Metabolism and Temperature Regulation

Question 1

How is glucose released into the blood stream after ingestion of a carbohydrate

Answer 1

• Ingested carbohydrates are of the forms:
  
  • starches - large polysaccharides
  • Sucrose (Fructose plus glucose)
  • Maltose (Two glucose molecules)
• Carbohydrates are digested within the gastrointestinal tract to the simple sugars glucose (80%), galactose (from lactose) and fructose
  
  • The reactions to digest the carbohydrates are catalysed by brush border enzymes together with amylase
• These sugars are absorbed into the portal blood stream and are transported to the liver
• In the liver, large amounts of glucose-6-phosphate together with other enzumes ensures that the majority of all monosacchraides are eventually converted to glucose
• Glucose is released by the hepatocytes as required or stored in the form of glycogen via the action of glycogen synthase
  
  • Glycogen phosphatase is the enzyme responsible for slow breakdown of glycogen to glucose for maintenance of blood glucose in between meals

Question 2

Describe the process that allows glucose to readily pass from the blood into the cellular cytoplasm

Answer 2

• Glucose is transported into most tissue cells by a process of facilitate diffusion
• Facilitated diffusion for glucose requires binding to a carrier protein
  
  • There are 14 GLUT proteins in humans
• Facilitated diffusion happens in both direction from high concentration to low concentration
• Binding of a single glucose molecule to the transporter protein enables a conformational change and passage of glucose into the cell

Question 3

Briefly describe the role and mechanism of action of insulin in cellular glucose transport

Answer 3

• Insulin greatly enhances the rate of transport by the GLUT membrane proteins
• Insulin binds to an insulin receptor on cells that are sensitive to the hormone including muscle and adipose cells
• Insulin binding activates a cascade of intermediate cellular products including numerous protein kinases such as protein kinase C
• The casecade leads to translocation of GLUT4 proteins to the cell membrane
  
  • Increased GLUT 4 proteins enhances the rate of glucose diffusion into the cell

Question 4

Briefly describe the process of glycogen formation

Where is glycogen formed and stored

Answer 4

• Glycogen is a large polymer of glucose molecules
  
  • Most cells in the body can store small amounts of glycogen
  • Liver cells can store large quantities of glycogen (5-8% of their total weight
  • Muscle cells can store 1-3% of their total weight as glycogen
• Glucose enters the cell via facilitated transport
• Cellular glucokinase or hexokinase converts glucose (and other monosaccharides to glucose-6-phosphate
• Glucose-6-phosphate is converted to glucose-1-phosphate
  
  • This is converted to uridine diphosphate glucose
• UDG is subsequently converted to glycogen for energy storage within the cell

Question 5

Briefly describe the process of glycogenolysis (glycolysis)

Answer 5

• Glycolysis is the proces by which glycogen is broken down to form pyruvic acid, ATP and H⁺
  
  • Pyruvic acid enters the citrate cycle to further generate energy in the form of ATP
  • H⁺ is converted to lactic acid for removal from the cell
• Glycolysis is a 10-step process catalysed by separate enzymes
• The process is initiated with splitting of glucose away from the polymer by the phosphorylase enzyme

Question 6

Which hormones can affect the rate of glycogenolysis and glycolysis

Answer 6

1. Epinephrine
   
   • activates adenylate cyclase and the formation of cAMP
   • cAMP activates PKA which in turn activates phosphorylase
     
     • Phosphorylase catalyses the initial step in glycogenolysis
2. Glucagon
   
   • Stimulates the formation of cAMP, primarily within liver cells
   • Similarly to epinephine, stimulates phosphorylase and the initial breakdown of glycogen to increase glucose release and utilisation
3. Insulin
   
   • Augments the cellular uptake of glucose via enhances translocation of GLUT to the cell membrane
   • Enhanced cell uptake of glucose promotes glycogen formation and storage of cellular energy
   • Insulin inactivates liver phosphorylase the initial rate limiting step of glycogenolysis
   • Insulin enhances both glycolysis and glycogen formation

Question 7

Briefly describe the process and importance of oxidative phosphorylation in energy generation

Answer 7

• Only small amounts of ATP are generated by the breakdown of glygoen to pyruvate - 2 ATP molecules
• Similar to above, only 2 molecules of ATP are generated through the citric acid cycle
• However, during these processes, hydrogen atoms are released for oxidation
• Hydrogen atoms are split to form hydrogen ions and electrons
• The free electrons eventually combine with dissolved O₂ and 2 x H₂O to form 3 x OH^-
• OH^- Then combine to form H₂O

This process generates large quantities of ATP and occurs solely within the mitochondria

Question 8

Briefly describe the initial absorptive process for fats from the gastrointestinal tract

Answer 8

• Fats enter the gastrointesintal tract and are initially digested within bile micelles to become monoglycerides and free fatty acids
• The have highly charge exterior surfaces and are soluble in the chyme
• The micelles move to the brush border of the intestinal surface and are immediately dissolved into the cell through the cell membrane
  
