggest that ClC-2 may be involved in estrogen-induced Cl(-) transport in rat kidney.

Exon repetition in mRNA

The production of different transcripts (transcript heterogeneity) is a feature of many genes that may result in phenotypic variation. Several mechanisms, that occur at both the DNA and RNA level have been shown to contribute to this transcript heterogeneity in mammals, all of which involve either the rearrangement of sequences within a genome or the use of alternative signals in linear, contiguous DNA or RNA. Here we describe tissue-specific repetition of selective exons in transcripts of a rat gene (SA) with a normal exon-intron organization. We conclude that nonlinear mRNA processing can generate tissue-specific transcripts.

Morphological and biochemical characterization of primary culture of rabbit proximal kidney tubule cells grown on collagen-IV coated Millicell-CM

The aim of this study was to better characterize rabbit proximal kidney tubule cells cultured on collagen IV-coated porous inserts, as compared to the same cells seeded in standard plastic wells. Total protein contents in confluent monolayers on permeable membranes were about twofold higher than those measured in confluent cultures in plastic wells. Microscopy examinations suggested that such a difference was probably due to a higher cell density and to an impressive development of the apical brush-border membrane. Moreover, measurement of unidirectional transport of p-aminohippuric acid and tetraethylammonium bromide confirmed the high polarization level of cultures on porous inserts. Results of methyl(alpha-D-[U-14C]glyco)pyranoside uptake suggested that cell phenotype was probably influenced by culture conditions. Analysis of different markers as a function of time in culture showed decreases of alkaline phosphatase (AP), gamma-glutamyltranspeptidase (GGT), and Na(+)-K(+)-ATPase activities as well as increases in LDH, ATP, and glutathione levels, similar to those formerly reported for cells cultured in standard plastic plates. However, comparative data from 6-d-old monolayers have shown that AP, GGT, Na(+)-K(+)-ATPase, glutathione reductase (GRED), and selenium-dependent glutathione peroxidase (Se-GPX) activities were 2.8-, 2.6-, 1.6-, 1.2-, and 2.1-fold, respectively, better preserved on precoated permeable membranes. On the other hand, this paper reports for the first time in the literature that GRED and SE-GPX, two phase II detoxification enzymes, were well maintained in cultures of rabbit proximal kidney tubule cells. Our results show that culturing rabbit proximal kidney tubule cells on collagen IV-coated porous membranes was accompanied by an improvement of both morphological and biochemical properties of the cells.

Activity and expression of Na(+)-H+ exchanger isoforms 1 and 3 in kidney proximal tubules of hypertensive rats

Increased activity of the cellular Na(+)-H+ exchangers (NHEs), especially isoform 1 (NHE-1), is a recognized intermediate phenotype of hypertension. NHE activity has been demonstrated to be increased in proximal tubules of the spontaneously hypertensive rat (SHR). However, with the recent cloning of other members of this family of transporters, it is unclear which isoforms may contribute to this increased activity. We have used specific antibodies raised against glutathione-S-transferase fusion proteins of rat NHE-1 and NHE-3 to determine the relative contributions of these isoforms to the NHE activity in freshly isolated and cultured proximal tubule cells from SHR and Wistar-Kyoto (WKY) normotensive control rats. In freshly isolated proximal tubule cells, NHE activity was elevated almost 3-fold in SHR cells (P < .001), and in both rat strains, the contribution from NHE-1 and NHE-3 was approximately equal. Western blots of membranes from these cells showed equal amounts of NHE-1 protein in SHR and WKY cells. However, NHE-3 protein expression was increased 50% in SHR cells (P < .001), and this may account for the elevated activity of this isoform in SHR. The effect of culturing these cells in vitro was then examined. Although total NHE activity in both cell types was decreased during culture, this was mainly due to loss of expression of NHE-3 protein. NHE-1 activity was persistently elevated in the SHR cells in culture. These findings suggest that elevated NHE activity in SHR proximal tubules could be mediated by two mechanisms: (1) increased NHE-1 activity without any increased NHE-1 protein content that persists despite culture and may resemble those changes described for extrarenal tissues and (2) increased NHE-3 activity due to increased expression of NHE-3 protein. Disappearance of NHE-3 during culture implies that our culture conditions did not replicate the in vivo environment and may have removed the factors contributing to the increased NHE-3 expression in SHR cells.

