Insulin

Insulin (Latin insula, "island", as it is produced in the Islets of Langerhans in the pancreas) is a polypeptide hormone that regulates carbohydrate metabolism. Apart from being the primary effector in carbohydrate homeostasis, it also has a substantial effect on small vessel muscle tone, controls storage and release of fat (triglycerides) and cellular uptake of both amino acids and some electrolytes. In this last sense, it has anabolic properties. Its concentration (more or less, prsence or absence) has extremely widespread effects throughout the body.

Actions on cellular and metabolic level

The actions of insulin on the global human metabolism level include:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• control of cellular intake of certain substances, most prominently glucose in muscle and adipose tissue (about 2/3 of body cells)
• increase of DNA replication and protein synthesis via control of amino acid uptake
• modification of the activity of numerous enzymes (allosteric effect)

The actions of insulin on cells include:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• increased glycogen synthesis – insulin forces storage of glucose in liver (and muscle) cells in the form of glycogen; lowered levels of insulin cause liver cells to convert glycogen to glucose and excrete it into the blood. This is the clinical action of insulin which is useful in reducing high blood glucose levels as in diabetes.
• increased fatty acid synthesis – insulin forces fat cells to take in glucose which is converted to triglycerides; lack of insulin causes the reverse
• increased esterification of fatty acids – forces adipose tissue to make fats (ie, triglycerides) from fatty acid esters; lack of insulin causes the reverse
• decreased proteinolysis – forces reduction of protein degradation; lack of insulin increases protein degradation,
• decreased lipolysis – forces reduction in conversion of fat cell lipid stores into blood fatty acids; lack of insulin causes the reverse
• decreased gluconeogenesis – decreases production of glucose from various substrates in liver; lack of insulin causes glucose production from assorted substrates in the liver and elsewhere
• increased amino acid uptake – forces cells to absorb circulating amino acids; lack of insulin inhibits absorption
• increased potassium uptake – forces cells to absorb serum potassium; lack of insulin inhibits absorption
• arterial muscle tone – forces arterial wall muscle to relax, increasing blood flow, especially in micro arteries; lack of insulin reduces flow by allowing these muscles to contract