  • The lipids are soluble within the cell membrane
  • This allows the bile micelles to continue digesting still more fat
• They are transported within the cell to the SER where they mainly form new triglycerides (3 x monoglycerides bound by one molecule of glycerol
  
  • These form into chylomicrons - triglycerides in the centre with some cholesterol surrounded by a phospholipid and apolipoprotein shell
• The chylomicrons once formed are secreted from the basolateral surface of the epithelial cell and taken up by the lymph

Question 9

Describe the process of chylomicron digestion to enable storage of fat within the body’s cells

Answer 9

• Lipoprotein lipase is expressed in many cells, especially adipose, skeletal muscle and cardiac cells
• This enzyme is transported to the surface of capillary endothelial cells
  
  • Hydrolyses the phospholipids and triglycerides within the chylomicrons as they contact the endothelial wall
• Hydrolysis of triglycerides releases free fatty acids and glycerol
• Fatty acids can dissolve in the cell membrane and diffuse across into the cytoplasm
  
  • These can then be used for energy generation or fat storage
• The fat storage cells synthesize glycerol which enables the conversion of free fatty acids back into triglycerides

Question 10

What is the purpose of triglyceride hydrolysis?

Describe the triggers for triglyceride hydrolysis

Answer 10

• Triglycerides are utilised throughout the body for the storage of energy
• When this energy is required in other parts of the body, such as during fasting, triglycerides must be released into the blood for transport
• Triglycerides are predominantly hydrolosed to free fatty acids and glycerol for transport to other parts of the body
• Reduced glucose availability to the storage cells will trigger hydrolysis
  
  • One of the glucose breakdown products is necessary for maintenance of the glycerol portion of triglycerides.
  • When this product (alpha-glycerophosphate) is low, the glycerol is hydrolysed
• Hormone sensitive lipase can also promote rapid hydrolysis

Question 11

Briefly describe the functions of the adipose tissue

Answer 11

1. Energy storage in the form of triglycerides
2. Heat / cold insulation
3. Hormone secretion
   
   • Leptin
   • Adiponectin
     
     • Both have an effect on appetite and energy expenditure

Question 12

Briefly describe the role of the liver in lipid metabolism

Answer 12

1. Degrade fatty acids into the small molecules that can be used for energy
   
   • acetyl-CoA
2. Synthesize triglycerides
   
   • Predominantly from carbohydrates
   • The liver is the primary source of unsaturated (or desaturated) triglycerids
     
     • Under the action of a dehydrogenase
3. Synthesize other lipids, mainly from fatty acids
   
   • cholesterol
   • phospholipid

Question 13

What is the role of carnitine in fatty acid oxidation

Answer 13

• Carnitine is present with the cytoplasm
• Carnitine acts as a carrier to transport fatty acids into the mitochondria from the cytosol
• Upon entry into the mitochondria, carnitine splits from the fatty acid
  
  • The fatty acid is then free to be degraded and oxidised

Question 14

Briefly describe the steps involved in beta oxidation.

Answer 14

1. Fatty acid is transported into the mitochondria in combination with carnitine
   
   • Carnitine is split away
2. The fatty acid combines with Coenzyme A and ATP to form fatty acyl-CoA
3. The second carbon from the CoA binds with oxygen
   
   • The beta carbon becomes oxidized
4. The 2 carbon portion of the fatty acid is released as acetyl CoA
5. Another CoA molecule binds to the new beta carbon and the process repeats
6. Acetyl CoA can enter the citric acid cycle for ATP and H⁺ generation

Question 15

The liver can produce acetyl CoA at a rate excessive to its metabolic demand. How is this additional acetyl CoA eventually utilised by other cells.

What are the intermediary products that are formed

Answer 15

• When the acetyl CoA is excessive to cellular needs, two molecules condense to form one molecule of acetoacetic acid
• Acetoacetic acid is released into the blood stream and can be transported throughout the body
• A part of the acetoacetic acid is converted to beta-hydroxybutyric acid and minute quantites are converted to acetone
  
  • Acetoacetate, beta-hydroxybutyric acid and acetone diffuse freely through the cell membrane
• These ketone bodies are produced readily and in large quantities
• However, as they are rapidly transported and very readily absorbed by the cells, the blood levels remain low

Question 16

Under what conditions does ketosis occur and in which circumstances will ketones become increased in the body

Answer 16

• Ketosis is the process by which ketones are produced during fatty acid metabolism
• Ketosis occurs when there are not enough carbohydrates or glucose within the body to provide adeuqate fuel for energy generation
• Ketones are produced after beta-oxidation of fatty acids
  
  • 2 molecules of acetyl-CoA combine to form acetoacetic acid
  • Acetoacetic acid is then partly converted to beta-hydroxybutyric acid and acetone
• Ketones can increase in the body whenever there is insufficient carbohydrate to provide energy
  
