Insulin W3

Question 1

Insulin

Answer 1

• Insulin is the main hormone involved in glucose metabolism. Also important in control of supply storage of fats and protein
• Insulin is a peptide hormone. It consists of 2 chains – α and β - linked by S-S bonds.
• Acts at tyrosine kinase receptors
• Secreted from β-cells in the Islets of Langerhans in the endocrine pancreas (β-cells are the most abundant cell type in the Islets) into the hepatic portal vein

Question 2

Insulin synthesis

Answer 2

synthesized as a pro- peptide by ribosomes on the RER
• Processing involves:
– Folding and formation of disulphide bonds
– Removal of the C peptide (31αα) to give 2 chains (A - 21αα and B
30αα) linked by disulphide bonds
• C peptide as well as insulin is secreted (Plasma C peptide level is a measure of insulin secretion as, unlike insulin, it is not cleared rapidly from the blood)
• It is stored until secretion is stimulated

Question 3

Control of insulin secretion - the major factor

Answer 3

major control factor for insulin secretion is level of blood glucose – a simple negative feedback pathway
– After a meal glucose concentration increases
– Glucose diffuses into the pancreatic β-cell (via GLUT-2)
– Increased glucose concentration in the cell causes secretion of
insulin
– Insulin causes a decrease in blood glucose
– Decreased glucose in the β-cell decreases insulin secretion

Question 4

Control of insulin secretion - other factors influencing secretion

Answer 4

Other factors can influence secretion including:
– Some amino acids (leucine, arginine)
– Autonomic nervous system (parasympathetic increases;
sympathetic decreases)
– Some gut hormones (incretins) - secreted in the GI tract in
response to food eg. GLP1

Question 5

Actions of insulin

Answer 5

• secreted following a meal due to increased BG
• acts at tyrosine kinease coupled receptors
• acts to decrease BG (and fatty acids and aminos)
• decreases BG by increasing the uptake of into cells and utilisation of glucose
• promotes storage of fuel mols

Question 6

Major target organs

Answer 6

Liver

Skeletal muscles

Adipose tissue

Question 7

Liver and insulin

Answer 7

insulin is secreted into the hepatic portal vein, transporting it directly to the liver. Liver removes 60% of the insulin before it enters the systemic
circulation

Question 8

Actions of Insulin – Glucose Homeostasis

Answer 8

Insulin lowers blood glucose by:
– increase glucose uptake into cells (skeletal muscle and adipose tissue)
– increase glycogenesis in the liver and skeletal
muscle
– decrease glycogenolysis
– decrease gluconeogenesis
– increase glycolysis

Question 9

Increased uptake of glucose is mediated by….

Answer 9

the insulin-dependant facilitated glucose h transporter, GLUT-4

Question 10

What does glucose require t move through the plasma membrane

Answer 10

Aa transporter

Question 11

What dos GLUT transporters allow

Answer 11

Facilitated diffusion of glucose down its conc grad

Question 12

What does GLUT-4 do in the presence of insulin

Answer 12

Moves from inside the cell to the cell membrane and therefore increases the uptake of glucose

Question 13

Where is GLUT-4 abundant

Answer 13

Skeletal muscle and adipose tissue

Question 14

Where is uptake by GLUT-2

Answer 14

Liver

Question 15

GLUT-2

Answer 15

Not insulin dependant

Question 16

Action of insulin in fats and proteins

Answer 16

Lowers blood fatty acids and increases triglyceride synthesis

• increases fatty acid uptake into adipose tissues
• increases uptake of glucose into adipose where it is used for de novo synthesis of fatty acids and glycerides
• decreases lipolysis

Question 17

Action of insulin in fats and proteins - after meal

Answer 17

Lowers blood amino acids and increases protein synthesis
- increases uptake of amino acids into cells
- increase protein synthesis
- decrease protein degradation