Term
| What is the major role of hepatocytes with regards to carbohydrate metabolism? |
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Definition
| Hepatocytes remove most of the dietary glucose from portal blood and store it as glycogen. Hepatocytes also synthesize glucose from pyruvate (gluconeogensis) as a source of fuel in fasting animals. |
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Term
| What are the three major roles that hepatocytes play in lipid metabolism? |
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Definition
Synthesize fatty acids, TGs, cholesterol, and apolipoproteins for lipoproteins
Esterify cholesterol for phospholipids
Degrade chylomicrons from portal and systemic blood and remove lipoprotein remnants from systemic blood |
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Term
| What is NH4+ fixation and what organ play a major role in this process? |
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Definition
| NH4+ is incorporated into urea or amino acids, this occurs primarily in hepatocytes |
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Term
| What are the three pathways through which bile is formed? |
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Definition
BA-dependent pathways: active secretion of bile acids creates an osmotic gradient increasing concurrent water excretion
BA-independent pathway: active transport of Na, glutathione, and HCO3- promotes bile formation by stimulating Na-K-adensoine 5'-triphosphatase
Secretin pathway: Secretin promotes bile formation by stimulating HCO3- and Cl- secretion into bile |
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Term
| What are the components of bile? |
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Definition
water, bile acids (accounts for 50% of bile solids), conjugated bilirubin, lecithin, cholesterol, fatty acids, electrolytes, and a variety of water soluble waste products.
**water can be partially reabsorbed in the gall bladder and bile ducts |
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Term
| What CBC abnormalities are associated with decreased hepatic function? |
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Definition
| Anemia, acanthocytes, codocytes, microcytes |
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Term
| What are the most common causes of decreased hepatic function? |
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Definition
portosystemic shunt
end stage chronic active hepatitis
toxic hepatopathy (e.g. steroid hepatopathy, anesthesia, tetracycline, carprofen, copper toxicity, hemochromatosis pyrrolizidine-alkaloid containing plants) |
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Term
| What are the substrates/precursor molecules for bilirubin vs. bile acid pathways? |
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Definition
Bilirubin is derived from heme degradation
Bile acids are derived from cholesterol degradation |
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Term
| What is the dominant form of bilirubin in the blood in the absence of disease? |
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Definition
Unconjugated bilirubin non-covalently associated with albumin
**conjugated bilirubin can also bind covalently to albumin (delta bilirubin) but concentrations of delta bilirubin are negligable in health |
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Term
| What species has a higher total serum [bilirubin] compared with other domestic species? |
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Definition
Horses
bilirubin is normally 0.7-2.0 mg/dL vs. <0.5mg/DL in other animals |
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Term
| What are the two components of conjugated bilirubin? |
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Definition
bilirubin monoglucoronide and/or bilirubin diglucuronide
**within hepatocytes unconjugated bilirubin is conjugated with glucoronide (and glucose in horses) |
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Term
| What is the rate limiting step in bilirubin secretion from hepatocytes? |
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Definition
Transport of conjugated bilirubin from hepatocytes into canaliculi
** energy dependent transport system for organic anions other than bile acids |
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Term
| Describe the methodology of conventional wet chemical assays for measuring serum [bilirubin] |
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Definition
these are spectrophotometric methods, they either measure total bilirubin or direct bilirubin (aka conjugated bilirubin)
*unconjugated bilirubin (indirect) reacts more slowly than conjugated bilirubin (direct) for most assays and therefor is not measured unless accelerant factors are added to the assay |
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Term
| Describe the methodology of dry chemical assays for measuring serum [bilirubin] |
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Definition
| Modified bilirubin diazo reaction in thin layer reagent films, can measure total, conjugates (direct), and unconjugated (indirect) bilirubin fractions |
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Term
| what environmental factor can degrade bilirubin? |
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Definition
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Term
| What interferences can affect measurement of serum [bilirubin]? |
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Definition
immunoglobulins (precipitate after adding acidic reagents and falsely increase results)