  • Starvation
  • Dietary manipulation - high fat / high protein, ultra-low carbohydrate
  • Diabetes mellitus - insulin deficiency leads to cellular depletion of glucose due to reduced GLUT facilitated transport

Question 17

Briefly describe the important steps in conversion of glucose to triglycerides (lipogenesis)

Answer 17

• Carbohydrates are converted into acetyl-CoA
• Acetyl CoA is converted to Malonyl CoA via a carboxylation reaction
• The malonyl-CoA undergoes esterification with additional carbon pairs added
• Once the fatty acids are ~14-18 carbons in length, they are transported to the SER where 3 FA are combined with one glycerol molecule by a dehydration reaction
• Glycerol is provided to the SER via glucose metabolism - production of alpha-glycerophosphate
  
  • Therefore, reduced glucose metabolism inhibits formation of triglycerides

Question 18

Note the major metabolic pathways that are interruptued in the absence of insulin

Answer 18

1. Reduced glucose transport into the cell
   
   • Reduced glucose metabolism into pyruvic acid
   • Reduced acetyl CoA to the citric acid cycle
   • Reduced alpha glycerophosphate - reduced glycerol for triglyceride formation
2. Increased proteolysis and catabolism
3. Increased beta-oxidation of triglycerides / fatty acids
   
   • Increased lipolysis
4. Reduced glycogen synthesis
   
   • Reduced activation of phosphofructokinase and glycogen synthase
5. Increased gluconeogenesis
6. Increases arterial smooth muscle tone

Question 19

Briefly explain the equilibrium between free fatty acids, triglycerides and the effect of changing glucose levels

Answer 19

Fats are present in adipose cells as two forms - triglycerides or free fatty acids

• When there is excess glucose available - increased alpha-glycerophosphate present (from glucose metabolism), more triglycerides are formed (fatty acids reduced)
  
  • This process effectively binds the fatty acids so they are less available to be used for energy generation
• Secondly, when glucose is available, acetyl CoA is rapidly produced
  
  • High quantities of acetyl CoA in conjunction with reduced free fatty acids promotes the formation of more fatty acids to balance the equilibrium
• Furthermore, the intermediaries of the citric acid cycle accelerate the acitivity of acetyl-CoA-carboxylase.
  
  • This enzyme catalyses the rate limiting step of carboxylation of acetyl-CoA into malonyl-CoA, the first step in fatty acid synthesis

Question 20

What is an adipokine?

List some of the known and important adipokines

Answer 20

• An adipokine is a cytokine secreted by adipose cells
• Over 100 biologically active adipokines have been discovered

1. Leptin
2. Adiponectin
3. Angiotensinogen
4. Apelin
5. Chemerin
6. IL-6
7. TNF-a
8. Plasminogen activator inhibitor
9. Visfatin
10. CRP

Question 21

What are the 2 major known actions of the adipokines

Answer 21

• Regulators of inflammation
  
  • Can be pro- or anti-inflammatory
• Affectors of energy balance by promoting insulin resistance

The adipokines can have local (apocrine) or systemic (endocrine) effects

Question 22

What are the roles of the adipokine hormone leptin?

Answer 22

1. Anti-obesity hormone
   
   • Leptin increases with increasing body condition
   • Binds to brain receptors (OB-R) and suppresses food intake, increases energy expenditure and influences food related behaviour
2. Stimulates angiogenesis
3. Inhibits apoptosis
4. Acts as a mitogen
5. Regulates immune and reproductive function
6. Modulates insulin sensitivity
7. Exerts an pro-inflammatory and pro-thrombotic effect
8. Inhibits aponectin

Question 23

Briefly describe the leptin levels in dogs and cats that are a good body condition or obese

How might those levels change after a meal?

Answer 23

• Most obese animals have higher serum leptin than those of thin animals
• Resistance to or lower sensitivity to leptin may be reduced or defective receptors or reduced tissue signalling
• In obese dogs, a study observed increased leptin levels and deficient leptin transport across the blood-brain barrier
• In dogs, leptin levels can double or triple after eating when compared to fasted dogs
• In cats, there is a mild increase after a meal only

Question 24

Briefly describe the proposed effects and regulation of adiponectin

Answer 24

• Adiponectin is most likely secreted by mature adipocytes
• Serum adiponectin is reduced in obese animals
  
  • Possibly due to inhibition by inflammatory cytokines
• Effects are target organ mediated
  
  • Increased insulin sensitivity
  • Lower serum glucose levels
  • Reduced liver and muscle triglyceride concentrations
    
    • Stimulates beta-oxidation and use of glucose together with reducing hepatic gluconeogenesis
  • Reduced inflammatory responses (anti-inflammatory)
  • Reduced atherosclerosis in humans