Comparative impact of cephaloridine on glutathione and related enzymes in LLC-PK1, LLC-RK1, and primary cultures of rat and rabbit proximal tubule cells

Among kidney tubular epithelial cell types, proximal tubule cells are one of the major renal targets for xenobiotics. Several in vitro culture models have been proposed for use of proximal tubule cells for in vitro pharmacotoxicology studies. This paper reports a comparative study of the response to cephaloridine exposure of two established cell lines from pig (LLC-PK1) and rabbit (LLC-RK1) kidneys and primary cultures of rat and rabbit proximal tubule cells. These cultured cells were first compared for their levels of activity of alpha-methylglucopyranoside transport, alkaline phosphatase, succinate dehydrogenase, and NADPH cytochrome c reductase, their glutathione-dependent activity levels, and their adenylate cyclase response pattern to stimulation by PTH and AVP. The results presented show major phenotypic differences between these four cellular models. The differences observed in glutathione-dependent mechanism activities and regulation may in part be responsible for the variability of the responses of these four cellular models when exposed to cephaloridine.

Biochemical discrimination between luminal and abluminal enzyme and transport activities of the blood-brain barrier

Luminal and abluminal membrane vesicles derived from bovine brain endothelial cells, the site of the blood-brain barrier, were fractionated in a discontinuous Ficoll gradient. A mathematical analysis was developed to determine the membrane distribution of membrane marker enzyme activities as well as the ratio of luminal to abluminal membrane in each fraction of the gradient. The results of this analysis indicate that gamma-glutamyl transpeptidase and amino acid transport system A are located on the luminal and abluminal membranes, respectively. Conversely, 5'-nucleotidase and alkaline phosphatase activities are evenly distributed between both membranes. Although Na+/K(+)-ATPase activity is primarily located on the abluminal membrane, approximately 25% of the activity is of luminal origin. Na+/K(+)-ATPase activities associated with each membrane showed different ouabain sensitivities, suggesting that different isoenzymes are located in luminal and abluminal membranes. The analytical procedure used in this study provides a quantitative means to determine the distribution of marker enzymes and transport proteins in partially purified membrane vesicle populations.

Platinum complex-induced dysfunction of cultured renal proximal tubule cells. A comparative study of carboplatin and transplatin with cisplatin

Platinum coordination complexes (PtCx) are potent against several types of cancer but are often nephrotoxic. With a view to developing a PtCx nephrotoxicity model, the toxicity of cisplatin (cDDP), transplatin (tDDP) and carboplatin (CBDCA) was studied in primary cultures of rabbit proximal tubule (RPT) cells and in the renal epithelial OK cell line. The cytotoxicity of these PtCx (10-3000 microM) was assessed after 24 h exposure of confluent monolayers in terms of LDH release; their effects at non-cytotoxic concentrations (1-1000 microM) on DNA and protein synthesis, glucose transport, marker enzymes and the total glutathione concentration were also determined, together with cellular platinum uptakes. The cytotoxicity ranking of the studied compounds differed for OK and RPT cells (cDDP > tDDP; cDDP > CBDCA and tDDP > cDDP; cDDP > CBDCA, respectively). Only results which were obtained in RPT cells corresponded to reported nephrotoxicity in vivo, making OK cells inappropriate for the study of PtCx nephrotoxicity in vitro. cDDP was about 10 times less cytotoxic for OK cells than for RPT cells because of lower cellular uptake. tDDP was unable markedly to inhibit biochemical and functional parameters in RPT cells below cytotoxic concentrations. At non-cytotoxic concentrations, cDDP and CBDCA depressed synthetic activity (mainly DNA) and, to a lesser extent, Na(+)-K(+)-ATPase activity and glucose transport in RPT cells. Total glutathione levels in RPT cells steadily increased during exposure to cDDP, tDDP and CBDCA, before the onset of cell death, arguing against an early role of glutathione depletion in PtCx toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of gentamicin-induced dysfunctions in vitro: the use of optimized primary cultures of rabbit proximal tubule cells