Hub can either falsely lower (wet methods) or increase (thin layer reagent slides) total [bilirubin] |
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Term
| In what conditions is hyperbilirubinemia associated with decreased uptake of unconjugated bilirubin by hepatocytes? |
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Definition
Fasting/anorexia (especially in horses)
Decreased functional hepatic mass |
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Term
| In what conditions is hyperbilirubinemia associated wit decreased conjugation of bilirubin? |
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Definition
| Decreased functional hepatic mass |
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Term
| In what conditions is hyperbilirubinemia associated with decreased excretion of conjugated bilirubin in bile? |
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Definition
obstructive cholestasis (hepatic and post hepatic)
Sepsis associated cholestasis (aka functional cholestasis) |
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Term
| What pattern of bilirubin fractions is most supportive of hemolytic anemia (aka increased bilirubin production) as the cause of hyperbilirubinemia in an individual? |
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Definition
unconjugated (indirect) > conjugate (direct)
**however, this is inconsistent and animals with hemolysis may not display this pattern. |
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Term
| What is direct bilirubin? |
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Definition
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Term
| What is indirect bilirubin? |
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Definition
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Term
| What processes other than RBC degradation can contribute to increased unconjugated (indirect) bilirubin? |
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Definition
destruction of erythrocyte precursors (e.g. ineffective erythropoiesis) and degradation of other heme proteins (myoglobin, cytochromes, peroxidase)
**these do not contribute enough to increased total bilirubin if they are the only cause of increased unconjugated (indirect) bilirubin |
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Term
| Describe the pathogenesis for hyperbilirubinemia (unconjugated-indirect) secondary to fasting in horses and cattle? |
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Definition
| increased lipolysis in adipocytes --> increased fatty acids in the blood interfere with uptake of unconjugated (indirect) bilirubin by hepatocytes |
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Term
| What receptors on hepatocytes are used for uptake of unconjugated (indirect) bilirubin? |
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Definition
Z-protien (fatty acid binding protein)
Y-protein (ligandin aka glutathione S-transferase B) |
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Term
| T/F reduction of hepatic clearance of bilirubin is greater than the reduction of hepatic clearance of bile acids in fasting |
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Definition
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Term
| What is the proposed mechanism explaining the increased incidence of fasting anorexia in horses? |
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Definition
| Horses primarily use glucose to conjugate bilirubin. Administration of glucose to anorexic horses lowered plasma concentrations of bilirubin and FFA so it's unclear if bilirubin concentrations were decreased due to to the availability of glucose for conjugation or the decreased [FFA] and lack of competition for bilirubin receptor proteins (Z and Y) |
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Term
| What pattern of bilirubin fractions is most supportive of fasting anorexia (aka decreased uptake of bilirubin by hepatocytes) as the cause of hyperbilirubinemia in an individual? |
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Definition
Unconjugated (indirect) > conjugated (direct)
Total bilirubin is usually only mildly elevated (<8 mg/dL) |
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Term
| In what breeds of sheep have hereditary deficiency in hepatic uptake of unconjugated (indirect) bilirubin been described? |
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Definition
Southdown and Corriedale* sheep
*Corriedale sheep also have hereditary deficiencies in hepatocellular excretion of conjugated (direct) bilirubin |
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Term
| What pattern of bilirubin fractions is most supportive of obstructive cholestasis (aka decreased bilirubin excretion in bile) as the cause of hyperbilirubinemia in an individual (dog or cat)? |
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Definition
Conjugated (direct) bilirubin > unconjugated (indirect)
** with time increased [Bc] in systemic and sinusoidal blood saturates hepatic bilirubin receptors also inhibiting uptake of unconjugated bilirubin, but [Bc] is expected to remain more increased than [Bu]
**in horses and cattle bilirubin fractions are more variable but if the pattern of Bd>Bi is present this is highly suggestive of obstructive cholestasis |
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Term
| Describe the pathogenesis of functional/sepsis associated cholestasis |
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Definition
TNF-alpha +/- endotoxin decreases amount of bile acid transport proteins in hepatocyte canalicular membranes.