Question 25

Adipose is an important source of angiotensinogen

Briefly explain how increased angiotensinogen contributes to morbidity in obesity

Answer 25

1. Angiotensinogen is the precursor molecule to angiotensin
2. Angiotensins modulate and upregulate adipose tissue lipogenesis
3. Angiotensin can down-regulate lipolysis
4. They also play a role in adipose cell differentiation
5. Enhanced activation of the local tissue RAAS may play a role in local tissue inflammation
6. May allow insulin resistance to progress
7. May increase the production of leptin
8. May supress the production of adiponectin

Question 26

Describe the function, regulation and production of ghrelin

Answer 26

• Ghrelin is known as the hunger hormone
• Synthesized in the oxyntic glands within the gastric fundus
• It is the only known orexigenic (appetite stimulating) hormone
• It is produced after fasting, crosses the blood-brain barrier and binds to receptors to:
  
  • Stimulate appetite
  • Stimulate production of growth hormone
• Maximal levels are produced after overnight fasting and reduce ~1 hour after eating
• Ghrelin levels are decreases in obese animals and increased in anorexic animals
• Ghrelin decreases less after high fat ingestion versus high carbohydrate ingestion
• The decreas in ghrelin levels is also less in obese versus lean (people)

Question 27

Describe the function, regulation and production of cholecystokinin

Answer 27

• CCK is an anorexigenic hormone produced by the L-cells of the SI
  
  • Production is stimualted in response to food intake
• CCK stimulates pancreatic and biliary secretions, thereby promoting digestion
• Exerts a negative feedback effect on appetite centrally
• Effects a delay in gastric emptying
• CCK increases more rapidly and to a higher level in lean versus obese (people)
• Post-prandial levels are higher afer a high-fat meal
• CCK is important in regul;ating food intake - there is no celar link with obesity

Question 28

Describe the function, regulation and production of glucagon like peptide-1 (GLP-1)

Answer 28

• GLP-1 is produced by the intestinal L-cells, plus lesser levels from the pancreas and hypothalamus
• Secretion is stimulated by:
  
  • Food in the small intestine
  • Glucose and fatty acid concentrations
  • Vagus nerve stimulation
• Stimulates beta-cell synthesis of insulin
• Inhibits alpha-cell secretion of glucagon
• Similar to CCK
  
  • Suppresses appetite in the CNS and decreases gastric emptying
  • Levels are higher after a high fat meal
• Post-prandial production is reduced in obese people, but increased in dogs with obesity

Question 29

What is the major pathophysiological cause of inflammation in obesity

Answer 29

• Increasing, especially rapidly increasing numbers of adipocytes causes a relative reduction in vascularisation and reduced local oxygen tension
• This leads to increased secretion of angiogenic and cytokines, some of which are pro-inflammatory mediators
• Monocyte chemo-attractant protein-1 (MCP-1) production is enhanced leading to increased numbers of macrophages in the expanding adipose tissue
• Macrophages can produce pro-inflammatory mediators such as TNF-a and IL-6.
  
  • An imbalance between pro-inflammatory and anti-inflammatory macrophage subtypes may be present with obesity

Question 30

Describe the role of the major obesity induced pro-inflammatory mediators

Answer 30

• Increased pro-inflammatory macrophages associated with obesity produce increased levels of TNF-a, IL-6 and CRP

TNF-a:

• Produced by numerous other cells in conjunction with macrophages
• Anti-tumour activity
• Blocks insulin receptor activation leading to insulin resistance
• Also involved in systemic inflammatory responses, auto-immune disorders, septic shock and fever

Interleukins: (especially IL-6)

• Increased IL-6 during obesity is released into the portal circulation and stimulates hepatic triglyceride secretion
• This leads to a reduction in hepatic insulin sensitivity

CRP:

• CRP is generated in the liver in response to increased IL-6 and TNF-a
• CRP can heighten the inflammatory response

Question 31

List the various metabolic or mechanical conditions that have been associated with obesity

Answer 31

1. Osteoarthritis
2. Insulin resistance and type II diabetes
3. Dyslipidaemia
4. Cardiovascular conditions
5. Kidney disease
6. Neoplasia
7. Respiratory disease
8. Neurological consequences

Question 32

Describe the endocrine pathways of insulin resistance (or development of type II diabetes) in association with obesity

Answer 32

The following changes occur with obesity:

• Increased fatty acid concentrations
  
  • This activates protein kinases that can adversely effect insulin receptor function
• Increased adipose derived hormones including leptin, adiponectin, resistin etc.
  
