Fate of glucose Flashcards
Describe the change in glucose in the fasting state
~2-4 hrs after a meal blood glucose levels return to basal levels and continue to decrease until the next meal
Insulin levels decline and glucagon levels rise which triggers release of fuels from the body stored
Describe how glucose is maintained in the fasting state
Liver produces glucose by glycogenolysis and by gluconeogenesis
What occurs in starvation?
animal has a prolonged period of low glucose levels
lack of energy sources to maintain vital cells, especially nervous system tissues which can only use glucose as a substrate => coma + death
What enzyme breaks down carbs in the mouth and SI?
amylase
What is the effect of high blood glucose?
detected by Islets of Langerhans in the pancreas
beta cells release insulin
Insulin causes tissues to store glucose as either glycogen or fats to return blood glucose to normal levels
What is the effect of low blood glucose?
Detected by islets of langerhans in pancreas
alpha cells release glucagon => mobilises glucose stores => returns blood glucose levels to normal
What happens to glucose when they enter hepatocytes?
- can be oxidised and be used as energy source for hepatocyte
- converted to glycogen and stored (influenced by insulin)
- converted to TAG and exported out the liver
what happens if TAGs get stored in the liver?
accumulation of fat in liver => prevents normal function => disease
e.g., fatty liver in cows, hepatic lipidosis in cats
How does the liver secrete TAGs?
packages them with proteins, phospholipids and cholesterol into lipoproteins - VLDL
VLDLs secreted into blood streams
What is the function of VLDLs (very low density lipoproteins)
Secreted in bloodstream from liver
some fatty acids from VLDL are taken up for immediate energy needs
Most are stored in adipose tissue as TAGs
Clinical relevance: glucose and the CNS
Brain and neural tissues can only use glucose as substrate for energy
Glucose is major precurse of neurotransmitters
Clinical signs of low blood glucose are usually neurological
How does the brain and other neural tissues use glucose for energy?
oxidize glucose via cellular respiration to generate ATP
What is the lactate equation?
Lactate produces in anaerobic respiration
Lactate + NAD+ <=> pyruvate + NADH + H+
Describe the cori cycle?
Lactate released from cells undergoing anaerobic glycolysis (usually muscle cells when O2 conc is low) taken up by liver and oxidised back to pyruvate
Pyruvate use to synthesise glucose (gluconeogenesis) which is returned to blood
what is the cori cycle?
Cycling of lactate and glucose between peripheral tissues and the liver
What is gluconeogenesis?
metabolic pathway that results in the generation of glucose from other products e.g., AAs, glycerol, fatty acids and lactate
Where do the substrates used in gluconeogenesis come from?
Glycerol - from lipolysis of adipose TAGs
AAs - from breakdown of protein
Lactate - from anaerobic glycolysis in RBC cytosol
Describe anaerobic glycoysis in the RBC
Glucose is only fuel for RBCs as they lack mitochondria
Glucose used to generate ATP in cytolosol via anaerobic glycolysis
Pyruvate form is converted to lactate and released into blood
Describe the fate of glucose in muscles
Muscles can use glucose from blood or from their own glycogen stores
Convert glucose into lactate through glycolysis or oxidising it to CO2 and H2O
Describe the fate of glucose in adipose tissues
Adipocytes oxidise glucose for energy
Adipocytes metabolise gluose to Acetyl CoA which can be turned into fat and stored
what is function and action of epinephrine on glucose?
Mobilised fuels during acute stress
Stimulates glucose production from glycogen
Stimulates fatty acid release from adipose tissue
What is the function and action of cortisol on glucose?
Provides for changing requirements over long-term
Stimulates AA mobilisation for muscle protein
Stimulates gluconeogenesis
Stimulates fatty acid release from adipose tissue
where does the body get energy from in the starved state?
Body limits amount of AAs used for gluconeogenesis in order to preserve muscle mass
Relies of ketones as source of energy
As there is limited glucose, the brain must rely on using intermediate energy source ketone bodies
=> elevated levels of ketone body in blood = ketosis
what is glucose sparing?
During starvation the brain uses ketone bodies as energy, therefore it needs less glucose
Glucose is still required for use by RBCs
Less glucose is used by the body to keep some ‘spare’
Liver needs to produce less glucose per hour during prolonged fasting than short periods
How is protein spared during starvation?
decreased rate of gluconeogenesis
Reserves proteins for essential functions such as biosynthetic function and new protein synthesis
Desribe the role of adipose tissue in starvation
Continues to bread down TAG stores to produce fatty acids and glycerol for energy
In liver fatty acids are converted to ketone bodies which are oxidised for energy
How does ketonemia occur?
The rate of ketone body production exceeds rate at which they can be used
Why is ketosis common in dairy cattle?
peak lactation has a high energy demand and cow is incapable of eating enough to support this
Body starts mobilising fats to provides energy => ketone production
How does the body adapt to the starvation state?
The breakdown of protein becomes tissue or protein specific to guard against vital proteins/cells becoming depleted
This occurs due to elevated levels of cortisol
Why do animals die of starvation?
Proteins become so depleted that heart, kidney etc stop functioning
Animal can develop infection and do not have adequate reserves to mount immune response
Deprived of vitamin and mineral precursors of coenzymes and other compounds necessary for tissue function
Lack of ATP and decreased electrolyte intake, electrolyte composition of cells or blood becomes incompatible with life