Compared to prior studies which frequently pinpoint the impairment of one parameter or function, this paper reports for the first time an extensive characterization of the toxic effects of gentamicin in a single model of primary cultured rabbit proximal tubule cells developed without insulin and glucose. Biochemical, functional and morphological approaches were used. Cellular response pattern was examined after a 72-h exposure during either the exponential growth phase or the stationary confluency phase of the culture to 0.2, 1, and 2.5 mM gentamicin. The biochemical study after gentamicin exposure showed increased activities for N-acetyl-beta-D-glucosaminidase and alkaline phosphatase, decreased activities for sphingomyelinase, cathepsin B, Na+/K(+)-ATPase, lactate dehydrogenase and NADPH cytochrome C reductase. Functional evaluation revealed decreased protein synthesis and alpha-methylglucose transport after gentamicin exposure. Morphometric study made it possible to show that the density of lysosomes, the cell fractional volume of the lysosomal compartment, and the mean size of the lysosomal profiles are increased in the cells. Intracellular accumulation of gentamicin in proximal tubular cells was dose dependent and reached high levels in cultured cells. In conclusion, this model compared to others in the literature allowed us to demonstrate in vitro a close response pattern to the in vivo situation after gentamicin exposure.

Biochemical, functional, and morphological characterization of a primary culture of rabbit proximal tubule cells

Primary cultures of renal rabbit proximal tubule cells were initiated from a pure suspension of proximal tubule fragments. Proximal tubule cells were grown in a hormone-supplemented, serum-free medium containing low concentrations of antibiotics. Confluent monolayers exhibited multicellular dome formation, indicating the presence of transepithelial solute and water transport. Ultrastructural examination revealed a monolayer of polarized epithelial cells with tight junctions and sparse membraneous microvilli facing the culture medium. Time course biochemical characterization was performed using a palette of 12 enzymes, representative of important metabolic functions or pathways. Brush-border-associated enzymes (gamma-glutamyl transpeptidase and alanine aminopeptidase) were moderately reduced throughout the culture whereas alkaline phosphatase was markedly decreased at confluency. Mitochondrial and lysosomal marker enzymes were well preserved over the culture period. Glutathione-S-transferase activity remained stable during the 16-day culture period investigated. Glycolysis enzyme activities (lactate dehydrogenase and hexokinase) were enhanced, as a function of culture age. Na(+)-K(+)-ATPase activity rise was concomitant with the increase of glycolysis marker enzymes. In contrast, the gluconeogenesis marker enzyme, glucose-6-phosphatase, fell dramatically to reach a low level equivalent to 4% of the activity measured in isolated proximal tubules. Primary cultures exhibited several differentiated functions of the proximal tubule cell: (a) PTH alone was able to induce a significant stimulation of adenylate cyclase activity, unlike isoproterenol, thyrocalcitonin, and arginine vasopressin, and (b) sodium-dependent alpha-methylglucoside (AMG) transport was detected. This AMG uptake was selectively inhibited by phlorizin (5 X 10(-3) M), which is a competitive inhibitor of glucose uptake at the apical membrane. Complete characterization made it possible to investigate hitherto unexplored aspects of in vitro cultured proximal tubule cells. This primary culture model could provide a useful and reliable tool to investigate in vitro renal proximal tubule function, under normal conditions or after a drug-induced toxicity.