  • Leptin can decrease insulin sensitivity, while adiponectin acts to increase sensitivity
  • Adiponectin production decreases in the obese
  • Resistin is increased in obesity and promotes the expression of the suppressor of cytokine signalling-3 (SOCS-3), a negative regulator of insulin sensitivity

Question 33

Describe the inflammatory pathways of insulin resistance (or development of type II diabetes) in association with obesity

Answer 33

• TNF-a and IL-6 levels are increased with obesity
• The Jun-terminal protein kinase 1 (JNK-1) is activated by TNF-a
  
  • This leads to a phosphorylation reaction that effectively weakens the action of insulin at the insulin receptor
  • JNK-1 leads to hyperglycaemia and hyperlipidemia by reducing Insulin repector substrate (IRS) and SOCS-3
• Retinol binding protein (RBP), produced int he liver and adipose cells increases with obesity
  
  • RBP inactivates GLUT-4 in adipose tissue

Question 34

List the various plasma lipoproteins

Note the major structural differences between the different lipoproteins

Answer 34

1. Chylomicrons
2. Very low density lipoprotein
3. Intermediate density lipoprotein
4. Low density lipoprotein
5. High density lipoprotein

Question 35

Note the major functional differences between the different lipoproteins

Answer 35

1. Chylomicron
   
   • ​Carry triglycerides from the gut to the liver
2. VLDL
   
   • Carry newly synthesized triglycerides from the liver to tissues
3. IDL
   
   • Intermediate between VLDL and LDL
4. LDL
   
   • Carry 3-6,000 fat molecules as phospholipids, cholesterol and triglycerides (and others)
5. HDL
   
   • Primarily collect fat from the body and transport it back to the liver
   • Produced within the liver as a complex of phospholipids and apolipoproteins
   • Initially cholesterol free, they scavenge cholesterol from the body

Question 36

Describe the major pathophysiological links between obesity and heart disease

Answer 36

• Obesity leads to an increase in effective circulatory volume and peripheral vascular resistance
  
  • This combination increases the incidence of hypertension
• There is subsequent eccentric hypertrophy or concentric hypertrophy depending on the combination of changes to ECV and PVR
• Abdominal obesity has been associated with diastolic dysfunction and reduced systolic dysfunction in dogs
  
  • Diastolic dysfunction can lead to portal congestion, portal vein thrombosis and myocardial hypoxia
• Note the apparent paradox that weight gain after the onset of heart failure has been associated with an improved survival
  
  • This may be more due to the lack of weight loss and cachexia as opposed to the actual weight gain

Question 37

Describe the pathophysiological association between obesity and renal disease

Answer 37

• Obesity leads to an increase in ECV, SNS activation, RAAS activation (production of angiotensinogen)
• Lipid metabolism may lead tp an increase in renal toxic metabolites
• Increased due to Na⁺ resorption - increased ECV and BP
  
  • Initially countered by an increased GFR and BP

With chronicity, the following can contribute to progressive renal damage:

• Renal vasodilation
• Renal hyperfiltration
• SNS and RAAS activation
• Metabolic disease
• Chronic inflammatory state

Leptin, which activates the SNS to exert its central function may also be important in raising BP in obese animals

Question 38

What are cachexia and sarcopenia?

List some of the more common disease processes known to contribute to cachexia

Answer 38

• Cachexia is the loss of lean body mass in association with disease
• Sarcopenia is the loss of lean body mass in the absence of a specific underlying disease

1. Neoplasia
2. Cardiac disease
3. Kidney disease
4. COPD

Question 39

Describe the four major aspects of cachexia pathophysiology

Answer 39

1. Increased energy utilisation
2. Decreased nutrient absorption
   
   • Altered GIT structure or function
3. Decreased nutrient intake
   
   • Medication side-effect
   • Dysregulation of the satiety signalling process
     
     • ghrelin, adiponectin, leptin, serotonin, insulin etc
     • eg. resistance to or reduced production of ghrelin or adiponectin
     • eg. increased sensitivity to the satiety signalling leptin
4. Altered metabolism
   
   • ​Essentially an imbalance between protein synthesis and catabolism
   • May be influenced by inflammatory mediators
     
     • Especially increases in TNF-a
   • Increasing metabolic demand such as in sepsis or with SNS activation as seen with CHF

Question 40

Briefly describe the major inflammatory mediators of cachexia

Answer 40

• Increased inflammatory cytokine production is a major cause of cachexia
• TNF-a, IL-1 and IL-6 are primary contributors to cachexia as they all:
  
  • Reduce appetite
  • Increase basal metabolic rate
  • Accelerate loss of lean body mass
• The activity of these cytokines is primarily mediated by activation of Nucelar Factor kappa-B (NF-kappa-B)
  
  • Which activates the ubiquitin proteosome pathway and leads to muscle cell atrophy and decreased muscle regeneration

Question 41

What is myostatin?

How are myostatin concentrations altered in chronic inflammatory disease and what is the effect?

