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Control of Blood Glucose Concentration

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The liver and the pancreas have a central role in the regulation of blood glucose concentration. The cells in the pancreas secrete the hormones which tell cells to take up glucose from the blood or not take it up. On demand, glucose is made from broken down glycogen in the liver.

 

The pancreas has alpha and beta cells. Alpha cells secrete glucagon which increases blood glucose concentration, while beta cells secrete insulin which decreases blood glucose concentration. People with type 1 diabetes have destroyed beta cells, so their lack of insulin makes them have to take it via injections.

 

Blood Glucose is too High

The pancreas detects this, so it secretes insulin. This stimulates the uptake of glucose from the blood by cells, and the storage of it in the liver once it’s converted to glycogen. This reaction is called glycogenesis. The stages are:

 

1. Insulin attaches to receptors on target cells
2. This triggers a change in how many channel proteins are included in the cell membrane
3. Separately, it also stimulates the activation of enzymes involved in converting glucose into glycogen

 

Blood Glucose is too Low

 

The pancreas detects this too, so it secretes its masterfully antidote: glucagon. This inhibits cells from taking up any more glucose from the blood, while initiating the breakdown of glycogen in the liver to produce more glucose. The glycogen is hydrolised (broken down in the presence of water) so the term for this reaction is glycogenolysis.

 

1. Glucagon attaches to receptors on target cells
2. This activates enzymes responsible for the conversion of glycogen into glucose
3. This activates enzymes responsible for the conversion of glycerol and amino acids into glucose

 

 

Another hormone involved in glucose regulation is adrenaline. This is secreted by the adrenal glands in times of stress or exercise. It has the same effect as glucagon. Both of these hormones act via a second messenger. That is, they bind to the plasma membrane of cells and exert their influence from the outside by causing a cascade of enzymatic reaction events inside the cell which ultimately end up in their response. They chicken out of it, don’t blame the messenger.

 

The second messenger model includes adenylate cyclase, cyclic AMP (cAMP) and protein kinase.

 

 

Thus glucagon (the first messenger) does not effect direct change, but acts via the second messenger pathway. Upon activation, the protein kinase adds phosphate groups to other proteins such as other enzymes. This activates them and can change other features such as their association with other proteins.

 

Diabetes

Type 1 diabetes is an auto-immune disease that develops early in life. The body destroys the pancreas islet cells responsible for the production of insulin. It is rectified with insulin injections and close monitoring of blood glucose. Advances are being made in recreating the missing cells either by transplantation, stem cell therapy or an implantable artificial device.

 

Unlike type 1, type 2 diabetes involves a desensitisation to insulin rather than its complete absence. It is associated with obesity and certain other health problems Treatment consists of an improved lifestyle with a better diet and more exercise. If it gets bad, insulin may be needed to supplement the patients’ own insulin.

 

 

 

 

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