Hepatic Flashcards
True or false: The horse does not have a gall bladder so bile flows freely almost continuously and unconcentrated in a direction opposite to blood.
True
Where are the spaces of Disse and what do they do?
Spaces of Disse lie beneath pores in the endothelial lining of the sinusoids. These connect with lymphatic vessels and freely absorb plasma but not RBCs.
A cleft, lies between the hepatocytes and the cells lining the sinusoids. It contains fluid similar to the composition of blood but does not contain erythrocytes.
What do stellate cells do?
Store fat soluble vitamins such as vitamin A.
What are the different zones of the liver lobule and what are the features of each?
Zone 1 (periportal) is located adjacent to the branching hepatic arteries and portal veins. It is the most oxygenated and hence most metabolically active. Responsible for oxidative liver functions such as gluconeogenesis and ß-oxidation of fatty acids and is the primary site for deposition of haemosiderin.
Zone 2 is the one typically affected by toxins such as creeping indigo.
Zone 3 is located adjacent to the central veins and has the highest cytochrome P450 mixed-function oxidase activity and the least favourable situation for oxygenation. It is most susceptible to toxins and hypoxic damage.
Where does the bile duct empty?
Major duodenal papilla (with the pancreatic duct)
When liver parenchyma cells are destroyed they are replaced with fibrous tissue that eventually contracts around the blood vessels. What is the effect of this?
Portal hypertension which can impede flow from the intestines and spleen resulting in leakage of fluid from the capillaries into the lumen and walls of the the intestine.
Which coagulation and fibrinolysis factors are formed in the liver?
Factors II, V, VII-XIII, fibrinogen, antithrombin III, protein C, plasminogen, plasminogen activatior inhibitor.
What is the role of hepcidin?
Reduces dietary iron absorption by reducing iron transport across the gut mucosa and reduces iron exit from macrophages and the liver.
What influence to glucocorticoids, glucagon and thyroid hormone have on the liver?
They increase gluconeongenesis, glycogenolysis and promote peripheral protein catabolism.
What is the role of the liver in eliminating ammonia?
- Synthesis of non-essential amino acids from α keto acids and ammonia via reversal deamination. Involves formation of glutamate from α-ketoglutarate and ammonia.
- Glutamate is used to form other amino acids and also participates in the conversion of more ammonia into glutamine which is delivered to the kidney to be converted back to ammonia and excreted (glutamine is non-toxic except in the brain where it causes cerebral oedema).
- Ammonia not excreted by the kidneys is returned to the liver where it synthesises urea via the Krebs-Henseleit cycle for excretion via the kidneys.
How are the majority of short-chain fatty acids (SCFA) delivered to the liver?
Incorporated into phospholipid or triglyceride by enterocytes and delivered via the portal blood.
What are the differences between VLDLs and HDLs and what happens to them once packaged?
VLDLs primarily contain triglycerides whereas HDLs primarily contain protein and phospholipids. They are released into hepatic sinusoids and once in the systemic circulation adipose tissue takes them up or endothelial lipases alter their composition by removing triglyceride, forming intermediate low density lipoproteins.
What are the roles of glucocorticoids and insulin on fatty acids in the liver?
Glucocorticoids increase fatty acid mobilisation from the periphery.
Insulin decreases adipose tissue release of fatty acids by activating lipoprotein lipase and inhibiting hormone sensitive lipase.
Insulin also acts on the liver to increase fatty acid synthesis from glucose.
What is the role of bile acids?
Amphoteric molecules that act as detergents to facilitate excretion of cholesterol and phospholipid from the liver into bile and facilitate digestion and absorption of lipid and lipid soluble compounds such as vitamin A, D, E and K from the intestinal tract.
What is the difference between conjugated and unconjugated bilirubin?
Macrophages in the spleen, bone marrow and liver engulf pigments, convert them to biliverdin, and then convert that to bilirubin and release it as free bilirubin - this is unconjugated for which is bound to albumin and delivered to the liver. Within the hepatocyte, the bilirubin is conjugated with glucuronide and is now water soluble and excreted into the bile canaliculi as conjugated bilirubin.
In normal circumstances, little conjugated bilirubin escapes into the circulation but with severe liver disease, increased amounts will escape and be freely filtered in the urine.
Microflora in the GIT reduce conjugated bilirubin to urobilinogen and stercobilinogen which impart a yellow-brown colour to faeces. Urobilinogen is absorbed by intestinal mucosa and returned to the liver.
A small amount of conjugated bilirubin in the intestinal lumen is hydrolysed to unconjugated bilirubin and reabsorbed. The live extracts most of the urobilinogen however a small amount spills over into the urine where it is concentrated and therefore detectable.
What are the phases of hepatic detoxification?
