Lecture 7: Liver and Kidney

the liver has a strategic location between the intestinal tract and the rest of the body that facilitates its task of

metabolic homeostasis in the body

the liver works to extract ingested nutrients, vitamins, metals, drugs, toxicants etc from the blood for

catabolism, storage, and or excretion into bile

bile formation is essential for uptake of

lipid nutrients, protection from oxidative insult, and excretion of endogenous xenobiotic compounds

the liver is a dominant site for

specific toxins

venous blood from the stomach and intestines flows via the portal vein through the liver before

entering the systemic circulation

scavenging or uptake processes in the liver extract minerals for

catabolism, storage, or excretion into bile

the liver is the first organ to

encounter ingested substances

blood flow to the liver

• 70% is oxygen depleted blood from the portal vein

• 30% is oxygenated blood from the hepatic artery

the hepatic artery feeds the

liver with oxygen

the portal vein brings blood from the

digestive tract

sinusoids are the channel between the hepatocytes where

blood travels on its way to central vein (CV)

CV (HV) drains blood from liver to

systemic circulation

there are many types of gradients like oxygen

zone 1= oxygen rich and zone 3= hypoxic

types of gradients

• oxygen

• bile salts

• bilirubin

• many organic ions

gradients of enzymes involved in detoxification

zone 1: glutathione

zone 3: cytochrome p450 proteins

sinusoids contain 3 major cells types

• endothelial cells

• kupffer cells

• ito cells

structural organization of liver: endothelial cells

line sinusoids; very porous to allow transfer of necessary proteins to hepatocytes

structural organization of liver: kupffer cells

resident macrophages (macrophage function: destroying infectious organisms that enter the body, clearing cellular debris, and wound healing) of liver

structural organization of liver: ito cells

fat storage cells (stellate cells); synthesize collagen and store vitamin A

bile

yellow fluid containing bile salts, glutathione, phospholipids, cholesterol, bilirubin, organic anions, proteins, metals, ions, xenobiotics

bile formation is essential for

• uptake of lipid nutrients from small intestine

• protection of small intestine from oxidative insult

• excretion of endogenous and xenobiotic compounds

hepatocytes transport

bile salts, glutathione, and other solutes into canalicular lumen

canaliculi are

channels between hepatocytes that drain into a common bile duct

canalicular lumen is sealed with

tight junctions

bile is concentrated and stored in

gall bladder before its release into duodenum

what is the major driving force for bile formation?

active transport of bile salts and other osmolytes into canalicular lumen

MDR (multi-drug resistance glycoprotein)

exports lipids and lipophilic drugs

CMOAT (canalicular multiple organic anion transporter)

exports conjugates of glutathione and glucuronides

bile excretion is very important in the

homeostasis of metals

metals are excreted into bile via

• facilitated uptake across sinusoidal membranes via facilitated diffusion or receptor- mediated endocytosis

• specific canalicular membrane transporters

bile is modified along is route to the gallbladder

epithelial cells lining bile ducts contain phase I and phase II enzymes to detoxify toxicants present in bile

hepatic response to chemical insult depends on

• intensity of insult

• cell population affected

• length of exposure (acute, chronic, etc)

hepatic injury: fatty liver (steatosis)

• results from disruptions in lipid metabolism

• lipids accumulate in hepatocytes

• commonly a response to acute exposure

• usually reversible

• caused by cycloheximide, ethanol

fatty liver (steatosis) is caused by

cycloheximide, ethanol

hepatic injury: cell death which can occur by

• apoptosis (lack of inflammation)

• necrosis (inflammation occurs)

  • ALT/AST

hepatic injury: cell death which can be

• focal (random)

• zonal (death in certain functional region)

• panacinar (widespread, massive cellular death)

• caused by acetaminophen, ecstasy, cocaine (oral exposure)

hepatic injury: canalicular cholestasis

results from

• decrease in functional integrity of sinusoidal and canalicular transporters

• diminished transcytosis

• diminished contractility of canaliculus

• weakened junctions between blood and canalicular lumen

  • solutes leak out of lumen

  • loss of charge and size gradient between canalicular lumen and blood

what can cause canalicular cholestasis?

cyclosporine

canalicular cholestasis

• decrease in bile formation

• Bile pigments often accumulate in skin and eyes when excretion of these pigments into bile is impaired – Jaundice

• can result in cell swelling, cell death, and inflammation

hepatic injury: bile duct damage

• damage to ducts that carry bile from liver to GI tract

• can result in loss of bile ducts (vanishing bile duct syndrome)

• Similar to symptoms seen with canalicular cholestasis

• caused by amoxicillin

hepatic injury: sinusoidal damage

occurs from:

• dilation of lumen

• blockage of lumen

• progressive endothelial destruction of endothelial cell wall of lumen

• extensive sinusoidal blockade or cell wall destruction results in liver becoming engorged with blood cells causing shock

• caused by anabolic steroids, acetaminophen

hepatic injury: cirrhosis

• Accumulation of extensive amounts of collagen fibers in response to injury or inflammation fibers in response to injury or inflammation

• Following repeated chemical insult, destroyed hepatic cells are replaced by fibrotic scars

• Architecture of the liver is disrupted

• Decreases liver’s capacity to perform its essential function

• NOT REVERSIBLE

• caused by repeated exposure to ethanol

hepatic injury: tumors

can arise from hepatocytes, bile duct cells, or cells of the sinusoidal lining (rare)

• aflatoxin

• thorotrast (radioactive thorium dioxide)

  • accumulates in Kupffer cells

  • emits radioactivity throughout its long half-life

Why is the liver the target site for so many toxins of diverse structures?

• because the liver has specialized uptake processes that result in higher exposure in the liver versus other tissues

• there is abundant capacity for bioactivation reactions

Why do many hepatotoxins preferentially damage one type of liver cell?

• specialized processes are located in the liver

• Example: Cocaine and acetaminophen cause Zone 3 hepatocellular necrosis

  • Zone 3 is site of high levels of cytochrome p450

  • P450 enzymes produce harmful metabolites of these two drugs

hepatocytes have perforated

epithelial layers

the liver is membrane rich and has the ability to

concentrate lipophilic compounds

the liver contains many sinusoidal transporters which

toxins may be substrates for

vitamin a hepatotoxicity initially affects sinusoidal ito cells which

extract the vitamin

cytochrome p450 enzymes may bioactivate many

toxins to free radicals

conditions in which cytochrome p450 is depleted has been show to decrease

liver damage during exposure to certain hepatotoxins

therapeutic doses of acetaminophen are not

hepatotoxic. however, fasting or other conditions that deplete glutathione may enhance acetaminophen hepatotoxicity

ethanol may increase Cytochrome P4502E1 causing

increased acetaminophen hepatotoxicity

Activation of Kupffer cells increases ROS and

reactive nitrogen species in the liver ex: LPS

In addition, migration (infiltration) of neutrophils, lymphocytes, and other inflammatory cells may occur to

combat infection but also may add to damage by depleting glutathione, etc., through release of excessive amounts of ROS and proteases, etc

Liver cells are vulnerable to same types of

insult that injure other tissues

Preferential liver damage occurs due to the

location of the liver and due to its high capacity for converting chemicals to reactive entities

other mechanisms of liver injury

• cytoskeleton disruption

• mitochondrial damage

cytoskeleton disruption

• phalloidin (mushroom)

  • Upon uptake into hepatocytes, prevents disassembly of actin filaments, affecting dynamic nature and integrity of the hepatocyte cytoskeleton

  • Leads to accentuated “actin web” resulting in dilation of the canalicular lumen

mitochondrial damage

• mitochondrial DNA codes for several proteins in the mitochondrial electron transport chain

• certain toxins affect mitochondrial DNA

  • mitochondrial DNA has limited capacity for repair

liver is susceptible to toxicological insult because of:

• The liver’s proximity and involvement with the GI tract

• The liver’s diverse and vital functions

  • bile formation

  • detoxification reactions

  • extraction of diverse substances

Kidney contributes to total body homeostasis

• excretion of metabolic waste

• synthesis of renin and erythropoietin

• Regulation of extracellular volume

• Acid/base balance

kidney receives relatively large levels of

xenobiotics

the kidney is divided into 3 major areas

• cortex

• medulla

• papilla

the functional unit of the kidney

nephron

nephron

• vascular element

• glomerulus

• tubular element (reabsorption/excretion throughout)

Nephron and Renal Vasculature

Flow rate to glomerulus is highly controlled and responds to nerve stimulation, hormones, signaling molecules, etc

Afferent Arteriole

Blood to glomerulus

Efferent Arteriole

• Blood leaving glomerulus

• Surrounds entire nephron for continual reabsorption and excretion

Glomerulus

Specialized capillary bed that filters a portion of the blood to an ultrafiltrate which enters the proximal tubule

Glomerular filtration is highly dependent on

transcapillary hydrostatic pressure, oncotic pressure, and permeability of the glomerular capillary wall.

