Insulin | Anatomy2Medicine
Insulin

Insulin

    • Metabolism during the fed or absorptive state
      • The fate of glucose in the liver
        • Excess glucose is stored in the liver as glycogen, which is used during periods of fasting to maintain blood glucose.
        • Excess glucose can be converted to fatty acids and a glycerol moiety, which combine to form triacylglycerols, which are released from the liver into the blood as very-low-density lipopro- tein (VLDL).
      • The fate of glucose in other tissues
        • The brain depends on glucose for its energy
        • Red blood cells, lacking mitochondria, oxidize glucose to pyruvate and lactate, which are released into the blood
        • Muscle cells
          • take up glucose by an insulin-stimulated transport process and oxidize glucose to CO2 and H2O to generate ATP for contraction.
          • Muscle stores glucose as glycogen for use during contraction.
        • Adipose cells take up glucose by an insulin-stimulated transport process and oxidize glucose to produce energy and convert it to the glycerol moiety used to produce triacylglycerol stores.
      • The fate of lipo proteins in the fed state
        • The triacylglycerols of chylomicrons (produced from dietary fat )and VLDL( produced from glucose by the liver) are substrates in capillaries for lipoprotein lipase to form fatty acids and glycerol.
        • The fatty acids are taken up by adipose tissue,converted to triacylglycerols,and stored.
      • The fate of amino acids from dietary proteins in the fed state.
        • The amino acids enter cells and are:
          • Used for protein synthesis

 

  • Used to make nitrogenous compounds such as heme, creatine phosphate, epinephrine, and the bases of DNA and RNA.

 

          • Oxidized to generate ATP.
    • Fasting
      • Energy reserves:fasting versus starvation (long-term)
        • Triglycerides
          • The principal energy reserves used for long-term food deprivation are triglycerides (stored in adipose tissue [fat]) for both lean and obese individuals.
        • Carbohydrate
          • The contribution of carbohydrate to total energy reserves is very small, yet it is the energy source called on first with heavy energy expenditures.
      • The liver during fasting
        • The liver produces glucose and ketone bodies that are released into the blood and serve as sources of energy for other tissues.
        • Production of glucose by the liver
          • The liver must maintain blood glucose levels
          • Glucose is required by the brain and red blood cells.
            • The brain oxidizes glucose to CO2 and H2O
            • red blood cells oxidize glucose to pyruvate and lactate.
        • Glycogenolysis
          • About 2 to 3 hours after a meal, the liver breaks down its glycogen stores by glycogenolysis amd free glucose is released into the blood
          • Glucose is then taken up by tissues and oxidized.
        • Gluconeogenesis
          • After about 4 to 6 hours of fasting, the liver begins the process of gluconeogenesis.

 

  • Within 30 hours, liver glycogen stores are depleted, leaving gluconeogenesis as the major process responsible for maintaining blood glucose

 

        • Production of ketone bodies by the liver
          • As glucagon levels rise, adipose tissue breaks down its triacylglycerol stores into fatty acids and glycerol, which are released into the blood.
          • Through -oxidation,the liver converts fatty acids to acetyl Co A.
          • Acetyl CoA is used by the liver for ketone body synthesis.
          • The ketone bodies are aceto- acetate and-hydroxybutyrate.
          • The liver cannot oxidize ketone bodies and releases them into the blood.
      • Adipose tissue during fasting

 

  • As glucagon levels rise,adipose triacyl glycerol stores are mobilized.

 

        • The liver converts the fatty acids to ketone bodies and the glycerol to glucose.
        • Tissues such as muscle oxidize the fatty acids to CO2 and H2O.
      • Muscle during fasting
        • Degradation of muscle protein

 

  • During fasting, muscle protein is degraded, producing amino acids that are partially
  • metabolized by muscle and released into the blood, mainly as alanine and glutamine (MCQ)

 

          • Tissues , such as gut and kidney,metabolize the glutamine (MCQ)
          • The products (mainly alanine) travel to the liver, where the carbons are converted to
          • glucose or ketone bodies and the nitrogen is converted to urea. (MCQ)
        • Oxidation of fatty acids and ketone bodies

 

  • During fasting, muscle oxidizes fatty acids released from adipose tissue, and ketone

 

          • bodies produced by the liver.
          • During exercise, muscle can also use its own glycogen stores as well as glucose, fatty acids, and ketone bodies from the blood.

 

  • Prolonged fasting (starvation)

 

    • Metabolic changes in starvation
      • When the body enters the starved state, after 3 to 5 days of fasting, changes occur in the use of fuel stores.
      • Muscle decreases use of ketone bodies and oxidizes fatty acids as its energy source.
      • Because of decreased use by muscle,blood ketone body levels rise.
      • The brain then takes up and oxidizes ketone bodies to derive energy. Consequently, the brain decreases its use of glucose, although glucose is still a major fuel for the brain.
      • Liver gluconegenesis decreases.
      • Muscle protein is spared
        • less muscle protein is degraded to provide amino acids for gluconeogenesis
        • Because of decreased conversion of amino acids to glucose, less urea is produced from amino acid nitrogen in starvation than after an overnight fast.
      • Fat: the primary fuel
        • The length of time that a person can survive without food depends mainly on the amount of fat stored in adipose tissue.