Phase 1: polar groups are added to the compound or existing polar groups are exposed by oxidation, hydroxylation, deamination or reduction. Some substrates are capable of saturating the enzymes responsible for this (usually of the P450 system) causing accelerated removal rates (eg barbiturates, bute, chlorinated hydrocarbons); others are inhibitors of these microsomal enzymes thus prolonging effects (eg chloramphenicol, cimetidine, organophosphates, morphine and quinidine)
Phase 2. The product of phase 1 is conjugated usually with glucuronate or sulphate.
True or false: substances for detoxification are usually water-soluble and biotransformation renders them more susceptible to renal or biliary excretion.
False. Substances for detoxification are usually water INSOLUBLE and biotransformation renders them more susceptible to renal or biliary excretion.
Biotransformation sometimes results in the formation of toxic metabolites from a non-toxic parent compound. Which group of horses may be more at risk of toxic effects when metabolising these compounds?
Young horses may have less ability to metabolise aromatic hydrocarbons for example.
What is the role of Kupffer cells?
They phagocytose and cleans the portal blood of bacterial endotoxins and other products absorbed from the GIT before they reach the systemic circulation and help cleanse the hepatic arterial blood of fibrin degradation products, tissue plasminogen activators, haemoglobin, microbes, foreign antigens and other particulate debris and help recycle iron from senescent or injured RBCs hence they accumulate haemosiderin.
What role does the liver have in vitamin D metabolism or synthesis?
Vitamin D is first converted in the liver to 25-hydroxycholecalciferol and exported to the kidney where it is transformed into 1,25 dihydroxycholecalciferol, the active form.
What role does the liver have in the foetus and how does this change with adulthood?
In the foetus, the liver is a site of haematopoiesis.
In adults it is an extra-medullary site only for intense conditions of erythrocyte regeneration or if a large portion of the bone marrow is destroyed.
In most conditions, what proportion of hepatic mass must be lost before the hepatic function is impaired?
80%+
Why do clinical signs rarely accompany focal hepatic injury?
Because sufficient hepatic reserve exists in unaffected regions to compensate.
The multifocal or generalised disease is more likely to result in clinically significant hepatic disease.
What is bridging necrosis?
Necrosis of contiguous hepatocytes that spans adjacent lobules in a portal to portal, portal to central or central to central fashion.
What are the gross features of zonal injury and common types of zonal injury?
Appears pale with an enhanced lobular pattern on the cut surface. Two most common types of zonal injury are centrilobular (primarily zone 3, such as severe acute anaemia, passive congestion from congestive heart failure and PA toxicity) and peri-acinar (involves only a wedge around the central vein).
Why are centrilobular and peri-acinar hepatocytes most susceptible to anoxic damage?
The normal oxygen tension is lowest in these regions and mixed function oxidase activity is the greatest in these areas.
What is a common cause of periportal (zone 1 acinar lobular) injury?
Infarction of hepatic vessels as may occur with verminous arteritis or exposure to gut-derived toxins.
List causes of acute generalised hepatic injury
- Infection (bacterial, viral, parasitic)
- Necrosis
- Apoptosis
- Inflammation (including immune disorders)
- Hepatotoxic agents
Acute inflammation most commonly accompanies necrosis and is characterised by what cells?
Neutrophils and lymphocytes in the area of cell death or surrounding portal triads.
What is cholangitis and what is the common source?
An inflammatory process primarily involving the biliary system. Usually the result of ascending infection from the GIT after cholestasis.
What role do stellate cells have in hepatic fibrosis and what stimulates them?
Kupffer cells produce TNF-a which stimulates the stellate cells to contract individually but proliferate in number. Once activated, they become chemotactic, decrease their vitamin A stores, secrete collagen and then become senescent.
What conditions commonly result in fibrosis?
Fibrosis often follows conditions resulting in chronic hypoxia, chronic inflammation, chronic cholangitis or cholestasis, metastatic neoplasia, trauma or ingestion of antimitotic agents such as PA.
What are the clinical signs of hepatic insufficiency in horses?
Common: depression, anorexia, colic, HEP, weight loss, orange-coloured urine and icterus.
Less: photosensitization, diarrhoea, bilateral pharyngeal paralysis, bleeding; ascites, dependent oedema;
Rarer: steatorrhea, tenesmus, generalised seborrhea, pruritus, endotoxic shock, polydipsia and haemolysis.
What are the stages (adapted from human medicine) of HEP?
Stage I: mild confusion, decreased attention, slowed ability to perform mental tasks, irritability
Stage II: Drowsiness, lethargy, obvious personality changes, inappropriate behaviour, disorientation
Stage III: Somnolent but rousable, marked confusion, amnesia, occasional aggressive uncontrolled behaviour
Stage IV: Coma
What is the pathogenesis of hepatic encephalopathy and proposed mechanisms?
