Question

Briefly explain the biphasic release of insulin into circulation. Include in your answer a description of the first and second phases.

Answer 1

Pancreas is both exocrine and endocrine gland.

Exocrine part produces enzymes for the digestion while the endocrine part secretes hormones.

Endocrine part consists of A(alpha) cells which secretes glucagon, B(beta) cells which secretes insulin and amylin, delta cells which secretes somatostatin and F cells

Insulin is a hormone released from the beta cells of islets of langerhans in the pancreas.

Rapid and sustained stimulation of beta cells with glucose induces biphasic insulin secretion whch cosists of a transient first phase and sustained second phase.

Important steps in the secretion of insulin from beta cell includes:

• Glucose enters the beta cell from the blood mainly via GLUT-2 receptors.
• Glucose undergoes glycolysis and TCA cycle inside beta cell resulting in ATP production.
• It causes closure of ATP sensitive potassium channels
• K+ ions cannot leave the cell and it causes depolarisation of the cell.
• As a result voltage gated calcium channels are opened.
• Ca2+ enters the cell
• It triggers insulin exocytosis

Several other factors like free fatty acids, glucagon like peptide-1(GLP1), Gastric inhibitory peptide(GIP), vasoactive intestinal polypeptide(VIP) also stimulate insulin secretion.

First phase

There will be rapid release of preformed insulin from the storage granules of beta cell following a meal. This is called first phase. It promotes peripheral utilization of glucose, suppression of hepatic glucose prduction an dlimits postprandial glucose elevation. It begins within 2 minutes of food intake and lasts for about 10-15 minutes.

Second phase

It is a prolonged phase of insulin secretion by pancreas when glucose enters the bloodstream which lasts for more than an hour.