INSULIN-1 Physiology Lectures Slides

Pancreas

• Pancreas secretes many hormones which are:

1. Insulin
2. Glucagon
3. Amylin,
4. Somatostatin, and
5. Pancreatic polypeptide

The islets of Langerhans, which secrete insulin and glucagon directly into the blood.

Increased Blood Glucose Stimulates Insulin Secretion

• At fasting level of blood glucose of 80 to
  90 mg/100 ml–the rate of insulin
  secretion is minimal.
• If the blood glucose concentration
  increased to a level two to three times
  normal and insulin secretion increases
  markedly.
• Release of preformed insulin from beta
  cells in 1st 2-3 minutes after increase in glucose concentration.

• However, in another 5-10 minutes, the
  insulin concentration decreases about
  halfway back toward normal.
• In next 2-3 hrs, insulin secretion rises a
  second time and reaches a new plateau
  because of synthesis of new insulin.

• Furthermore, the turn-off of insulin secretion is almost equally as rapid, occurring within 3 to 5 minutes after reduction in blood glucose concentration back to the fasting level.

Factors That increase or decrease insulin secretion

1.Increase glucose uptake into muscles:

• Under resting conditions, resting muscle membrane is slightly impermeable to glucose → muscle use fatty acids for energy, not glucose.
• Between meals, small insulin is released →small quantity of glucose enter
  into the cell.
• Moderate/Heavy exercise →increase muscle contraction →increase translocation of Glucose transporter 4 (GLUT 4) to cell membrane
  →Glucose diffuse into the cell(But again, this process does not require
  large amount of insulin.
• Few hours after meal →↑↑ blood glucose →increase release of insulin
  →rapid transport of glucose into muscle cells →now muscles use glucose
  not fatty acids.

2.Increase glucose storage into muscles:

• Extra glucose that is not used by muscle →converted into Glycogen(Glycogenesis) for later use of energy provision.

3.Increase glucose uptake into liver:

• Insulin increase the uptake of glucose from the blood by the liver
  cells.
• When glucose diffuses into the liver cells → enzyme glucokinase
  causes phosphorylation of glucose (phosphorylated glucose become
  trapped now as now it cannot diffuse back through the cell
  membrane).

4.Increase glucose storage into liver:

• Insulin cause conversion of glucose into glycogen by increase the activity of enzyme glycogen synthase.

5.Inhibit breakdown of Glycogen (Glycogenolysis)

• Insulin inactivates liver phosphorylase, the principal enzyme that causes liver glycogen to split into glucose.

6.Inhibit Gluconeogenesis

• Insulin also inhibits gluconeogenesis.
• It does this mainly by decreasing the quantities and activities of the liver enzymes required for glucose synthesis.

7.Increase conversion of Glucose into Fatty Acids

• When the quantity of glucose entering the liver cells is more than can be stored as glycogen or can be used for local hepatocyte metabolism, insulin promotes the conversion of all this excess glucose
  into fatty acids.

Something important to remember!!!

• The BRAIN CELLS are also quite different from most other cells of the body in that they normally use ONLY GLUCOSE for energy
  and can use other energy substrates, such as fats, only with difficulty.
• Therefore, it is essential that the blood glucose level always be maintained above a critical level, which is one of the most important functions of the blood glucose control system.
• When the blood glucose falls too low, into the range of 20 to 50 mg/100 ml, symptoms of hypoglycemic shock develop, characterized by progressive nervous irritability that leads to fainting, seizures, and even coma.