Insulin is a hormone produced by the beta cells of the pancreas. This hormone is primarily responsible for maintaining glucose homoeostasis in the human body.
The human body depends on glucose for most of its energy needs. In fact, glucose is the primary source of energy for all living cells. However, most human tissues cannot utilise glucose in the absence of insulin. The brain is an exception. The brain can use glucose even in the absence of insulin.
Let us briefly discuss human digestion. The food we eat consists of carbohydrates, proteins and fats. The amylase present in saliva starts the digestion of carbohydrates in food. The pancreatic amylase completes the process of digestion of carbohydrates in the small intestine. The pancreatic amylase completely breaks down the carbohydrates into glucose and fructose. Glucose and fructose are six carbon molecules that are the simplest of the sugars. Glucose enters the bloodstream after absorption from the small intestine.?
Blood then supplies glucose to tissues, and the excess glucose is converted into glycogen in the liver. Glycogen is the energy reserve of the body. When the body is fasting, glycogen is degraded into glucose and utilised by the cells.
As soon as glucose enters the bloodstream, it stimulates the release of insulin from the beta cells of the pancreas.Amino acids also stimulate the release of this hormone from the pancreas (in fact, amino acids are a more potent stimulus).
The insulin acts primarily on the liver, muscle and adipose tissues. Other tissues are not dependent?on this hormone to utilise glucose.
Insulin is a peptide hormone consisting of two polypeptide chains- A & B. A disulphide bond links the two chains. It is first synthesised as a single polypeptide chain- preproinsulin. This polypeptide chain is processed by the endoplasmic reticulum to produce a 39 amino acid residue. During this process, a polypeptide chain called C-peptide is released. We can check the functioning of the pancreas by checking the level of C-peptide in the blood.
The synthesis and release of this hormone are regulated at several levels- transcription of the gene, translation and post-translational modifications.
Insulin is an anabolic hormone. It induces the synthesis of many substances. It also regulates growth. The following effects are known:-
Glycogen synthesis- The cells of the liver and the muscles are the primary targets of insulin. Insulin promotes uptake of glucose by the hepatocytes and myocytes. It also promotes synthesis of glycogen in the liver and muscle. When the body is fasting, the blood level of insulin decreases and this promotes degradation of glycogen. The degradation of glycogen maintains blood glucose at a constant level.
Lipid synthesis- This hormone acts on the adipocytes or fat cells and promotes synthesis of triglycerides and fatty acids. Treatment with insulin leads to weight gain due to increased production of fat by adipose tissue (apart fro fluid retention). It also promotes esterification of fatty acids.
Decreases the degradation of proteins
Decreased degradation of fats
Decreases production of glucose- Apart from degradation of glycogen, the human body has mechanisms to produce glucose (gluconeogenesis) from amino acids. Gluconeogenesis helps maintain blood glucose levels when the body is fasting, and the glycogen stores are depleted.?
Increases uptake of amino acids. Also promotes the production of proteins.
Increased potassium uptake by cells- Insulin promotes uptake of potassium by the cells. Therefore, serum potassium levels can fall following treatment with insulin. The ability to reduce serum potassium levels is harnessed therapeutically to reduce elevated serum potassium. This hormone increases uptake of potassium by increasing the concentration of Na-K ATAase on the cell surface by translocation of the receptors from the cytosol to the cell membrane.
Effect on arterial muscles ? Has a significant impact on the arterial muscles. This hormone causes relaxation of the arterial muscles, thus, reducing arterial tone. The reduction in the arterial tone improves microcirculation.
Increases secretion of hydrochloric acid in the stomach
Reduces sodium excretion by the kidneys. Therefore, treatment with insulin can lead to fluid retention.
Disorders due to over or underproduction of insulin
Insulin plays a critical role in glucose homoeostasis. Therefore, over or underproduction can result in many disorders. Diabetes Mellitus is a disorder caused by underproduction. Lack of insulin leads to impaired utilisation of glucose by body tissues. Therefore, blood glucose levels rise and result in some problems- kidney damage, damage to blood vessels and nerves, etc.?
Until the development of injectable insulin, diabetes was a disease with high morbidity and mortality. In 1920. Scientists Banting and Best purified insulin and postulated that injections can be used to treat diabetes. Use of insulin to treat diabetes has revolutionised the management of this chronic disorder. For their work, Banting and Best were awarded the Nobel Prize in Physiology in 1923.
Overproduction leads to dropping in blood glucose levels. When blood glucose level drops (hypoglycaemia) below a specified threshold symptoms like weakness, dizziness, palpitations, sweating and seizures can occur. Overproduction is the hallmark?of a disorder?called insulinoma.