Control of blood glucose is very important. Sufficient glucose must be delivered to cells for respiration, but too much glucose can eventually result in coma or even death. The concentration of glucose is affected by factors such as eating and exercise, and must be brought back to the normal level of about 4mmol/L to 5.5mmol/L. The body uses negative feedback mechanisms involving insulin and glucagon to tightly control the concentration of glucose in the blood.
Role of insulin
Insulin is a hormone secreted by beta cells in the pancreas. It acts to lower blood glucose concentration in response to raised blood glucose. Insulin is transported in the blood to reach specific receptors on liver cells (hepatocytes) and muscle cells. The responses triggered in these cells by insulin are as follows:
- Activation of enzymes needed to catalyse glycogenesis (the conversion of glucose to glycogen). Glycogen is a glucose storage molecule.
- Increased permeability of cell surface membranes to glucose due to increased inclusion of glucose specific channel proteins, such as GLUT4, in the cell surface membrane (so glucose uptake is increased).
Role of glucagon
Glucagon is a hormone secreted by alpha cells in the pancreas. It acts to raise blood glucose concentration in response to low blood glucose. Glucagon specific receptors are found on the surface of liver cells. When glucagon binds these to receptors, the following responses are triggered:
- Activation of enzymes needed to catalyse glycogenolysis (the break down of glycogen to glucose).
- Activation of enzymes needed to catalyses gluconeogenesis (the conversion of glycerol and amino acids into glucose).
Role of adrenaline
Adrenaline is a hormone secreted by the adrenal glands. It acts to raise blood glucose in response to low blood glucose, and will also act in response to exercise or stress. Adrenaline binds to specific receptors found on the surface of liver cells to trigger the following responses:
- Activation of enzymes needed to catalyse glycogenolysis (the break down of glycogen to glucose).
- Inhibition of glycogenesis.
How do beta cells secrete insulin?
A question you probably ask yourself on a daily basis. This is how beta cells respond to the stimulus (high blood glucose) to secrete insulin:
- Increased glucose enters the beta cells by facilitated diffusion, causing an increased rate of respiration.
- Increased ATP from respiration triggers potassium ion (K+) channels in the cell surface membrane to close.
- K+ ions accumulate inside the cell which depolarises the membrane.
- This triggers calcium ion (Ca2+) channels to open and Ca2+ diffuse into the cells.
- Vesicles containing insulin fuse with the cell surface membrane and insulin is secreted by exocytosis. (This is similar to what happens at a synapse.)
How do glucagon and adrenaline activate glycogenolysis?
Another burning question which everyone thinks about at least once a week. Well, they act via a second messenger in a neat little bit of cell signalling:
- The hormone binds to a specific receptor on the cell surface membrane of a liver cell.
- This activates an enzyme called adenylate cyclase, which converts ATP into cAMP (cyclic AMP – the second messenger).
- cAMP activates another enzyme called protein kinase A.
- Protein kinase A activates a whole complicated cascade of reactions which eventually ends with the conversion of glycogen to glucose.
Summary
We’ve covered a lot of content in this article. Here is a summary diagram to show the negative feedback mechanisms involved in the control of blood glucose.
