Insulin function physiology | Actions of insulin | Endocrine physiology
Channel: Physiology Open
Date: 2020-08-02

Physiology lecture on endocrine system physiology details insulin functions.

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The main function of insulin is to store the excess fuel during times of their availability for future use. So how does insulin promote the storage of nutrients ? By affecting the entry of nutrients into cells and their metabolic cycles.

Insulin inhibits Glycogenolysis and Gluconeogenesis

Liver: 
In liver it increases the activity of enzyme glucokinase which converts glucose to glucose 6 phosphate.  Then it promotes glycogenesis by increasing the activity of glycogen synthase while it inhibits glycogenolysis by inhibiting glycogen phosphorylase. Also, excess glucose which has not been converted into glycogen enters into glycolysis pathway and produces acetyl CoA in liver. Now this acetylCoA  is used for synthesis of fatty acids since insulin promotes the activity of the enzyme Acetyl CoA carboxylase . Also, in the process of synthesis of fatty acids, malonyl CoA is formed which inhibits carnitine palmitoyltransferase enzyme and hence inhibits beta oxidationof fatty acids.  The formed fatty acids should now go and get stored in their depots i.e adipose tissue. So they are transported from liver to adipose tissue as triacylcglycerols in VLDL via blood…Insulin increases the expression of lipoprotein lipase in the walls of capillaries of adipose tissue. This lipoprotein lipase releases fatty acids from triacylglycerols in VLDL which then enter into the adipose tissue.

Skeletal muscles: 
Now insulin also promotes the entry of glucose in most cells except neurons and RBCs. It does this, by causing fusion of GLUT 4 transporters which are present inside the cell to the membrane. This increases their number on the membrane. Since glucose is in higher concentration in blood, it enters into the cells along its concentration gradient via these GLUT 4 transporters by facilitated diffusion. So most cells start using glucose as a fuel in presence of insulin and hence usage of fats as fuel is spared.  In skeletal muscles, excess glucose is also stored as glycogen .. 

Adipose tissue:
The glucose acts as a substrate for glycerol portion of triacylglycerol in adipose tissue. So the glycerol and fatty acids combine to form triacylglycerol in adipose tissue. Also insulin  inhibits hormone sensitive lipase in fat cells. This lipase breaks down stored triacylglycerol into fatty acids and glycerol. Thus by inhibiting hormone sensitive lipase, insulin prevents hydrolysis of triglycerides in adipose tissue. 

Effects on protein metabolism:
 Insulin also promotes amino acid uptake by cells and promotes protein synthesis, simultaneously inhibiting their breakdown.

Watch insulin signal transduction pathways: https://youtu.be/QAiOK9NMJCA
Watch pathophysiology pof diabetes mellitus: https://youtu.be/JcdHHS_nxPc

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