Ans. is 'c' i.e., Induction of lipogenesis ACTIONS OF INSULIN Insulin affects the metabolism of carbohydrates, lipids and proteins. Effects on carbohydrate metabolism The overall effect of insulin is to decrease blood glucose level. Insulin increases the utilization of glucose and decreases its production by its following actions : - i) Stimulation of oxidation of glucose by glycolysis especially in the liver and skeletal muscle. ii) Stimulation of glycogenesis i.e., insulin ours conversion of glucose into its storage form, glycogen. This action is seen in both liver and muscles. iii) Inhibition of gluconeogenesis. Insulin stimulates the uptake of glucose by myocytes (skeletal muscle, cardiac muscles), adipocytes (adipose tissue) and hepatocytes. Tissues that do not depend on insulin for glucose uptake include brain, erythrocytes (RBC), the epithelial cells of kidney & intestine, Liver, and Cornea & lens of eye. The mechanism through which insulin increases glucose uptake is different in different tissues. In the muscle and adipose tissues, insulin increase facilitated diffusion by increasing glucose transpoer (GLUT4 ) on the cell membrane. In the liver, insulin stimulates glucose entry into hepatocytes indirectly by induction of glucokinase so that the glucose entering the liver cells is promptly conveed to glucose - 6 - phosphate (glucose trapping). This keeps the intracellular glucose concentration low and ours entry of glucose into the liver. Thus, though the liver do not depend on insulin for glucose uptake, insulin stimulates glucose entry into hepatocytes. That means glucose entery can occur in liver without the action of insulin, but this is facilitated by insulin. On the other hand, myocytes (skeletal and cardiac muslces) and adipocytes (adipose tissue) are dependent on insulin for glucose uptake. Increased uptake of glucose in the glucose has following effects :- i) T Glycolysis :- It is due to induction of key enzymes of glycolysis by insulin. These key enzymes are glucokinase, phosphofructokinase and pyruvate kinase. ii) Increased glycogen synthesis (glycogenesis) :- It is due to induction of glycogen synthase. iii) Decreased glycogen break-down (Glycogenolysis) : - It is due to inhibition of enzyme phosphorylase. iv) Decreased gluconeogenesis :- It is due to inhibition of enzymes Pyruvate carboxylase, PEP carboxykinase, fructose 1, 6-bisphosphatase, glucose - 6 - phosphates. Effects on lipid metabolism Insulin induces lipogenesis by inducing enzyme acetyl CoA carboxylase, the rate limiting enzyme in fatty acid synthesis. Triglyceride synthesis is increased by induction of lipoprotein lipase. Lipolysis (13-oxidation) is decreased due to inhibition of hormone sensitive lipase, so that the fat in the adipose tissue is not broken down. Thus free fatty acid and glycerol are decreased. Because of antilipolytic action insulin decreases ketogenesis. There are two impoant lipases : - i) Lipoprotein lipase : - It hydrolysis the triglycerides of chylomicrons and VLDL into free fatty acid and glycerol in the vessels of skeletal muscles, cardiac muscles and adipose tissue. There FFA is taken up by the cells of these tissue and is conveed back into the triglyceride and the FFA is stored as triglyceride. So, lipoprotein lipase is involved in the synthesis of triglyceride. Lipoprotein lipase is stimulated by insulin, therefore insulin stimulates triglyceride synthesis. ii) Hormone sensitive lipase : - It is involved in lipolysis and cause degradation of stored triglyceride of adipose tissue into FFA and glycerol. FFA comes out into the blood raising the FFA levels of blood. Insulin inhibits hormone sensitive lipase therefore decreases FFA levels of blood. Effects on protein metabolism Insulin stimulates synthesis of protein (anabolism) and inhibits protein breakdown (catabolism). Insulin increases the active transpo of many amino acids into the tissue. In addition insulin increases protein synthesis by increasing the rate of synthesis of mRNA.