Answer 41

• Myostatin is a member of the transforming growth factor beta superfamily
  
  • Negatively regulates muscle mass
• Exercise decreases myostatin levels, whereas chronic inflammation can increase myostatin
• TNF-a and angiotensin II can increase myostatin levels in CHF, augmenting the effects of cachexia

Question 42

In addition to the inflammatory cytokines, which hormones are known to contribute to cachexia in CHF

Answer 42

• Catcholamines
  
  • Activation of the SNS
• RAAS activation
• Cortisol
• Atrial natriuretic peptide
• Brain type natriuretic peptide

Question 43

Briefly describe the absorption and distribution of amino acids within the body

Answer 43

• Amino acids are absorbed directly from the gastrointestinal tract (SI)
  
  • Ingested protein is digested by proteases such as trypsin and absorption across the luminal epithelial membrane occurs via facilitated or active transport
• The amino acids are in part stores within the epithelial cells and then transported to the liver via the portal veins. As protein digestion and absorption is slow, there is minimal increase in systemic blood amino acids following a protein meal
• The liver cells take up amino acids and produce proteins (polymerised amino acids - via peptide linkages) in the RER.
  
  • These proteins can be rapidly digested by cellular lysosomes to free amino acids
• Most cells produce their own proteins from amino acids absorbed directly from the blood

Question 44

Briefly note the importance of the liver in maintaining protein and amino acid equilibrium within the body

Answer 44

• Amino acids are supplied directly to the liver following protein digestion and absorption in the gut
• Amino acids are readily transported into the liver cells
• The liver cells rapidly synthesize proteins from the available amino acids
  
  • These proteins in part serve as a storage pool for the amino acids
• The liver cells can rapidly degrade the cellular proteins (fusion with lysosomes) to provide amino acids to the circulation
• Amino acids are maintained at a fairly constant level in the systemic circulation

Question 45

Describe the potential fate of amino acids after they enter the body

Answer 45

• The majority of amino acids are utilised by the body’s cells to produce proteins
• Each cell has an upper limit for protein storage and excess amino acids can be converted and used for energy generation
  
  • Deamination is the initial step for protein amino acid conversion and occurs almost entirely within the liver
  • Deamination converts the amino acids to keto-acids
  • This process generates ammonia as a waste product which is excreted primarily in the form of urea
• Following deamination, the keto acids are converted to substances such as pyruvic acid or acetyl CoA
  
  • This can then be:
    
    • used for energy generation in the citric acid cycle
    • Stored as triglycerides
    • Stored as glycogen

Question 46

Note the major hormones involved in protein regulation

Note each hormones basic mechanism of action on protein metabolism

Answer 46

1. Growth hormone
   
   • Increased tissue protein quantity
     
     • Likely due to increased amino acid transport and acceleration of DNA and RNA transcription and translation in the ribosomes
2. Insulin
   
   • Increases protein synthesis by increasing amino acid transmembrane transport
   • Ensures adequate glucose within cells for energy generation, reducing protein degradation
3. Glucocorticoids
   
   • Decrease protein content in most tissues by increasing the rate of breakdown
   • Increase plasma amino acid concentrations
   • Increases liver and plasma proteins
4. Testosterone
   
   • Encourages increased protein deposition especially in the contractile proteins of muscle cells
   • Unlike GH, there is a limit to the increased protein content that can be caused by testosterone
5. Estrogen
   
   • Similar to testosterone but less effective
6. Thyroxine
   
   • Increases the basal metabolic rate
   • Leads to increased and rapid protein degradation in the absence of adequate glucose
   • Leads to rapid growth and protein synthesis as long as there is adequate other energy sources
     
     • eg. thyroxine deficiency during the growth phase causes marked stunted growth

Question 47

List the different factors that contribute to heat production in the body

Answer 47

Heat production is determined by the metabolic rate

1. Basal metabolic rate
2. Extra metabolism caused by muscle activity
3. Extra metabolism caused by the effect of thyroxine
   
   • and other hormones including GH and testorsterone
4. Extra metabolism caused by sympathetic activity, NE and epi
5. Extra metabolism caused by increased chemical activity
6. Extra metabolism needed for digestion, absorption and food storage

Question 48

Describe the various physiological mechansims by which heat is lost from the skin

Answer 48

1. Radiation - loss via infrared heat rays
2. Conduction
   
   • Energy is transferred to the air via conduction of kinetic energy in the form of vibrations
   • Once the surrounding air reaches body temperature, conduction from the body ceases unless convection occurs
3. Convection
   
   • Air movement removes the heat loss to the air via conduction
4. Heat loss to water
   
   • The ability of water to absorb heat is many thousand times greater that air. No insulation can occur and heat loss is much greater
5. Evaporation
   
   • Heat loss via sweat is in addition to the insensible lossed from the skin and lungs