Glomerular capillary wall

• Permits high rate of fluid filtration

• Provides a barrier to the transglomerular passage of macromolecules

the nephron: proximal tubulue

• Reabsorbs approximately 60-80% solutes, small proteins, and water filtered at the glomerulus

  • Numerous transport systems

  • Specific endocytotic protein reabsorption processes

the nephron: loop of henle

• reabsorbs Na+/K+ and water

• possesses Na+/K+/2Cl- co-transporters

• water is freely permeable in descending limb

• ascending limb is impermeable to water

the nephron: distal tubule/collecting duct

• Sensitive to physiologic triggers that may cause a decrease glomerular filtration rate (GFR)

  • To prevent massive loss of fluid/electrolytes if impaired tubular reabsorption occurs

• Collecting duct performs final adjustments to urinary volume and composition

  • Responsive to ADH (increased ADH = increased permeability of collecting duct to water = increased water reuptake)

Acute Renal Failure

• Characterized by low glomerular filtration rate GFR and azotemia (buildup of nitrogenous wastes in the blood)

• Drug may precipitate within kidney causing obstruction

• drug may cause vasoconstriction

Impaired Tubular Integrity

• Chemical may compromise cell to cell adhesion in kidney tubules

• Results in gaps in cell lining causing back-leak of filtrate and decreased GFR

• detached cells may cause obstruction of tubules

Kidney has a remarkable ability to compensate for loss in functional renal mass

example:

• Following unilateral nephrectomy, GFR of the remaining kidney increases 40-60%!

• In addition, compensatory increases in all other functions of the nephron occur (reabsorption, etc.)

If a chemical induced changes in renal function

problem may not be detected until compensatory mechanisms are overwhelmed

Chronic Renal Failure

• May occur from long-term exposure to various chemicals

• Adaptation following nephron loss causes increased GFR in functional neurons

  • whole kidney GFR is maintained

with time in chronic renal failure

adaptations can be maladaptive

in chronic renal failure glomerulosclerosis eventually develops leading to

tubular atrophy and interstitial fibrosis

in chronic renal failure, mechanical damage occurs as a result of

chronically increased GFR

Kidneys constitute 0.5% total body weight, but receive

25% of resting cardiac output

Therefore, any drug or toxin in the systemic circulation will be delivered to the kidney in

relatively high concentrations

The kidney concentrates urine and may concentrate toxicants in tubular fluid which drives

passive diffusion of toxicants into tubular cells

the kidney is very sensitive to circulating vasoconstrictors and prostaglandins (vasodilators). any interference with these substances=

renal involvement

Proximal Tubular Injury

• Most common site of toxicant-induced renal injury

• Proximal tubule has leaky epithelium that favors the flux of compounds into the tubule

Loop of Henle/Distal Tubule/Collecting Duct Injury

• Amphotericin B (anti-fungal), cisplatin (chemotherapeutic): cause impaired concentrating ability

Papillary Injury

Agents that inhibit vasodilatory prostaglandins compromise renal blood flow the medulla/papilla and result in tissue ischemia

Glomerular Injury

• initial site of chemical exposure in the nephron

• Cyclosporine, Amphotericin B (antifungal)

  • Impair glomerular filtration by causing renal vasoconstriction and decreasing glomerular filtration

  • injury may occur to glomerular cells walls (cyclosporine)

Assessment of Renal Function: non invasive

• Urine volume measurement

• Osmolality

• pH

• Urinary composition

• GFR determination (via measurement of creatine clearance)

Biochemical Mechanisms of Renal Cell Injury: cell death

• apoptosis: organized, usually affects scattered, individual cells

• oncosis: affects many contiguous cells, cells rupture releasing cellular contents, inflammation follows

• as toxicant concentration increases, process usually shifts from apoptosis to oncosis

A chemical can initiate cellular injury by

a variety of mechanisms

biochemical mediators of toxicity: Cell Volume and Ion Homeostasis

• Both tightly regulated and critical for reabsorptive properties of tubular epithelial cells

• Toxicants can affect these parameters by increasing ion permeability and disrupting cell volume, or by disrupting ATP production

biochemical mediators of toxicity: Cytoskeleton and Cell Polarity

• Toxicants may disrupt membrane integrity by:

  • Alteration of cytoskeletal components

  • Disruption of energy metabolism or calcium and phospholipid homeostasis

biochemical mediators of toxicity: mitochondria

Nephrotoxins may compromise cellular respiration and ATP production causing mitochondrial dysfunction

biochemical mediators of toxicity: lysosomes

Exposure to unleaded gasoline induces cellular injury through rupture and release of lysosomal enzymes

biochemical mediators of toxicity: ca2+ homeostasis

• free cytosolic ca2+ (ca2+ pool) is critical in renal cells

• ca2+ level is maintained by a series of pumps located on the endoplasmic reticulum

• Certain nephrotoxins may disrupt these mechanisms

  • High calcium levels may cause activation of degradative calcium-dependent enzymes (phospholipases) which may degrade cellular components

specific nephrotoxicants: heavy metals

• Different metals have different primary targets in the kidney

• Most metals bind to sulfhydryl groups of critical proteins, inhibiting their normal functions and causing renal cell injury

  • mercury

  • cadmium

  • lead