Subcellular localization of celiptium-induced peroxidative damage in rat renal cortex

Celiptium (N2-methyl-9-hydroxyellipticinium) is an antitumor agent of the ellipticine series. We have shown a dose-dependent nephrotoxicity in rats and demonstrated a lipid overload in proximal tubular cells (unsaturated free fatty acid accumulation). We have also shown an increase in thiobarbituric acid reactive substances (TBARS), namely the 4-hydroxyalkenals, that is paralleled by a decrease in phosphatidylethanolamine in rat kidney cortex. In the present study, peroxidative damage was localized in mitochondria, microsomal and brush-border membranes of kidney cortex. Female Wistar rats were injected with a single i.v. dose of 20 mg/kg celiptium and sacrificed on day 8. Subcellular fractionation studies showed that celiptium induced alterations: 1) in mitochondria (slight increase in aldehydes), 2) in microsomal membranes (increase in free fatty acids (FFA) with in particular rises in oleic (18:1) and linoleic (18:2) acids), 3) in brush-border membranes or BBM (decrease in protein and phospholipid contents); residual membranes showed an increase in oleic and linoleic acids and a decrease in the polyunsaturated fatty acids, arachidonic (20:4) and docosahexaenoic (22:6) acids, 4) in cytosol (increase in FFA and TBARS content). Thus, celiptium induces peroxidative damage in kidneys through lipid abnormalities which predominantly occur in brush-border membranes and consist of an increase in free fatty acids and aldehydes in cytosol.

Inhibition of catalase and epoxide hydrolase by the renal cystogen 2-amino-4,5-diphenylthiazole and its metabolites

Subchronic feeding of 2-amino-4,5-diphenylthiazole (DPT) to rats results in the development of renal cysts and has been used as a model system to study polycystic kidney disease. Because previous studies revealed changes in renal enzymes following DPT administration, a possible direct effect of DPT and its phenolic metabolites on catalase and a related enzyme, epoxide hydrolase, was examined. Experiments with three in vitro systems (suspensions of rabbit renal tubules, rat kidney homogenates, and commercially obtained bovine liver catalase) revealed direct inhibition of catalase activity by the diphenolic metabolite (diOH- DPT: 2-amino-4,5di(4'-hydroxyphenyl)-thiazole), the known renal cystogen nordihydroquaiaretic acid (NDGA) 2-amino-4(4'-hydroxyphenyl),5-phenyl-thiazole (4OH-DPT), and the known catalase inhibitor 3-amino-1,2,4-triazole; DPT did not inhibit catalase activity. Following oral administration to rats of the DPT congeners, 4OH-DPT caused the greatest decrease in both renal catalase and cytosolic epoxide hydrolase activities and the shortest time to onset of cystic lesions. In vitro, mouse liver cytosolic epoxide hydrolase activity was substantially inhibited by 4OH-DPT and dioH-DPT, and NDGA, but not by 2-amino-4-phenyl,5-(4'-hydroxyphenyl)-thiazole (5OH-DPT) or DPT itself. Microsomal epoxide hydrolase (mEH) activity was inhibited by 4OH-DPT, unaffected by DPT or dioH-DPT, and stimulated 2-fold by 5OH-DPT. Finally, mEH activity was substantially higher in samples of normal human kidney than in samples of kidney derived from a patient with autosomal recessive polycystic kidney disease; no differences were observed in cEH activity in these samples. Although the role of altered catalase and epoxide hydrolase activities in cystogenesis is unknown, DPT-induced cyst formation is associated with loss of these enzyme activities in kidney tissue. To our knowledge, this is the first report of an in vivo diminution of cytosolic epoxide hydrolase activity by xenobiotics.

Adenylate cyclase responses and biochemical characterization of primary rabbit proximal tubular cell cultures and LLC-PK1 cells

Freshly isolated rabbit proximal tubules (PT), confluent primary rabbit proximal tubule cultures (PTC) and LLC-PK1 cells were characterised. Brushborder enzyme activities were lower in PTC than in LLC-PK1: ratios were 0.026 for alkaline phosphatase (AP), 0.458 for alanine aminopeptidase (AAP) and 0.514 for gamma-glutamyl transpeptidase (GGT). PT/PTC ratios were 79.7 for AP, 7.96 for AAP and 3.45 for GGT. Specific activities of hexokinase (HK) and lactate dehydrogenase (LDH) were high in cultured cells as compared to PT: PT/PTC ratios were 0.063 and 0.033, while PTC/LLC-PK1 ratios were 0.406 and 1.19 for HK and LDH respectively. PTC/LLC-PK1 ratios were 2.21 for Na/K ATPase, 2.07 for succinate dehydrogenase, 1.12 for cathepsin B, 0.607 for N-acetyl-beta-D-glucosaminidase and 8.98 for glutathione-S-transferase. Adenylate cyclase response to parathormone (PTH), was similar in PTC and PT, but stimulated/basal ratios were higher in PT than in PTC. LLC-PK1 cells were stimulated by thyrocalcitonin (SCT), arginin-vasopressin (AVP) and PTH; stimulated/basal ratios ranked AVP greater than PTH greater than SCT. Differences between both types of cultures affect the choice of in vitro model for nephrotoxicity studies.

