Regulation of Energy Metabolism and Growth: Endocrine System (Chapter 21c)

The Endocrine System: Regulation of Energy Metabolism and Growth

Overview

This section explores the hormonal regulation of metabolism during the absorptive and postabsorptive states, focusing on the roles of insulin, glucagon, and other regulators such as the sympathetic nervous system and epinephrine. Understanding these mechanisms is essential for grasping how the body maintains energy balance and responds to metabolic challenges.

Regulation of Absorptive and Postabsorptive Metabolism

Hormonal Regulation

• Insulin: The primary hormone of the absorptive state, secreted by β cells in the pancreatic islets of Langerhans. It promotes glucose uptake and energy storage.

• Glucagon: The primary hormone of the postabsorptive state, secreted by α cells in the pancreatic islets. It promotes the breakdown of energy stores and glucose sparing.

• Other regulators: The sympathetic nervous system and epinephrine also influence metabolic states, especially during stress or exercise.

The Role of Insulin

Insulin: Structure and Function

• Peptide hormone produced by β cells in the pancreatic islets of Langerhans.

• Promotes glucose uptake by body cells.

• Stimulates synthesis of energy storage molecules such as glycogen and triglycerides.

Stimuli for Insulin Secretion

• Direct stimulation:

  • Increased plasma glucose concentration

  • Increased plasma amino acids concentration

• Feedforward mechanisms:

  • Presence of food in the GI tract

  • Parasympathetic nervous system activation

  • Glucose-dependent insulinotropic peptide (GIP)

  • Glucagon-like polypeptide 1 (GLP-1)

• Inhibition during postabsorptive state:

  • Sympathetic nervous system activity

  • Epinephrine

Actions of Insulin

• Anabolism: Builds up energy stores by promoting glycogen and triglyceride synthesis.

• GLUT4 transporter insertion: Increases glucose uptake by most cells (except liver and CNS cells).

• Promotes glucose metabolism for energy production.

• Increases amino acid transport into most cells.

• Decreases catabolism (breakdown of molecules).

Mechanism of Glucose-Stimulated Insulin Secretion

• High plasma glucose enters β cells via GLUT2 transporters.

• Glucose is converted to pyruvate through glycolysis, then enters the mitochondria.

• Oxidative phosphorylation produces ATP.

• ATP binds to ATP-sensitive K+ channels, causing them to close and blocking K+ efflux.

• Membrane depolarization opens voltage-gated Ca2+ channels; Ca2+ influx triggers insulin exocytosis.

Equation:

• Glycolysis: $\text{Glucose} \rightarrow \text{Pyruvate}$

• ATP production: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

The Role of Glucagon

Glucagon: Structure and Function

• Peptide hormone produced by α cells in the pancreatic islets of Langerhans.

• Promotes breakdown of energy storage molecules (glycogenolysis, lipolysis).

• Promotes glucose sparing: body cells utilize non-glucose energy sources.

• Antagonist of insulin: counteracts insulin's effects.

Stimuli for Glucagon Secretion

• Postabsorptive state:

  • Sympathetic nervous system activity

  • Epinephrine

  • Decreased plasma glucose concentration

  • Increased plasma amino acids concentration

  • GIP and GLP-1

• Inhibition during absorptive state:

  • Increased plasma glucose concentration

Table: Factors Affecting Insulin and Glucagon Release

Factor	Effect on Insulin Secretion	Effect on Glucagon Secretion
↑ Plasma [glucose]	Increase	Decrease
↑ Plasma [amino acids]	Increase	Increase
↑ Plasma [GIP]	Increase	Increase
↑ Parasympathetic activity	Increase	Increase
↑ Sympathetic activity	Decrease	Increase
↑ Plasma [epinephrine]	Decrease	Increase

Negative Feedback Control of Blood Glucose Levels

Blood Glucose Ranges

• Normal: 70–110 mg/dL

• Hyperglycemia: >140 mg/dL (may indicate diabetes mellitus)

• Hypoglycemia: <60 mg/dL

Blood glucose levels are maintained primarily by the actions of insulin and glucagon through negative feedback mechanisms.

Absorptive State

• Most cells use GLUT4 transporters for glucose uptake.

• Liver and muscle perform glycogenesis to store glucose as glycogen.

Postabsorptive State

• Liver: gluconeogenesis and glycogenolysis to release glucose.

• Adipose tissue: lipolysis releases fatty acids for energy.

• Sympathetic stimulation promotes glucose sparing.

Effects of Increased Amino Acid Concentration

• Occurs after high-protein, low-carbohydrate meals.

• Stimulates insulin release (increases amino acid and glucose uptake).

• Can be dangerous if carbohydrate intake is low, potentially causing hypoglycemia.

• Also stimulates glucagon release, counteracting insulin to maintain blood glucose.

Effects of Epinephrine and Sympathetic Nervous Activity on Metabolism

Role in Postabsorptive State

• Suppresses insulin release.

• Stimulates glucagon release.

• Augments glucagon action under normal conditions.

• Becomes critical during stress (fight or flight, exercise, disease, tissue repair).

Epinephrine Actions

• Liver: Stimulates glycogenolysis and gluconeogenesis (also direct sympathetic stimulation).

• Skeletal muscle: Stimulates glycogenolysis (no direct sympathetic stimulation).

• Adipose tissue: Stimulates lipolysis (also direct sympathetic stimulation).

Diabetes Mellitus

Types and Pathophysiology

• Type 1 Diabetes Mellitus (IDDM): Insulin deficiency due to autoimmune destruction of β cells.

• Type 2 Diabetes Mellitus (NIDDM): Deficient insulin target cell response (insulin resistance).

Primary Sign: Hyperglycemia

• Normally, high glucose inhibits α cell glucagon secretion.

• Decreased insulin or response reduces α cell permeability to glucose, falsely signaling low glucose.

• α cells increase glucagon release, worsening hyperglycemia.

• Consequences:

  • Hyperlipidemia and ketosis

  • Metabolic acidosis (ketoacidosis)

  • Glucosuria (glucose in urine)

Example: Negative Feedback in Blood Glucose Regulation

• After a meal, increased blood glucose stimulates insulin release, promoting glucose uptake and storage.

• During fasting, decreased blood glucose stimulates glucagon release, promoting glucose production and release.

Additional info: The notes above expand on the original slides by providing definitions, mechanisms, and clinical relevance for each hormone and metabolic state, ensuring a comprehensive understanding suitable for exam preparation.