IL-6 also interferes with bile salt transport
Impaired excretion of conjugated (direct) bilirubin may be a direct defect associated with sepsis OR may arise from canalicular membrane damage as a result of retained bile acids |
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Term
| What bacteria have been implicated in functional/sepsis associated cholestasis? |
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Definition
| E coli and Staph intermedius |
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Term
| Why is the plasma from herbivores more yellow than plasma from carnivores? |
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Definition
| carotenoid pigments from plants may impart a yellow discoloration to plasma from herbivores |
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Term
| What is the minimum serum [Bt] expected prior to visible icterus? |
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Definition
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Term
| T/F enterohepatic recirculation of bile acids is highly efficient in health |
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Definition
| True- nearly all bile salts are returned to liver via intestinal absorption and portal blood flow |
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Term
| Describe the physiology of increased postprandial [BA] compared with baseline? |
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Definition
| ingestion of a meal stimulates gall bladder contraction with increased [BA] entering the intestine --> after intestinal absorption of bile acids the [BA] in portal blood may exceed liver's ability to extract BA --> increased [BA] in systemic blood |
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Term
| What is the major stimulus for gall bladder contraction? |
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Definition
| cholecystokinin (secreted by endocrine cells in the duodenum in response to food) |
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Term
| What are the four major bile acids? |
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Definition
chalk acid
chenodeoxycholic acid
deocycholic acid
lithocholic acid
**at normal pH (7.4) they disassociate and the anionic molecules dominate (aka bile salts) |
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Term
| Describe the methodology of the bile acid assay? |
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Definition
spectrophotometric assay that uses 3alpha-hydroxysteroid dehydrogenase
*there are also radioimmunoassays or enzyme immunoassays that may be used to measure concentrationc of conjugated bile acids |
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Term
| What are the two major pathologic processes that contribute to increased [BA] in fasting dogs? |
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Definition
decreased BA uptake from portal blood (decreased functional hepatic mass OR portosystemic shunt)
decreased BA secretion in bile (obstructive or functional/sepsis associated cholestasis) |
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Term
| Describe the molecular mechanism for increased [BA] in obstructive cholestasis |
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Definition
| down regulation of canalicular bile acid transport proteins so hepatocytes may pump BA into sinusoidal blood rather than into canaliculi BA are also toxic and when they accumulate can cause hepatocellular damage and release of other substances from hepatocytes and canaliculi into sinusoids and systemic circulation |
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Term
| What cytokine is associated with decreased bile acid transport proteins in hepatocyte canalicular membranes (and hence functional cholestasis and increased [BA])? |
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Definition
| tumor necrosis factor alpha |
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Term
| What factors (other than hepatic function and normal portal vasculature) can affect fasting serum [BA]? |
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Definition
spontaneous gall bladder contraction (increased [BA])
intestinal disease may impair absorption of BA from the gut (decreased [BA]) |
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Term
| What might explain a higher resting [BA] vs. post-prandial [BA]? |
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Definition
variations in GI motility and intestinal absorption
animal may not have eaten the provided food, or may have a poor cholecystokinin response
gallbladder may have contracted prior to feeding |
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Term
| In what species is urine bile acid:creatinine ratio most useful for diagnosising hepatic disease? |
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Definition
Cats
**in dogs using the reported decision threshold urine Ba:Cr had poor negative predictive value due to frequent false negatives. The ratio is pretty good in cats and dogs at positively identifying disease but negative results are suspect. |
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Term
| Why is most of the ammonia in blood ammonium (NH4)? |
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Definition
| At body temp and pressures ammonia (NH3) is a gas (i.e. not dissolved in the plasma) |
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Term
| Where does the ammonium (NH4) in the blood come from? |
