3.6.4.2 Control of blood glucose concentration

Okay so to start we should know a little bit about hormones:

• They are all produced in glands which secrete the hormone directly into the blood (endocrine glands)
• carried in the blood plasma to the cells on which they act (target cells). These cells have specific receptors on their cell-surface membranes that are complementary to a specific hormone
• are effective in very low concentrations but often have widespread and long lasting effects

One mechanism of hormone action is the secondary messenger model. This is used by two hormones involved in the regulation of blood glucose concentration (adrenaline and glucagon).

The mechanism involving adrenaline is as follows:

• adrenaline binds to a transmembrane protein receptor within the cell-surface membrane of a liver cell
• this binding causes the protein to change shape on the inside of the membrane
• this protein shape change leads to the activation of an enzyme called adenyl cyclase. The activated adenyl cyclase converts ATP to cyclic AMP (cAMP)
• The cAMP acts as a second messenger that binds to protein kinase enzyme changing its shape and therefore activating it
• the active protein kinase enzyme catalyses the conversion of glycogen to glucose which moves out of the liver cell (by facilitated diffusion) and into the blood.

The pancreas

The pancreas is a gland that is situated in the upper abdomen (behind the stomach). It produces enzymes for digestion (protease, amylase, and lipase) and hormones for regulating blood glucose concentration (insulin and glucagon). Scattered throughout the cells that produce digestive enzymes are groups of hormone-producing cells known as islets of Langerhans. These include:

• alpha cells (which are larger and produce glucagon)
• beta cells (which are smaller and produce insulin)

The liver

This is located below the diaphragm. It is made up of cells called hepatocytes. The hormones that the pancreas produces (glucagon and insulin) have an effect in the liver. There are three processes regarding blood sugar which take place in the liver that we need to learn:

• Glycogenisis
• This is the conversion of glucose to glycogen
• when blood glucose concentration is higher than normal the liver removes glucose from the blood and converts it into glycogen.
• Glycogenolysis
• This is the break down of glycogen to glucose
• when blood glucose concentration is lower than normal the liver can convert stored glycogen to glucose which diffuses into the blood to restore the normal blood glucose concentration
• Gluconeogenesis
• This is the production of glucose from sources other than carbohydrate (e.g glycerol and amino acids) when the supply of glycogen is exhausted.

The beta cells of the islets of Langerhans in the pancreas have receptors that detect a rise in blood glucose concentration. They respond by secreting the hormone insulin directly into the blood plasma. 

Factors that affect blood glucose concentration include:

• diet - in the form of glucose absorbed following hydrolysis of other carbohydrates (e.g starch/maltose/lactose/sucrose)
• glycogenolysis - the break down of glycogen to glucose
• gluconeogenesis - the break down of sources other than carbohydrates into glucose

Insulin

The beta cells of the islets of Langerhans in the pancreas have receptors that detect the stimulus of a rise in blood glucose concentration and respond by secreting insulin. Glycoprotein receptors on cell-surface membranes bind specifically with insulin molecules bringing about:

• a change in the tertiary structure of the glucose transport carrier proteins which causes them to change shape and open allowing more glucose into the cells by facilitated diffusion allowing more glucose in by facilitated diffusion
• an increase in the number of carrier proteins responsible for the transport of glucose
• at low insulin concentrations the protein from which these channels are made is part of the membrane vesicles. A rise in insulin concentration results in these vesicles fusing with the cell-surface membrane so increasing the number of glucose transport channels
• activation of the enzymes that convert glucose to glycogen and fat (glycogenesis)

As a result blood glucose concentration is lowered by:

• increasing the rate of absorption of glucose into the cells
• increasing the respiratory rate of the cells which uses up more glucose thus increasing their uptake of glucose
• increasing the rate of conversion of glucose into glycogen in the liver and muscle cells
• increasing the rate of conversion of glucose to fat

This subsequent lowering of the glucose in the blood causes the beta cells to reduce their secretion of insulin

Glucagon

Alpha cells of the islets of Langerhans detect a fall in blood glucose concentration and respond by secreting the hormone glucagon. Glucagon...

• attaches to specific protein receptors on the cell surface membrane of liver cells
• activating enzymes that convert glycogen to glucose
• activate enzymes involved in the conversion of amino acids and glycerol into glucose

Adrenaline

At times of excitement/stress adrenaline is produced by the adrenal glands above the kidneys. This raises the blood glucose by:

• attaching to protein receptors on the cell-surface membrane of target cells
• activating enzymes that causes the breakdown of glycogen to glucose in the liver

Insulin and glucagon act antagonistically. This means that the concentration of glucose is not constant but fluctuates around an optimum point.

Diabetes

There are two forms of diabetes:

• Type 1
• due to the body being unable to produce insulin
• Control: it is controlled by injections of insulin. 
• Type 2
• due to glycoprotein receptors on body cells being lost/losing their responsiveness to insulin
• Control: regulate the intake of carbohydrate in the diet and match this to an amount of exercise