Endocrinology 4 - Insulin Secretion and Intermediary Metabolism

Question 1

Explain why glucose levels must be tightly controlled.

Answer 1

• Glucose is very important as a substrate, especially in the brain, which almost completely relies on glucose.
• Low blood glucose concentration (hypoglycaemia) causes the brains function to become impaired

Question 2

What happens if the blood glucose concentration is less than 2mM?

Answer 2

Hypoglycaemia - unconsciousness, coma and ultimately death.

Question 3

Briefly describe the structure of the pancreas.

Answer 3

• 98% is associated with exocrine secretions to the small intestine
• 2% is made up of small clumps of cells called the islets of Langerhans
• Islets of langerhans are joined by gap junctions, allowing passage of small molecules, and tight junctions which form intracellular spaces.

Question 4

Describe the three cell types in the islets of langerhans, and their function.

Answer 4

• alpha cells secrete glucagon
• Beta cells secrete insulin
• Delta cells secrete somatostatin

Question 5

Explain how insulin, somatostatin and glucagon affect blood glucose leves and growth and development.

Answer 5

• Insulin stimulates growth and decreases blood glucose levels
• Somatostatin inhibits everything - insulin/glucagon production as well as growth
• Glucagon increases blood glucose.

Question 6

Which factors affect insulin production?

Answer 6

• High blood glucose
• High amino acids
• Some gastrointestinal hormones
• Glucagon
• Sympathetic activity (a receptors) inhibits insulin, parasympathetic activity (beta receptors) increases insulin.

Question 7

What does insulin cause?

Answer 7

• Increased glycogenesis
• Increased glycolysis
• Increased glucose transport into cells via GLUT4

Question 8

Which factors affect glucagon production?

Answer 8

• Decreased blood glucose
• Amino acids
• Gastrointestinal hormones
• Both parasympathetic and sympathetic activity
• Insulin and somatostatin are inhibitory

Question 9

What does glucagon cause?

Answer 9

• Increased lipolysis
• Increased amino acid transport into the liver
• Increased gluconeogenesis
• Increased glycogenolusis

Question 10

Describe the process of insulin secretion

Answer 10

• Glucose enters the beta cell via the GLUT 2 transporter.
• It is then converted to glucose 6-phosphate by glucokinase (this is the rate limiting step)
• ATP is produced in metabolic reactions, which opens ATP sensitive K+ channels.
• This causes Ca+ channels to open, which stimulates insulin release.

Question 11

What is produced from proinsulin?

Answer 11

Insulin and c-peptide in equal amounts

Question 12

Describe the role of glucagon like peptide-1.

Answer 12

• Produced in response to nutrients in the gut, it stimulates insulin production and suppresses glucagon
• Increases satiety (feeling of fulness)
• Rapidly degraded

Question 13

Describe the structure of the insulin receptor

Answer 13

• There are 2 alpha subunits
• There are 2 beta subunits, which are transmembrane and communicate with the inside of the cell
• B subunits have tyrosine kinase domains