Endocytosis and lysosomal hydrolysis of proteins in proximal tubules

The techniques and basic protocols described above can be readily reproduced by investigators with experience in perfusion of isolated nephron segments. They can be modified and adapted by the investigator to address specific issues. In particular, isolated perfused nephron segments have also been successfully used for elucidation of biochemical and morphological aspects of endocytosis and lysosomal hydrolysis of macromolecules, proteins, and polypeptides. The reader is directed to the references cited under Methodological Approaches in this chapter for a description of these techniques. Although studies on the endocytic uptake and metabolism of proteins and polypeptides using isolated perfused nephron segments have made significant inroads in our understanding of these fascinating and important biological processes, much remains to be learned. Hopefully, future uses of the technique will further advance our knowledge in this field.

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.

Investigations in intracellular drug storage: localization of disobutamide in lysosomal and nonlysosomal vesicles

In the investigation of the dynamic nature of intracellular drug-induced storage disorder associated with clear cytoplasmic vacuoles (CCV), rat urinary bladder carcinoma cells (RBT CC-8) were cultured with [14C]disobutamide. Cell monolayers were then harvested and analytical cell fractionation techniques were employed to examine the association of disobutamide with the various subcellular fractions. Disobutamide distributed into two modes: as a free, organelle-independent fraction and as a light membrane-associated fraction that overlapped with markers for the plasma membrane, endoplasmic reticulum, and lysosomes. The similarity in buoyant densities of these organelles derived from RBT CC-8 cells precluded resolution of these structures by isopycnic centrifugation. In additional experiments, disobutamide was incubated in vitro with a suspension of isolated rabbit renal proximal tubules. In these cells, analytic fractionation showed that the drug localized predominantly to lysosomes and to a separate light membrane fraction that was clearly resolved from the markers for the endoplasmic reticulum, brush border, mitochondria, and peroxisomes; this fraction overlapped with the most buoyant aspects of the Golgi apparatus and basolateral plasma membrane. The buoyant density of this disobutamide-associated nonlysosomal fraction was 1.11 g/ml. Electron microscopy of the disobutamide-exposed tubules showed a substantial formation of apical vesicles, especially small, smooth-surfaced vesicles, typical of the endocytic apparatus. From these findings and based on the physicochemical properties of the cationic moiety of disobutamide, we conclude that the drug localizes in lysosomal and nonlysosomal acidic vesicles.

Modulation of gentamicin nephrotoxicity by chronic inhibition of angiotensin-I-converting enzyme in rat

Perindopril, a new specific and potent inhibitor of angiotensin-I-converting enzyme, was used to evaluate the possible participation of inhibition of the renin-angiotensin system in the development of aminoglycoside-induced renal failure. Kidney function, morphology and biochemistry were evaluated at regular intervals throughout the study. Perindopril was given orally to rats at a daily dose of 2 mg/kg for 15 days prior to and during 15-day gentamicin treatment given intraperitoneally at a daily dose of 50 mg/kg. Perindopril treatment alone induced no modification in renal function or structure. Gentamicin treatment alone induced typical renal lesions which were scored as moderate and a slight but significant decrease in ACE blood levels. Concurrent treatment with perindopril and gentamicin induced a greater drop in ACE blood levels than after the administration of perindopril alone and produced more marked renal impairment than after the administration of gentamicin alone. These observations suggest that the integrity of the renin-angiotensin system may play an important role in limiting kidney injury during aminoglycoside-induced nephrotoxicity.

