Table - Regulation of Metabolism and Hormonal Control of Blood Glucose Levels

Hormonal Control of Blood Glucose Levels

Hormone

Source

Stimuli

Target Tissue

Insulin

β cells in the pancreatic Islets of Langerhans

↑ [glucose] in the blood, ↑ [AA] in the blood, ↑ ps, ↑ GIP, ↑ gastrin (inhibited by sympathetic activity)

Primarily muscle and adipose tissues; facilitates Glut 4 transporter insertion. Liver has Glut 2 transporter (insulin not needed).

Glucagon

α cells in the pancreatic Islets of Langerhans

↓ [glucose] in blood, ↑ sympathetic activity, ↑ [AA] after a pure protein meal

Liver: effects include proteolysis, glycogenolysis, gluconeogenesis, ketone synthesis from fatty acids. Skeletal muscle: glycogenolysis. Kidney: gluconeogenesis.

NE/EPI

Adrenal medulla (90% EPI)

Emergency and exercise stimuli

Same mechanisms as glucagon.

Cortisol

Adrenal cortex

Stress axis stimuli

Same mechanisms as glucagon, with decreased immune function.

Metabolism is a complex set of biochemical reactions that balance anabolic (building) and catabolic (breaking down) processes to maintain energy homeostasis in the body. These reactions are crucial for growth, repair, and energy production.

Absorptive State

  • The absorptive state occurs shortly after eating, typically within 4 hours post-feeding. During this phase, the body primarily engages in anabolic reactions. Major processes include:

    • Glycogenesis: The conversion of glucose to glycogen for storage in the liver and muscle.

    • Lipogenesis: The synthesis of fatty acids from excess glucose and their storage as triglycerides in adipose tissue.

    • Protein Synthesis: The formation of proteins from amino acids, crucial for muscle repair and growth.

Post-absorptive State

  • The post-absorptive state begins approximately 4 hours after the last food intake and predominantly involves catabolic reactions, which serve to mobilize energy stores. Key mechanisms include:

    • Glycogenolysis: The breakdown of glycogen into glucose to be released into the bloodstream.

    • Lipolysis: The breakdown of triglycerides into glycerol and free fatty acids, which can be used for energy.

    • Proteolysis: The breakdown of proteins into amino acids to be used for energy or gluconeogenesis.

    • Gluconeogenesis: The synthesis of glucose from non-carbohydrate precursors, such as lactate and amino acids, primarily occurring in the liver.

Hormonal Regulation

  • Several hormones play vital roles in regulating metabolism and maintaining blood glucose levels:

    • Insulin: Produced by β cells in the pancreatic Islets of Langerhans, insulin is released in response to increased blood glucose, elevated amino acids, and several gastrointestinal hormones. It primarily targets muscle and adipose tissues, facilitating the insertion of the Glut 4 transporter to enhance glucose uptake. The liver utilizes the Glut 2 transporter and does not require insulin for glucose uptake.

    • Glucagon: Secreted by α cells in the pancreatic Islets of Langerhans, glucagon is triggered by low blood glucose levels or high sympathetic activity. It prompts the liver to initiate gluconeogenesis, glycogenolysis, and ketone synthesis, while also stimulating skeletal muscle to break down glycogen. Furthermore, it assists the kidneys in gluconeogenesis during fasting states.

    • Norepinephrine/Epinephrine (NE/EPI): Produced in the adrenal medulla, these hormones are released during stress or exercise. They share similar mechanisms with glucagon, enhancing the mobilization of glucose and free fatty acids for immediate energy use.

    • Cortisol: Released from the adrenal cortex during stress, cortisol acts to increase blood glucose by promoting gluconeogenesis and ketogenesis, while also decreasing immune function, which is a part of the body’s stress response.

Homeostasis of Plasma Glucose

  • Maintaining homeostasis of plasma glucose levels is crucial for providing a consistent fuel supply to the brain and central nervous system. Variations in glucose levels can lead to:

    • Hypoglycemia: Characterized by low blood sugar, which can cause symptoms like dizziness, confusion, and, in severe cases, loss of consciousness.

    • Hyperglycemia: Elevated blood sugar levels can lead to long-term complications such as cardiovascular disease, nerve damage, and kidney failure.

Additional Notes

  • Insulin promotes cellular glucose uptake, which is essential for energy production and metabolism.

  • Type I Diabetes: This autoimmune disorder leads to the destruction of insulin-producing beta cells, resulting in an inability to produce insulin.

  • Type II Diabetes: This condition is characterized by insulin resistance, where the body's cells do not respond effectively to insulin, often associated with obesity and lifestyle factors. A video resource provides a more detailed differentiation of Type I and Type II diabetes, emphasizing their pathophysiology and management strategies.