Principles of Human Physiology: The Endocrine System

Principles of Human Physiology Overview

Chapter Overview

• Title: The Endocrine System: Regulation of Energy Metabolism and Growth
• Author: Cindy L. Stanfield, Sixth Edition
• Publisher: © 2017 Pearson Education, Inc.

Learning Outcomes

• Describe the hormonal control of metabolism during absorptive and postabsorptive states.

21.5 Regulation of Absorptive and Postabsorptive Metabolism

Hormonal Regulation

• Insulin: Referred to as the hormone of the absorptive state.
• Glucagon: Identified as the hormone of the postabsorptive state.
• Other Regulators: Includes the sympathetic nervous system and epinephrine.

The Role of Insulin

• Type: Peptide hormone.
• Source: Produced by beta () cells in the pancreatic islets of Langerhans.
• Functions:
  • Promotes glucose uptake by body cells.
  • Facilitates the synthesis of energy storage molecules.

Mechanism of Insulin Action

• Beta Cells in Pancreas: Secrete insulin when stimulated.
  • Effects on Various Tissues:
    • Most tissues (except brain, liver, exercising muscle):
    • ↑ Glucose uptake
    • ↑ Amino acid uptake
    • ↑ Protein synthesis
    • ↓ Protein breakdown
    • Adipose tissue:
    • ↑ Fatty acid and triglyceride synthesis
    • Promotes lipolysis (the breakdown of fats).
    • Liver and Muscle:
    • ↑ Glycogen synthesis
    • ↓ Glycogenolysis (the breakdown of glycogen).
    • Liver:
      • ↑ Fatty acid and triglyceride synthesis
      • Facilitates gluconeogenesis.

Factors Affecting Insulin and Glucagon Release

• Increasing Insulin Secretion:

  • ↑ Plasma [glucose]
  • ↑ Plasma [amino acids]
  • ↑ Plasma [GIP (Gastric Inhibitory Peptide)]
  • ↑ Parasympathetic activity

• Decreasing Insulin Secretion:

  • ↑ Sympathetic activity
  • ↑ Plasma [epinephrine]

• Effect of Glucagon Secretion:

  • ↑ Plasma [glucose]: Decreases glucagon secretion.
  • ↑ Plasma [amino acids]: Increases glucagon secretion.
  • ↑ Parasympathetic activity: Increases glucagon secretion.
  • ↑ Sympathetic activity: Increases glucagon secretion.
  • ↑ Plasma [epinephrine]: Increases glucagon secretion.

Mechanism of Insulin Release

• Stimulus: High plasma concentration of glucose.
• Glucose enters beta cells through GLUT2 transporters.
• In the cell:
  • Glucose is metabolized through glycolysis to form pyruvate.
  • Pyruvate enters the mitochondrial matrix for further processing.
  • Calcium Trigger:
  • Cell depolarization opens voltage-gated Ca²⁺ channels leading to calcium influx.
  • Increased intracellular calcium triggers insulin release via exocytosis.
• Depolarization of the cell is further attributed to the closure of ATP-sensitive K⁺ channels by ATP, preventing potassium outflow.

Actions of Glucose on Insulin Secretion

1. High glucose concentration outside the cell causes entry via facilitated diffusion.
2. Glucose is converted to glucose-6-phosphate.
3. Leads to increased ATP generation through oxidative phosphorylation.

Negative Feedback Control of Blood Glucose Levels by Insulin and Glucagon

Blood Glucose Levels

• Normal Range: 70-110 mg/dL
• Hyperglycemia: Levels exceed 140 mg/dL, can indicate diabetes mellitus.
• Hypoglycemia: Levels fall below 60 mg/dL.

Absorptive State

• Process:
  • Increase in plasma glucose leads to insulin secretion from beta cells.
  • Insulin promotes glucose uptake by most cells and glycogen synthesis in the liver and muscle.
  • Decrease in plasma glucose as a feedback control mechanism.

Postabsorptive State

• Processes:
  • The liver engages in gluconeogenesis and glycogenolysis to maintain blood glucose levels.
  • Adipose Tissue: Engages in lipolysis, resulting in elevated plasma fatty acids, thereby sparing glucose.
  • Alpha Cells in Pancreas: Stimulate glucagon secretion in response to decreasing plasma glucose levels.