Carrier-mediated transport of pyroglutamyl-histidine in renal brush border membrane vesicles

These studies were performed to determine if a transmembrane carrier for pyroglutamyl-histidine (pGlu-His) is present in the luminal membrane of renal proximal tubular cells. Previous studies have suggested the intact transepithelial transport of pGlu-His, a dipeptide formed by the hydrolysis of luteinizing hormone-releasing hormone by enzymes associated with the brush border in the proximal nephron. With the use of a renal brush border membrane vesicle preparation, pGlu-His showed H+-stimulated, Na-independent, saturable transport into an osmotically active space. High-pressure liquid chromatographic analysis of both the intravesicular and extravesicular fluids indicated intact uptake of the dipeptide. The transport constant (Kt) and Vmax for pGlu-His transport were 9.3 X 10(-8) M and 6.1 X 10(-12) mol.mg-1.min-1, respectively. Transport of pGlu-His was not inhibited by the dipeptides glycyl-proline, glycyl-sarcosine, and N-beta-alanyl-L-histidine, which have been previously shown to be transported into renal brush border vesicles via a single, low-affinity, high-capacity, Na-independent, and H+-stimulated peptide carrier. In addition, the gamma-glutamyl-containing peptides gamma-glutamyl-histidine and N(N-L-gamma-glutamyl-L-cysteinyl)glycine and the tripeptide pyroglutamyl-histidyl-prolinamide were without an inhibitory effect. In contrast, transport of pGlu-His was inhibited by the dipeptide pyroglutamyl-alanine. This study demonstrates the existence of a high-affinity, low-capacity H+ cotransport system for pGlu-His in the proximal tubular luminal plasmalemma, which appears to be specific for pyroglutamyl-containing dipeptides. The data indicate that multiple dipeptide carriers are present in the proximal nephron.

Lysosomes of the renal cortex: heterogeneity and role in protein handling

Rate sedimentation of the kidney cortical mitochondrial/lysosomal (ML) fraction yields two distinct classes of lysosomes: the large lysosomes or protein droplets and a heterogeneous broad band of smaller lysosomes. The protein droplets which are recovered as a well defined zone of high purity also sediment as a homogeneous band after equilibrium banding at a density of 1.235 g/ml in sucrose. The small lysosomes co-sediment with other subcellular organelles as a broad band, indicated by the distribution of various acid hydrolases, which exhibit subtle heterogeneity among these small lysosomes. The distribution of renin containing granules indicates that in size they represent a distinct subpopulation of small lysosomes. Further fractionation of small lysosomes by equilibrium banding separates two distinct populations at densities 1.20 (small light) and 1.235 g/ml (small dense). Comparison of lysosomal populations fractionated in these studies with the distribution of lysosomal acid hydrolases along the different segments of the nephron suggests that large and small dense lysosomes probably originate from the proximal tubule while the small light lysosomes may contain lysosomes from the distal tubule. Very small, lysosome-like organelles subfractionated from the 'microsomes' may constitute a mixture of small light lysosomes, lysosomal fragments and endocytic vesicles from a variety of cell types. Time course studies with 3H labelled Cd-thionein, following intravenous administration, suggests that uptake in the kidney cortex is very rapid and that catabolism takes place in two distinct phases: rapid breakdown starting in the endosome compartment and slower breakdown in lysosomes. From the association of labelled lysozyme (125I) and Cd-thionein (109Cd) it appears that all the different lysosomal populations identified are at some stage involved with uptake and catabolism of these two proteins.