Question 49

Describe the mechanism of panting in animals and why it is necessary

Answer 49

• Panting is a necessary thermoregulatory system in dogs (and cats) as they have fur that minimises heat loss via conduction and convection and they have no sweat glands in the skin preventing significant evaporation
• Panting si triggered by the thermoregulator centre in the hypothalamus
• Panting is controlled by the panting centre in the pneumotaxic respiratory centre in the pons
• Air rapidly passes over the respiratory secretions, especially the saliva on the tongue and oral mucosal surfaces
• Alveolar minute ventilation is not significantly altered as the breaths are very shallow and mainly involves movement of the dead space air

Question 50

Describe the role of the hypothalamus in maintenance of a normal body temperature

Answer 50

• Heat sensitive neurons present in the anterior-preoptic hypothalamus (and ~1/3 as many cold-sensitive neurons)
• These neurons increase their firing rate with an increase or decrease of body temperature respectively
• Temperature receptors in the skin are much more adept at sensing cold versus warmth
  
  • Similarly, the temperature sensors in the deeper tissues detect mainly cold rather than warmth
• The peripheral and anterior hypothalamic signals are transmitted to the posterior hypothalamus
• Heat and cold signals from the body and centrally are combined and integrated in the posterior hypothalamus to effect heat preserving or heat producing reactions of the body

Question 51

Describe the major mechanisms for reducing the body temperature

Answer 51

• Vasodilation of the skin blood vessels
  
  • Increase blood flow to the skin increases the rate of heat conduction away from the body
  • Caused by inhibition of the sympathetic centres in the posterior hypothalamus
• Sweating
  
  • Less important in animals
• Panting
  
  • Increased convection of air across moist mucosal surfaces of the oral cavity and tongue
• Decrease heat production
  
  • Minimise chemical thermogenesis and activities such as shivering

Question 52

Describe the mechanisms by which the body can increase temperature in response to cold

Answer 52

• Vasoconstriction of the skin blood vessels
  
  • Mediated by sympathetic stimulation of the posterior hypothalamus
• Piloerection
  
  • Sympathetic stimulation of the arrector pili muscles
  • These muscles attach to the hair follicles and cause the hairs to stand upright
  • This increases air trapping around the body and reduces conductive and convective heat loss
• Increased thermogenesis
  
  • Shivering causes heat production in the muscles
  • Heat production by sympathetic stimulation
  • Thyroxine stimulation - increases metabolic activity

Question 53

Describe the physiological process of shivering

Answer 53

• Shivering is triggered by the posterior hypothalamus
• The shivering centre is triggered when heat signals from the anterior hypothalamus reduce and cold signals from the periphery increase
• A signal is sent from the shivering centre through bilateral tracts of the brain stem and lateral spinal cord and eventually to the anterior motor neurons and skeletal muscles
• The signals and non-rhythmic and cause an increase in skeletal muscle tone
• When the tone rises above a criticl level, shivering begins due to feedback oscillation of the muscle spindle strethc reflex mechanism
• Shivering can increase heat procution by four to five times normal

Question 54

Define fever

What are the major causes of fever?

Answer 54

• Fever refers to a body temperature that is above the usual range of normal
• Exercise
• Environmental / heat stroke
• Exogenous pyrogens - bacterial infection
• Brain lesions

Question 55

What is a pyrogen?

Explain the mechanism of action of pyrogens in causing a fever

Answer 55

• A pyrogen is any substance that can cause a rise in the set-point of the temperature control centre in the hypothalamus
• Some pyrogens act directly on the hypothalamus, however, most act through the production of cytokines in concert with the immune system
• Bacteria or bacteria breakdown products are ingested by macrophages or LGLs (killer T cells) which subsequently produce cytokines in response - as for the innate and adaptive immune systems
• The cytokines, especially IL-1, circulate through the body fluids
  
  • IL-a can effect a change to the hypothalamic set-point within 8-10 minutes
• IL-1 causes fever first by stimulating production of PGE₂ or other prostaglandin
  
  • The protaglandin acts on the hypothalamus to elicit the fever reaction

Question 56

Define heat stroke

Answer 56

• Heat stroke is a severe elevation in body temperature from 40.5 C to 43.0 C after an animal has been exposed to elevated ambient temperatures or performed strenuous exercise

Question 57

Briefly outline the major pathophysiological changes that can occur during heat stroke

Answer 57

• Initially, panting is triggered in the panting centre (within the respiratory centre in the pons).
  
