Metabolic Effects of Insulin

Insulin promotes the storage of nutrients as glycogen, TAG, and protein and inhibits their mobilization.
1. Effects on carbohydrate metabolism: The effects of insulin on glucose metabolism promote its storage and are most prominent in three tissues: liver, muscle, and adipose. In liver and muscle, insulin increases glycogen synthesis. In muscle and adipose, insulin increases glucose uptake by increasing the number of glucose transporters (GLUT-4) in the cell membrane. Thus, the IV administration of insulin causes an immediate decrease in blood glucose level. In the liver, insulin decreases the production of glucose through the inhibition of glycogenolysis and gluconeogenesis. [Note: The effects of insulin are due not just to changes in enzyme activity but also in enzyme amount insofar as insulin alters gene transcription.]
2. Effects on lipid metabolism: A rise in insulin rapidly causes a significant reduction in the release of FA from adipose tissue by inhibiting the activity of hormone-sensitive lipase, a key enzyme of TAG degradation in adipocytes. Insulin acts by promoting the dephosphorylation and, hence, inactivation of the enzyme . Insulin also increases the transport and metabolism of glucose into adipocytes, providing the glycerol 3-phosphate substrate for TAG synthesis .
Expression of the gene for lipoprotein lipase, which degrades TAG in circulating chylomicrons and very-low-density lipoproteins ([VLDL] ), is increased by insulin in adipose, thereby providing FA for esterification to the glycerol. [Note: Insulin also promotes the conversion of glucose to TAG in the liver. The TAG are secreted in VLDL.]
3. Effects on protein synthesis: In most tissues, insulin stimulates both the entry of amino acids into cells and protein synthesis (translation). [Note: Insulin stimulates protein synthesis through covalent activation of factors required for translation initiation.]