Biochemical effects of gentamicin on rat kidney cortex. I. Analytical subfractionation of control tissue

As a first step in studies of the molecular mechanism(s) underlying gentamicin toxicity, rat kidney cortex has been subfractionated using differential centrifugation. An analytical, rather than preparative approach was used. DNA was used as a marker for the nuclei, cytochrome oxidase for mitochondria, acid phosphatase for lysosomes, catalase for peroxisomes, NADPH-cytochrome c reductase for the endoplasmic reticulum, p-nitrophenyl-alpha-mannosidase (at pH 5.5) for the Golgi apparatus, AMPase for the plasma membrane in general, and alkaline phosphatase for the brush border, and lactate dehydrogenase for the cytosol. In addition, electron microscopy was performed on the subfractions obtained. The distributions of subcellular markers obtained here for the rat kidney cortex closely resemble the corresponding distributions reported for rat liver. This procedure can now be used to look for biochemical and/or toxic changes which might be reflected in an altered distribution pattern for marker enzymes.

Aminoglycoside-induced alterations of phosphoinositide metabolism

There exists a strong interaction between aminoglycosides and phosphoinositides, and these membrane lipids are even considered as the drug receptors. To shed some light on the role of such an interaction in the drug nephrotoxicity, we have investigated the influence of aminoglycosides on phosphoinositide metabolism in kidney proximal tubules where these compounds accumulate. Experiments were carried out by measuring 32P labelling of phosphatidylinositol 4-phosphate (PI-P) and of phosphatidylinositol 4,5-bisphosphate (PI-P2) after incubation of homogenates of isolated proximal tubules with [gamma-32P] ATP. The treatment of rabbits with neomycin, gentamicin and amikacin (50, 50 and 300 mg/kg/day, respectively for seven days) promoted a decrease in 32P-PI-P2 and an increase in 32P-PI-P, when compared to the respective values observed in tubules from untreated rabbits. Under these conditions, the extent of modifications in lipid labelling was similar with the three drugs tested. In in vitro experiments, the exogenous addition of the above aminoglycosides to the incubation medium containing tubule homogenates from untreated rabbits also produced, in a dose dependent manner, a decrease in 32P-PI-P2 and an increase in 32P-PI-P. In the in vitro experiments, however, amikacin and gentamicin appeared to be less potent than neomycin. The results indicated moreover that phosphoinositide metabolism was more sensitive to the in vivo (vs. in vitro) action of the drugs. Phosphoinositides are involved in Ca2+ transport and/or mobilization processes, and aminoglycosides are known to interfere with the Ca2+ binding to membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Impairment of lysosome-pinocytic vesicle fusion in rat kidney proximal tubules after treatment with gentamicin at low doses

Gentamicin, an aminoglycoside antibiotic, is known to accumulate within the kidney cortex and to elicit nephrotoxic reactions due to the necrosis of proximal tubules. Female Sprague-Dawley rats, treated for 9 days with gentamicin at a low dose (10 mg/kg ip, once a day), were used to determine the fate of the antibiotic within the proximal tubular cells and its effects on the functional properties of the lysosomes. The analysis of the lysosomes by isopyknic equilibration in sucrose gradient (density 1.10-1.24 g/ml) revealed that gentamicin remains associated with these organelles (marker enzymes sulfatase B and cathepsin B) throughout the treatment duration. Gentamicin treatment markedly decreased the buoyant density of the lysosomes. As was shown by electron microscopic examination of the subcellular fractions collected from the sucrose gradient, the shift of the lysosomes toward lower densities was a result of overloading with undegraded phospholipids (myelin-like figures). The effect of gentamicin on the functional properties of the lysosomes was examined by using horseradish peroxidase (HRP) as a marker of endocytic activity and of the processing by tubular cells of exogenous proteins. Treatment with gentamicin did not significantly modify the intracortical accumulation of HRP, which was estimated to be 2.2% of the amount injected. HRP was shown by isopyknic equilibration to be mostly associated with the lysosomes. This was confirmed by electron microscopic examination of proximal tubular cells after cytochemical demonstration of HRP with diaminobenzidine and H2O2. In rats not exposed to gentamicin, more than half of the lysosomes contained HRP activity. In animals treated with gentamicin, one-third of the lysosomes that retained a normal appearance exhibited HRP activity. In contrast, lysosomes overloaded with phospholipids (identified by the presence of myelin-like figures) were very seldom labeled with HRP activity. Taken altogether, the present observations suggest that the alterations induced by gentamicin treatment impair their ability to fuse with incoming endocytic vesicles.