Several mechanisms may be involved:
- GIT-derived neurotoxins (ammonia most commonly incriminated)
- Augmented activity of GABA
- Altered expression of benzodiazepine receptors
- Increased neurosteroid synthesis
- False neurotransmitter accumulation after plasma amino-acid imbalance (>AAA /
- Increased blood manganese concentrations
- Increase inflammatory mediators expression (cytokines)
- Increased permeability of the BBB and cerebral hypertension
- Impaired CNS energy metabolism.
In the face of liver failure ammonia is insufficiently metabolised via the urea cycle and with increasing concentrations, it enters the CNS and may cause encephalopathy. It has a toxic effect on cell membrane neurons by inhibiting Na/K ATPase activity in nerve cell membranes causing depletion of ATP. It also interferes with CNS energy production through alteration of the tricarboxylic acid cycle resulting in α-ketoglutarate formation and increased synthesis of glutamine which when it accumulates in astrocytes, causes swelling of the cell and generation of cerebral oedema.
Because glutamate is the major excitatory neurotransmitter in the mammalian brain, in acute liver failure increased synaptic release results in overaction of glutamate receptors and hence hyperexcitability. In chronic liver failure downregulation of glutamate receptor activity or increased GABA activity contributes to decreased excitatory transmission. Hyperammonaemia also induces NO and ROS which lead to the accumulation of peroxides, oxidative stress and nerve cell damage. Other gut-derived neurotoxins such as Mercaptans, short-chain fatty acids, oxindole and phenols are likely involved.
Histologically what cell type confirms HEP?
Alzheimer type II changes to astrocytes (enlarge nuclei and lack of cytoplasm).
What mechanisms might induce increased GABAergic tone in HEP?
- Hyperammonaemia may increase GABAergic tone
- The GABA receptor has binding sites for 3 classes of synergistic ligands (GABA and agonists, benzodiazepines, barbiturates); Increased brain levels of natural benzodiazepines may increase GABA-ergic tone in HEP
- There is evidence for a functional increase in GABAergic tone mediated allosterically through the benzodiazepine receptor by an endogenous diazepam-like substance. Clinical response to flumazenil (benzodiazepine receptor antagonist) supports this.
What is the proposed role of inflammatory products associated with hepatic injury in the development of HEP?
- Breakdown products of hepatocytes and inflammatory mediators (cytokines, free radicals etc) increase the permeability of the BBB
- Toxic events in the brain result in both cytotoxic (swelling without alteration in BBB) and vasogenic (increased permeability of the BBB) brain oedema
- TNFα is increased in patients with acute and chronic liver disease and the level correlates with severity of HEP and prognosis.
- TNFα increases ammonia diffusion into the brain, inhibits astrocyte uptake of glutamate and inhibits glutamate synthetase, increases expression of benzodiazepine receptors and increases brain capillary fluid leakage and cerebral oedema formation.
- Excess manganese (as occurs with hepatic disease) potentiates the production of TNFα.
- Alterations in cerebral blood flow and BBB permeability leading to cerebral hypertension and oedema and changes in cerebral metabolism are present in patients with HEP.
What does icterus signify?
Hyperbilirubinaemia
What broad categories cause hyperbilirubinaemia and for each, what are the main causes?
- Increased production: Haemolysis (intra or extravascular) or intracorporeal haemorrhage and subsequent reabsorption of RBC products; Occurs regardless of liver function as the rate of production exceeds the ability of the liver to excrete. Results in increased unconjugated bilirubin, although a mild increase in conjugated bilirubin may occur concurrently.
- Impaired hepatic uptake/conjugation: Results in increased unconjugated bilirubin (retention or hepatic icterus); Most common form in horses. Certain drugs, anorexia or prematurity can impede hepatic uptake and conjugation of bilirubin. Congenital deficiency in enzymes required to conjugate bilirubin may result in ‘Gilbert’s Syndrome’ (human condition, but suspected in some TBs)
- Impaired excretion of bilirubin: Blockage of bile can accompany cholangitis, hepatitis, obstructive cholelithiasis, neoplastic infiltration, fibrosis, physical obstruction of the biliary tract or hyperplasia of the biliary tract. Conjugated >30%.
Is hepatocellular disease most commonly associated with increases in conjugated bilirubin, unconjugated bilirubin or both?
Usually both, although the majority is usually from unconjugated. Increases in the conjugated fraction less than 25% of the total are usually indicative of predominant hepatocellular disease and increases greater than 30% are usually indicative of cholestasis.
Describe the pathogenesis of hepatogenic photosensitisation.
Phylloerythrin is a GIT product of bacterial degradation of chlorophyll that is conjugated and excreted by the liver. During hepatic insufficiency blood concentrations increase. Exposure of the phylloerythrin to UV light causes activation of electrons within the molecule to an excited state, with the resultant free radical formation and cell membrane damage and necrosis.
