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Physiological effects

The following section has been copied from page 108 of book the book “Thrive with Diabetes” from Laurence Chalem.

''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 directly useful in reducing high blood glucose levels as in diabetes.citation needed]''

I suggest change it for:

Stimulates the uptake of glucose -  Insulin decreases blood glucose concentration by inducing intake of glucose by the cell. This is possible because Insulin causes the insertion of the GLUT4 transporter in the cell membranes of muscle and fat tissues which allows glucose to enter the cell. http://www.diabetesincontrol.com/handbook-of-diabetes-4th-edition-excerpt-4-normal-physiology-of-insulin-secretion-and-action/

Induce glycogen synthesis - When glucose levels are high, insulin induces the formation of glycogen by the activation of the hexokinase enzyme, which adds a phosphates group in glucose, thus resulting in a molecule that cannot exit the cell. At the same time, insulin inhibits the enzyme glucose-6-phosphatase, which removes the phosphate group. These two enzymes are key for the formation of glycogen. Also, insulin activates the enzymes phosphofructokinase and glycogen synthase which are responsible of glycogen synthesis. http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/pancreas/insulin_phys.html

 

Signal transduction

The following section is very confusing.

''The manner in which the presence of insulin in the extracellular fluids affects metabolic changes inside its target cells is strangely complex and counterintuitive. Insulin binds to the extracellular portion of cell membrane-bound insulin receptors. Before insulin binds to its receptor, however, the receptor consists of two identical protein monomers, which remain separate until insulin binding occurs.[49] In the presence of insulin in the extracellular fluid, two of these monomers come together, bind the insulin molecule (also a protein) to their unified extracellular domains, resulting in the conversion of the intracellular domain of the resulting protein complex into an active enzyme, tyrosine kinase. This enzyme then phosphorylates itself, as well as a cytosolic protein named insulin receptor substrate 1 (IRS-1), which becomes bound to activated insulin receptor.[50] The resulting complex sets a variety of intracellular phosphorylation cascades in motion which ultimately result in the insertion of glucose transporter type 4 (GLUT4) molecules into the cell membranes of fat and muscle tissues, increasing the rate of glucose transport into these cells (making their glucose uptake “insulin dependent”).[49] It also results in the activation of the enzyme, glycogen synthase, causing liver and muscle cells to convert glucose into glycogen. A similar activation of the glycolytic enzymes, and of acetyl CoA carboxylase, stimulates liver, adipose and lactating mammary gland tissue to synthesize triglycerides (fats).[51][52] Another phosphorylation cascade that is activated by insulin binding to its receptor is the multistep Erk and MAP kinase pathways, which ultimately phosphorylate a number of substrates important for cell proliferation, cell cycle progression, cell division and differentiation (RSK kinases, Elk-1 transcription factor, etc.)''

I suggest the following:

The effects of insulin are initiated by its binding to a receptor present in the cell membrane. The receptor molecule contains an α- and β subunits. Two molecules are joined to form what is know as a homodimer. Insulin binds to the α-subunits of the homodimer, which faces the extracellular side of the cells. The β subunits have tyrosine kinase enzyme activity which is triggered by the insulin binding. This activity provokes the autophosphorylation of the β subunits and subsequently the phosphorylation of proteins inside the cell known as insulin receptor substrates (IRS). The phosphorylation of the IRS activates a signal transduction cascade that leads to the activation of other kinases as well as transcription factors that mediate the intracellular effects of insulin.

http://www.diabetesincontrol.com/handbook-of-diabetes-4th-edition-excerpt-4-normal-physiology-of-insulin-secretion-and-action/