Quantitative requirement for ATP for active transport in isolated renal cells

We investigated the quantitative relationship between cellular ATP concentration and Na+-K+-ATPase activity as measured by ouabain-sensitive 86Rb influx in rabbit proximal renal cells. Cellular ATP was reduced in a stepwise manner by rotenone (10(-7) to 10(-5) M) and was increased by 10 mM adenosine. During these maneuvers, ouabain-sensitive 86Rb influx was linearly related to cellular ATP and did not saturate up to 9.9 mM ATP. In contrast, Na+-K+-ATPase activity in membranes prepared from these cells saturated at 2.0 mM ATP at various sodium (10-100 mM) and potassium (4-100 mM) concentrations. Sodium-dependent phosphate uptake and alpha-methylglucoside (alpha-MG) uptake were both inhibited to a similar degree when cellular ATP was reduced. We conclude that 1) the ATP requirement for saturation of Na+-K+-ATPase is higher in intact renal cells than in the membranes, and 2) the uptake of phosphate and alpha-MG are similarly influenced by reduction in ATP. This effect of ATP on phosphate and AMG uptake is most likely an indirect one and is secondary to changes in the sodium gradient across the cell.

Renal handling of plasma high density lipoprotein

Renal tubular reabsorption and hydrolysis of plasma high density lipoprotein (HDL3) were studied. Rabbit proximal straight nephron segments were microperfused in vitro with iodinated HDL3. Progressive luminal uptake and cellular accumulation of radiolabeled material were observed during an initial phase, followed by a reduction in sequestration and the appearance of 125I-label in the bathing medium. To detect proteolysis, collected perfusates and bathing media were analyzed for trichloracetic acid soluble radioactivity. 125I-HDL3 in the luminal fluid was intact, but metabolites appeared in the bathing medium. Electron microscopic radioautography demonstrated endocytic uptake of 125I-HDL3 at the luminal membrane of the proximal tubule and movement of grains into lysosome-like dense bodies. Incubation of radiolabeled HDL3 in the presence of renal homogenates resulted in proteolytic activity with an acidic pH optimum. Analytical cell fractionation studies indicated that hydrolysis of the protein component is associated with lysosomes derived from proximal kidney tubules. Collectively, the data show that plasma HDL3 can be reabsorbed in the proximal nephron by a mechanism involving endocytosis at the luminal membrane, followed by proteolysis at lysosomes.

Early effects of gentamicin, tobramycin, and amikacin on the human kidney

The early alterations at the level of the proximal tubule of the human kidney caused by the three most currently used aminoglycosides, gentamicin, tobramycin, and amikacin, were studied. A prospective, randomized, and comparative approach using multidisciplinary methods was used. The patients received either no treatment or one of the three aminoglycosides at a therapeutic dose for 4 days preceding nephrectomy for neoplasia partly involving one kidney. The three aminoglycosides studied induce an early lysosomal phospholipidosis. Gentamicin and tobramycin cannot be distinguished on the basis of drug tissue accumulation, lysosomal overloading, or effect on lysosomal phospholipase A1. Amikacin induces significantly lower lysosomal overloading and no loss of phospholipase A1 activity.

Gentamicin nephrotoxicity in cadmium, lead and mercury pretreated rats

The effect of a previous chronic exposure to cadmium, lead or inorganic mercury on the nephrotoxic potential of gentamicin was investigated in female Sprague-Dawley rats. A daily dose of 10 mg gentamicin/kg body weight/day was administered for 21 days to rats having a renal load of 168 micrograms Cd, 35 micrograms Pb or 129 micrograms Hg/g whole kidney. Urine analysis suggests an attenuation of the nephrotoxic potential of gentamicin while a microscopical examination of kidneys indicates a superimposition of the effects of the metals and the antibiotics. The only clear interaction observed consists in a reduction of gentamicin accumulation in the cortex of cadmium-treated animals. It is concluded that none of the metal pretreatments potentiates the nephrotoxic effects of gentamicin.