Unpigmented areas absorb the most UV light hence the lesion distribution. Lesions are initially erythematous and oedematous and progress to pruritus, pain, vesiculation, ulceration, necrosis and sloughing.
Explain why colic might be seen in horses with acute hepatocellular disease?
Pain is due to acute change in hepatic size (due primarily to hepatic swelling) and/or biliary obstruction (cholelithiasis).
Signs may include anorexia, bruxism, dog-sitting, recumbency and rolling.
Palpation on the right, particularly behind the last rib may elicit a pain response.
A proportion (10/25 in one report) of horses with liver disease and colic may also have gastric impactions.
What is the suspected pathogenesis of the development of diarrhoea with hepatic disease, although uncommon?
Alterations in the intestinal microflora, portal hypertension (rare but can increase hydrostatic and oncotic pressure resulting in water and protein loss into the lumen), intestinal hyperema, and deficiency of bile acids may be involved.
Why is steatorrhea rare in horses with hepatic disease?
Due to the low-fat equine diet.
Which factors associated with coagulation and fibrinolysis are sensitive to hepatic disease and why?
- Fibrinogen
- Vitamin K dependent factors (II, VII, IX, X and protein C) have short half-lives. Factor VII half-life is 4-5hours; others range from 4-5 days.
- Vitamin K is fat-soluble so requires bile acids for intestinal absorption, hence any disorder that results in reduced bile acid excretion will limit Vitamin K dependent factors.
- Antithrombin III activity should be increased with liver disease, hence promoting bleeding.
- Failure to remove activated coagulation factors promotes coagulation, and FDP’s interfere with platelet function and fibrin clot formation
What causes pruritus and seborrhea in horses with liver disease?
Retention of bile acids and accumulation in the skin.
True or false: Ponies with hyperlipaemia may develop dependent abdominal oedema after vascular thrombosis or as a result of increased blood flow and hydrostatic pressure in the subcutaneous abdominal vein caused by a partial obstruction of the caudal vena cava by the rapidly enlarged liver
True
Why is oedema secondary to hypoalbuminaemia a rare finding in horses with acute hepatic disease?
The half life of albumin is long (19-20 days) hence hypoalbuminaemia and resultant oedema is rare in acute disease.
What is hepatorenal syndrome?
Acute azotaemia and anuria occur in ponies with hyperlipaemia and hepatic lipidosis. Thought to be due to reduced effective circulating volume, decreased hepatic inactivation of renin and endotoxaemia.
What is the proposed mechanism of PU/PD in cases of chronic hepatic disease?
- Increased aldosterone concentrations occur due to reduced hepatic biotransformation and decreased effective circulating volume due to portal hypertension and hypoalbuminaemia, resulting in Na and H20 retention.
- Na raises osmolality of the extracellular fluid, thereby stimulating the thirst centre and PD/PU ensues.
List the hepatocellular derived enzymes, biliary derived enzymes and indicators of hepatic function.
Hepatocellular enzymes: sorbitol dehydrogenase (SDH), AST, GLDH, ARG, ALP (more biliary, mild hepatocellular), LDH, ALT, ICD
Biliary enzymes: GGT, ALP
Hepatic function: Bile acids, conjugated bilirubin, ammonia, partial thromboplastin time, prothrombin time
True or false: magnitude of elevation of hepatocellular-derived enzymes may not correspond to functional abnormalities and therefore should be interpreted as a measure of disease rather than a test of function
True
List non-hepatic causes of increased unconjugated bilirubin.
- Haemolysis (up to 1,368umol/L)
- Anorexia (up to 135umol/L)
- Intestinal obstruction
- Cardiac insufficiency
- Gilbert’s syndrome (inherited bilirubin-uridine diphosphate glucuronyl transferase deficiency)
- Drug administration (corticosteroids, heparin, halothane)
What is the proposed mechanism for higher bilirubin concentration in foals?
Turnover of foetal haemoglobin to adult haemoglobin and concurrent deficiency of liver-binding and conjugating enzymes relative to adults.
True or false: a normal bilirubin value precludes a diagnosis of hepatic insufficiency.
False.
In a horse with detectable urinary bilirubin concentrations, is a hepatocellular or cholestatic disease more likely and why?
Chlolestatic.
Conjugated bilirubin is water soluble but only excreted in the urine when it reaches levels that surpass the renal threshold. If conjugated bilirubin is contributing more than 30% to the total bilirubin then cholestatic disease is suspected, hence when the value increases sufficiently to cause renal overflow, cholestatic disease should also be suspected
Is detection or failure to detect urobilinogen in urine an indicator of reduced bilirubin excretion?
Failure to detect urobilinogen - this is usually present in urine and indicates a patent bile duct.
