Structure and Function of the Kidney • The functional unit of the kidney is the nephron • The major functions of the kidney are to maintain extracellular fluids, to eliminate wastes resulting from normal metabolism, and to excrete xenobiotics and their metabolites • Mammalian kidneys have 10,000-1,000,000 nephrons per kidney
Structure and Function of the Kidney (cont) • The glomerulus yields an ultrafiltrate of plasma that represents 20% of the renal blood flow, ie. 2-3% of cardiac output • Endothelial surface is negatively charged and contains fenestrae • The glomerular basement membrane is sandwiched between the epithelial cells and contains anionic sialoglycoproteins, glycoproteins and collagen IV • The mesangium provides support • The outer capsule is Bowman’s capsule
Structure and Function of the Kidney (cont) • The tubule resorbs greater than 99% of the glomerular filtrate • The proximal tubule has extensive resorption and selective secretion (convoluted - S1 and S2, straight - S3). S2 is primary site for low MW protein resorption and S3 is primary site for P450. • Thin loop of Henle - resorption of fluids • Distal tubule - resorption of fluids and acid-base balance • Collecting duct - resorption of fluids, antidiuretic hormone and acid-base balance
Structure and Function of the Kidney (cont) • Produces erythropoietin, which regulates RBC production • Hydroxylates 25-OH-cholecalciferol (vitamin D metabolite), to promote bone resorption and calcium and phosphorus absorption from the gut • Releases renin to regulate the peripheral renin-angiotensin-aldosterone system (juctaglomerular apparatus)
Assessment of Kidney Function:Morphologic Evaluation • Urinalysis • Gross evaluation of the kidney at necropsy • Histopathology of the kidney • Electron microscopy of the kidney
Assessment of Kidney Function:Urinalysis • Proteinuria - indicates glomerular damage • Glycosuria - indicates tubular damage • Urine volume and osmolarity • pH • Enzymes - indicates tubular damage • Microscopic examination - casts, crystals, bacteria, etc.
Assessment of Kidney Function:Blood Chemistries • Blood urea nitrogen (BUN) • Creatinine • Electrolytes - Ca, Mg, K, P • Glomerular filtration rate - determines the clearance of inulin, creatinine and BUN • Renal clearance - measures the clearance of p-aminohippuric acid by filtration and secretion
Glomerular Disease: Toxicities due to Alteration of Anionic Charge • Hexadimethrine - polycationic molecule reduces anionic charge, which permits escape of anionic molecules such as albumin and IgG • Polynucleoside of puromycin - damages epithelial foot processes
Glomerular Disease:Immune Complex Disease • Anti-GBM mediated glomerulonephritis is induced by heterologous antibodies • Antibodies due to exogenous antigens - cationized molecules such as lysozyme, IgG and BSA bind to anionized surfaces; Concanavalin A binds to sugars in the GBM
Glomerular Disease:Immune Complex Disease (cont) Deposition of circulating immune complexes • Drug or toxin-induced T-cell dependent polyclonal B-cell activation - mercury in Brown Norway rats • Unknown mechanism - gold salts, D-penicillamine, hydralazine • Antibodies to heterologous proteins - safety evaluations of recombinant proteins in laboratory animals
Nephrosis: Damage to the renal tubule • Halogenated hydrocarbons - chloroform, hexachlorobutadiene, trichloroethylene, dibromochloropropane, & bromobenzene • Heavy metals - cadmium, mercury & lead • Antibiotics - cephalosporins & aminoglycosides • Mycotoxins - ochratoxin A & citrinin • Ethylene glycol • Antineoplastic drugs - cisplatinum • Alpha2u-globulin nephropathy
Haloalkane Nephrosis • Chloroform is metabolized by P450 to an electrophile, phosgene, which is a potent cytotoxicant. • Carbon tetrachloride is metabolized to free radicals and phosgene. • P450 is localized in the proximal tubule. • This results in nephrosis with necrosis, enzyme, glucose and protein excretion in urine, and increased BUN and creatinine concentrations in serum.
Haloalkene Nephrotoxicity • 1,1-Dichloroethylene, trichloroethylene and tetrachloroethylene are metabolized by P450 to electrophilic metabolites and or free radicals. • These metabolites can be cytotoxic and/or genotoxic. • Nephrotoxicity is exacerbated when glutathione is depleted.