  • This reflex starts the process of heat dissipation as long as the relative humidity is not too high
• As the body temperature rises, metabolic rate also rises, compounding the heat gain if dissipation cannot occur
  
  • Behavioural mechanisms such as seeking shade or cool surfaces help minimise heat gain
• With continuation, metabolic acidosis can occur
• • Peripheral vasodilation occurs
    
    • To help maintain blood pressure, splanchnic constriction occurs (SNS mediated)
• Increased SNS activation and circulating catecholamines lead to an increased heart rate and cardiac output
• With progression and development of metabolic acidosis, cardiac output will reduce and perfusion especially to vital organs will reduce
• Likely due to over-production of nitric oxide, splanchnic vasodilation eventually occurs
  
  • This leads to blood pooling, reduced venous return and a marked drop in cardiac output
• This triggers the initial signs of shock with decreased perfusion, intestinal ischaemia, hypoxia and endothelial cell damange - leads to increased vascular permeability
• Direct hyperthermia can also trigger inflammatory, hemostatic and tissue damage processes
  
  • SIRS which can progress to MODS
  • Coagulation defects such as DIC
  • Ischaemic necrosis - further precipitates the above

Question 58

List the potential major pathophysiological complications following a severe bout of heat stroke

Answer 58

• SIRS
• DIC
• ARDS
• Rhabdomyolysis
• Acute kidney injury
• Hepatic damage
• Neurological damage
• Acute pancreatitis

Haemodynamic deterioration and pulmonary lesions are the major causes of death

Question 59

Briefly describe the clinical neurological abnormalities seen in dogs with heat stroke.

What are the underlying pathophysiological mechanisms for the neurological abnormalities?

Answer 59

• Neurological abnormalities are commonly seen with moderate to severe heat stroke
  
  • Altered consciousness or disorientation
  • Stupor
  • Seizures
  • Coma
• In pathological studies of fatal heat stroke the following findings have been reported:
  
  • cerebral oedema
  • haemorrhage
  • hyperemia
  • neuronal necrosis
• Direct thermal injury may play a minor role in the clinical signs
• More likely, the abnormalities are caused by shock and cerebral hypoperfusion / cerebral hypoxia
• MODS and metabolic derangements such as metabolic acidosis and respiratory alkalosis, hypoglycemia and haematological/coagulation abnormalities including DIC

Question 60

Describe the potential haematological derrangements that can be seen secondary to moderate and severe heat stroke

Answer 60

• Initial sympathetic drive causes widespread superficial vasodilatation and reduction to peripheral vascular resistance
• Diffuse thermal injury to the vascular endothelium triggers many aspects of the coagulation cascade
  
  • Vascular endothelial injury exposes tissue factor
  • TF combines with F VII to activate the TF (extrinsic) pathway
  • Kallikrein and Factor XII are also activated leading to activation of the contact activated (intrinsic) pathway
• Diffuse multi-organ themal or hypoxia induced necrosis also stimulates the coagulation cascade
• Direct hepatocellular injury may further precipitate coagulation abnormalities as both clotting factors and anti-thrombotic proteins are reduced
• Elevated temperatures enhances platelet activation, coagulation and fibrinolysis
  
  • This combination leads to increased utilisation of platelets and coagulation factors - the initiating process for DIC development
• Note: increase in PT, aPTT, and reduced protein C together with hypofibrinogenemia at 12-24 hours post presentation was significantly associated with death.

Question 61

Describe the pathophysiological mechanisms for the development of acute kidney injury in moderate to severe heat stroke

Answer 61

• At presentation, renal function issues may be difficult to detect
• The mechanism of renal injury is likely multifactorial
  
  • hypoperfusion - distributive shock and dehydration
  • direct thermal injury
  • endotoxemia
  • myoglobinemia
  • release of cytokines and vasoactive substances
  • vascular injury due to microthrombi, especially with DIC

Question 62

Briefly comment on the availability of biomarkers to detect early AKI in dogs

Answer 62

• Early AKI is difficult to detect as injury and reduced GFR occur prior to the onset of elevated serum creatinine
• Early detection may be helpful in cases where AKI might be anticipated and contribute to mortality if not appropriately addresses early in the course of a disease
  
  • Eg. shock, heat stroke, any cause of DIC, sepsis etc.
  • AKI is invariably present in dogs with heat stroke

Biomarkers: all with creatinine ratios

• urine neutrophil gelatinase associated lipocalin
• urine retinal binding protein
• urine protein creatinines ratio - median of 4.8 at the time of presentation
• Fractional excretion of sodium is invariably reduced
• GFR reduced
• Serum creatinine normal to mild increases early in disease

Question 63

Describe the pathophysiological consequences to the gastrointestinal tract following moderate to severe heat stroke together with the more widespread implications

Answer 63

• The primary inciting cause for GIT injury following heat stroke is blood flow re-distribution
  
  • Cutaneous vasodilatation is combined with visceral vasoconstriction
  • The high metabolic activity of gastrointestinal epithelial cells makes them particularly susceptible to periods of reduced blood flow, hypoxia and ischemia
• Splanchnic vasoconstriction and local hyperthermia stimulates production of nitric oxide with resultant vasodilatation
  
  • Splanchnic vasodilatation in this manner occurs immediately before vascular collaspe as it is associated with a marked reduction in venous return due to blood pooling
• Epithelial damage leads to impaired mucosal barrier function and increased permeability
  
  • Bacteria and endotoxin translocation
  • endotoxin can worsen SIRS and can potentially lead to sepsis, MODS and death