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Definition
Most comes from intestinal bacteria (dietary protein digestion and other metabolites)
Less ammonium (NH4) is produced by deamination of amino acids (many cells) and deamination of adenosine monophosphate (muscle) |
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Term
| How is ammonium (NH4) excreted? |
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Definition
enters portal blood and is taken up by hepatocytes --> made into urea, amino acids, or proteins
--> any NH4 that isn't or can't be taken up by hepatocytes is filtered by the glomerulus and is either excreted in urine or fixed into glutamine by renal tubular cells*
*in response to academy glutamine is deaminated by renal tubular cells into glutamate and NH4 is excreted to neutralize excess excreted H+ |
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Term
| Describe the three methods for assaying [NH4+] |
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Definition
enzmatic method: NH4 + alpha-ketoglutarate + NADPH --> glutamate +NADP
Dye binding method: alkylating agent is added to convert NH4 to NH3 which binds to a dye
Colorimetric or ion specific electrodes |
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Term
| What is the effect of sample type on serum ammonium concentrations? |
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Definition
| [Nh4] serum > [NH4] plasma |
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Term
| How does delayed processing of a room temp sample affect [NH4]? |
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Definition
increases because of:
degeneration of labile proteins and amino acids
delayed separation of plasma from RBC/WBC allows NH4 to be produced by these cells
- In some cases NH3 can escape from plasma in tubes that aren't stoppered or are incompletely filled resulting in decreased [NH4] |
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Term
| What congenital deficiency (reported in a dog) has been associated with increased [NH4]? |
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Definition
argininosuccinate synthetase deficiency
*involved in the urea cycle |
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Term
| What are some congenital deficiencies involving components of the urea cycle that have been associate with increased [NH4]? |
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Definition
These cause increased [NH4] because it is not incorporated into urea in the liver
Dogs:
argininosuccinate synthetase deficiency
Horses:
defective mitochondrial transport of ornithine (morgan fillies)
Cats:
Ornithine carbamoyltransferase deficiency |
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Term
| What acquired disorder can be associated with decreased NH4 clearance from portal blood as a result of disruption of the urea cycle? |
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Definition
cobalamin deficiency
ultimately results in decreased activity of carbamoyl phosphate synthetase |
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Term
| Is NH4 higher in arterial or venous blood? |
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Definition
arterial
**however, animals that had increased [NH4] in fasting arterial blood samples almost always had increased [NH4] in venous samples |
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Term
| What conditions are associated with increased NH4 production? |
|
Definition
postprandial
urea toxicosis in cattle
strenuous exercise (horses and dogs)
Urease producing UTI with concurrent obstruction
intestinal disease in horses (unpredictable) |
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Term
| Describe the pathogenesis of increased [NH4] in cattle with urea toxicosis? |
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Definition
| Urea is hydrolyzed in the rumen --> NH3 combines with H+ to form NH4 --> as the rumen fluid becomes more alkaline (pH >8) the NH3:NH4 ratio shifts towards NH3 which diffuses into plasma where it then combines with H+ to increase plasma [NH4} |
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Term
| Describe the pathogenesis of increased urea production secondary to strenuous excise in horses and dogs? |
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Definition
| During exercise 2 ADP combine to form ATP and AMP (adenosine monophosphate), AMP is deaminated into inosine monophosphate and NH3 --> NH3 diffuses into plasma, combines with H+ to increased [NH4] |
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Term
| What is the indication for doing a NH4 tolerance test? |
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Definition
used when fasting [NH4] is WNL but there is still high clinical suspicion of shunt or decreased functional hepatic mass (e.g. production of NH4 is not sufficient to overwhelm a less functional liver or impaired urea cycle)
*can contribute to encephalopathy |
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Term
| What time point is best to assess increased [NH4] when running a post prandial NH4 tolerance test? |
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Definition
6 hrs post ingestion of high protein food
**good at identifying dogs with shunts, dogs with other liver disease did not have significantly different pre or post [NH4], but did have pre [NH4] higher than healthy dogs |
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