Glutathione-mediated Nephrosis • Glutathione conjugates of haloalkanes can form episulfonium ions. • Primary route for 1,2-dichloroethane, 1,2-dibromoethane and 1,2-dibromo-3-chloro-propane. • These can alkylate macromolecules and cause cytotoxicity and genotoxicity.
Cystine Conjugate -lyase Activation • Stable cystine conjugates from glutathione can be formed in the liver from trichloro-ethylene, tetrafluoroethylene and hexa-chlorobutadiene and transported to the kidney. • They are further metabolized by -lyase in the kidney to generate reactive thiols.
Biotransformation of Trichloroethylene + GSH GSH-transferase Cytochrome P450 -Lyase Chlorothioketene
Trichloroethylene-associated renal cell carcinomas • Highly exposed workers exhibit nephrosis and an increased incidence of renal cancer • TCE and other renal neoplasms have a high incidence of mutations in the von Hipple-Lindau (VHL) tumor suppressor gene. This requires mutation of one allele and deletion of the other • TCE mutations include a hot spot at nucleotide 454 (CT) • VHL gene is located at 3p25 • 3p25.5 is a fragile site at the chromosome telomere • ogg1 mutations are also associated with renal cancer and it is also located at 3p25
Lead Nephropathy • Lead induces acute nephrosis of the proximal tubule and affects blood pressure to the kidney. • Low molecular weight proteins bind Pb2+, leading to resorption by endocytosis. • Cellular and mitochondrial swelling, karyomegaly, mitosis and intranuclear inculsion bodies are seen. • Decreased tubular resorption of glucose, phosphate and amino acids +/- proteinuria • Chronic exposure causes tubulo-interstitial disease.
Cadmium Nephrotoxicity • Metalothionein (Mt) is a low molecular weight protein that transports cadmium (Cd) to the kidney. • Mt is taken up in the lysosomes and Cd is released. Cd t ½ is 10-30 years in humans. • Free Cd stimulates synthesis of renal Mt, which binds Cd. When renal Mt is depleted, toxicity occurs in S1 and S2. • Proteinuria, calciuria, aminoaciduria and glucosuria are seen. • Chronic exposure leads to tubulo-interstitial disease. There appears to be a threshold of 200 ppm Cd in the kidney before renal disease occurs.
Mercury Nephropathy • Hg2+ and methylmercury cause damage to the S3 and S2/S3 segments of the proximal tubule. • Hg2+ causes damage to the cell membrane and mitochondria, and loss of cellular control over intracellular Ca++. • Chronic exposure results in anti-GBM and immune-complex disease of the kidney. • Methylmercury is highly lipid soluble and concentrates in the proximal tubule, causing damage to mitochondria and lysosomes.
Chromium Nephropathy • Cr+6 causes acute nephrosis of S1 and S2. • It damages the brush border, causing decreased absorption of proteins, glucose and amino acids. • Large membranous myeloid bodies form as a result of disrupted lysosomal function. • Decreased glomerular filtration leads to increased BUN and creatinine.
Ethylene Glycol Nephropathy • Toxicity first discovered in humans when ethylene glycol was used as a drug solvent. • Causes acute renal toxicity in animals after exposure to anti-freeze. • Ethylene glycol is metabolized to oxalic acid. • Calcium oxylate crystals form in the lumen of the tubules.
Renal Papillary Necrosis • Caused by non-steroidal anti-inflammatory drugs – phenacetin and acetaminophen. • Highest concentration of drugs are in the renal papilla. • Prostaglandin hydroperoxidase is highest in the medulla and is thought to metabolize these drugs to reactive quinoneimines.
Urinary Bladder Toxins • Chemicals that cause bladder stones and calcium phosphate precipitates frequently cause bladder cancer in rodents. • Aromatic amines, tobacco smoke and parasitic infections cause bladder cancer in humans.
Saccharin • Non-genotoxic • Induces bladder epithelial tumors in male rats at >1% in the diet • Must be administered to neonatal animals • Can act as a promoter for genotoxic agents • Only the sodium salt of saccharin is carcinogenic • Mechanism requires the formation of calcium phosphate-containing precipitates