The following points highlight the top four effects of insulin. The effects are:- 1. Effects on Carbohydrate Metabolism 2. Effects on Protein Metabolism 3. Effects on Lipid Metabolism 4. Effects on Tissue Growth.
1. Effects on Carbohydrate Metabolism:
Insulin shows anabolic effects on carbohydrate metabolism by different ways:
(a) Glucose transport:
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Insulin increases the glucose transport from extracellular fluid into muscle cells, adipose tissue, mammary glands and other extra-hepatic cells by stimulating the translocation of glucose transporter from the cytoplasm to the plasma membrane, the site of facilitated diffusion. Thus, insulin increases the number of glucose transporter in the cell membrane.
In liver cells, insulin increases hepatic uptake of glucose in a different way. The hormone increases the synthesis of glucokinase enzyme which converts glucose into glucose 6-phosphate in liver cells. Increased concentration of this enzyme lowers the hepatic cellular concentration of glucose; this, in turn, enhances simple diffusion of glucose into hepatic cells.
(b) Glycogenolysis:
The breakdown of glycogen into glucose or glycogenolysis is inhibited following insulin action. Thus, insulin decreases release of glucose into blood from liver. It inactivates glycogen phosphorylase and glucose 6-phosphatase, the rate limiting and final enzymes of the glycogenolysis, respectively.
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(c) Gluconeogenesis:
Gluconeogenesis or formation of glucose from non-carbohydrate materials is inhibited by insulin. Insulin represses the regulating enzymes of gluconeogenesis including pyruvate carboxylase, fructose 1, 6-bisphosphatase, and glucose 6-phosphatase.
(d) Glycolysis:
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Insulin stimulates glycolysis or breakdown of glucose in liver, muscles, adipocytes and other tissues by inducing many enzymes of that pathway like pyruvate kinase, hexokinase II etc.
(e) Glycogenesis:
Synthesis of glycogen in liver and muscle is stimulated by insulin because this hormone activates glycogen synthase, the rate limiting enzyme of glycogenesis and also other enzymes of that pathway.
(f) Pentose phosphate pathway:
Insulin enhances the conversion of pentose from glucose by inducing glucose 6-phosphate dehydrogenase and 6-phosphogluconate enzymes of pentose phosphate pathway.
2. Effects on Protein Metabolism:
Insulin exerts anabolic effects on protein metabolism which is observed in different physiological events like:
(a) Amino acid uptake:
In muscle cells, insulin increases the amino acid influx by affecting the membrane transporters of amino acids.
(b) Synthesis of new proteins:
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Insulin regulates the transcription of specific mRNAs, and ribosomal translation of many structural as well as enzymatic proteins in its target organ tissues. Insulin also promotes the entry of amino acids into cells and modulates protein component of 40S ribosomal subunit.
(c) Breakdown of proteins:
Insulin decreases protein breakdown, amino acid catabolism in different cells and urea formation.
3. Effects on Lipid Metabolism:
Insulin acts as a lipid anabolic hormone. Its activities on lipid metabolism are pronounced in the following processes:
(a) Lipogensis:
Synthesis of lipids from carbohydrates is enhanced by insulin.
The hormone facilitates such lipogenesis:
(i) By promoting glucose entry into adipocytes,
(ii) By inducing different enzyme activities that produce NADPH for lipogenesis like glucose 6-phosphate dehydrogenase, malic enzyme etc.,
(iii) By promoting the activities of enzymes that catalyse the formation of acetyl CoA to be used in fatty acid synthesis,
(iv) Insulin lowers the circulating levels of fatty acids.
(b) Fatty acid uptake:
In adipose tissues insulin stimulates the lipoprotein lipase activity on the capillary endothelial membrane. The activated enzymes then hydrolyse circulating triacylglycerols to free fatty acids which are then taken up by adipocytes to be used in fat synthesis in adipose tissues.
(c) Lipolysis in adipose tissues:
In contrast to above functions, insulin decreases adipose tissue lipolysis. It performs inhibition of lipolysis by:
(i) Reducing the activity of triacylglycerol lipase enzyme,
(ii) Activating the lipase phosphatase enzyme, and
(iii) Lowering the blood free fatty acid level and consequently decreasing β-oxidation and ketone body formation in adipose tissues.
4. Effects on Tissue Growth:
Insulin stimulates DNA replication, gene transcription, ribosomal translation and potentiates the action of many growth factors. All such effects are reflected in cell proliferation and growth of many tissues like liver, connective tissues, skin, adrenal and mammary glands.