Glucose homeostasis Flashcards
What is the only energy source used by the brain?
Glucose is the only source of energy used by the brain.
The brain needs around 120g of glucose per day
Discuss glycogen as an energy store?
- Stores glucose as glycogen in the liver and skeletal muscle
- Easily available to be broken down to glucose when needed
- Limited size- can only store about 190g- wouldn’t even last a day
Discuss fat as an energy store?
- The most efficient store
- Fat is stored as triglycerides in adipose tissue
- Unlike glycogen, we can store as much as we want- therefore any excess food eaten caused weight gain
Discuss protein as an energy store?
- Proteins stored in skeletal muscle and can be used in the brain if really needed
- Amino acids ca be converted to glucose via gluconeogenesis in the liver
What is glycogenesis?
Synthesis of glycogen from glucose
What is glycogenolysis?
Breakdown of glycogen into glucose
What is lipogenesis?
Synthesis of fats
What is lipolysis?
The breakdown of fats
What is gluconeogenesis?
The formation of glucose from amino acids
Why is insulin the most important hormone in blood glucose regulation?
Because it is the only hormone that can decrease blood glucose
Where is insulin secreted from?
Beta cells in the islet of langerhans in the pancreas into the hepatic portal vein leading to the liver
Where is glucagon secreted from?
Alpha cells in the islet of Langerhans in the pancreas
What is the structure and formation of insulin?
Two polypeptide chains ( an alpha and beta chain) joined by disulphide bridges
alpha- 21 aa
beta- 30 aa
Synthesised in ribosomes on the rough endoplasmic reticulum
The chains are three-folded and joined by disulphide bridges
The C-protein is then removed which is important as it has a longer half-life than insulin so is a good measure of insulin secretion.
What receptors does insulin bind to?
Act at tyrosine kinase receptors
How is insulin secretion controlled and what is the mechanism of secretion?
Via a negative-feedback loop
- When food is eaten, the blood glucose increases
- Glucose diffuses into the pancreatic beta cells via the GLUT-2 transporter
- An increased glucose concentration inside the cell, causes the secretion of insulin
- The release of insulin causes a decrease in blood glucose
- This leads to a negative feedback loop, as a decrease in blood glucose causes the decrease in insulin secretion
Mechanism of secretion:
- An increase in glucose after eating, causes glucose to enter the cell via a GLUT-2 receptor via facilitated diffusion- down it’s concentration gradient
- once inside the cell, the glucose is broken down by glycolysis, krebbs cycle, citric acid cycle etc
- This leads to an increase in ATP
- ATP binds to the ATP-gated potassium channels causing them to be closed
- This means less K+ ions can leave the cell, causing a positive charge inside the cell.
- This causes depolarisation of the membrane
- This causes the opening of the voltage-gated Ca2+ ion channels casing an influx of calcium ions into the cell. Also, will lead to calcium induced calcium released from the endoplasmic reticulum
- If there is an increase in calcium ions, it increases exocytosis of secretory granuls containing insulin leading to insulin release
- Additionally, insulin release can be moderated by:
- leucine- if high protein, can be converted to amino acids which can be metabolised to ATP
- Parasympathetic ns- Acetylcholine acts at muscarinic receptors = Phospholipase C- PIP2- IP3- increases ca2+ release
- Sympathetic ns- Noradrenaline and adrenaline inhibits adenyl cyclase- less cAMP production
- GLP-1- activates g alpha s, increases the activity of adenyl cyclase, increase cAMP production via PKA and EPAC- increase Ca2+ induced exocytosis
