Endocrine System - March 24, 2025

Endocrine System Overview

• Basic Concepts

• Hormones trigger responses by binding to receptors.

• Signaling mechanisms involved in endocrine functions.

Types of Hormones

• 1. Amines

• Small, water-soluble (hydrophilic) molecules.

• Derivatives of amino acids tyrosine or tryptophan.

• 2. Proteins/Peptides

• Also water-soluble (hydrophilic) molecules.

• Examples: Insulin, Glucagon.

• 3. Steroids

• Derived from cholesterol; mostly lipid-soluble (hydrophobic).

• Examples: Cortisol, Testosterone, Estradiol.

Hormone Actions and Mechanisms

• Hydrophilic Hormones (Amines, Proteins)

• Bind to receptors on the plasma membrane.

• Trigger intracellular signaling pathways (often involve second messengers).

• Activation of second messenger pathways leads to cellular responses.

• Example: Epinephrine stimulates acute stress response.

• Thyroid Hormones

• Exception: Thyroxine (T4) binds to intracellular receptors, transported through membrane by transporters.

• Steroid Hormones

• Bind to intracellular receptors (cytosol or nucleus).

• Affect gene transcription (upregulate or downregulate), requiring more time to elicit responses.

Mechanism of Action

• Non-Steroidal (Hydrophilic) Response Mechanism

• Hormone binds to receptor on membrane, activating G-proteins.

• G-protein activates adenylyl cyclase, converting ATP to cAMP, acting as a second messenger.

• Can lead to activation or deactivation of enzymes, influencing cell function.

• Steroid Hormone Mechanism

• Lipophilic, pass through plasma membrane.

• Hormone-receptor complex enters nucleus, binds to DNA, affecting gene expression.

• Response Rates

• Hydrophilic hormones: quick response.

• Steroid hormones: slower, as they involve gene transcription and protein synthesis.

Synthesis and Regulation

• Hydrophilic Hormones

• Stored in secretory vesicles until stimulation triggers release via calcium-mediated exocytosis.

• Steroid Hormones

• Synthesized and released immediately; not stored.

• Feedback Mechanisms

• Maintain hormone levels homeostatically.

• Hormones released at specific times ensure proper physiological responses; excess hormones can be detrimental.

Example: Thyroid Regulation

• Pathway

• Hypothalamus secretes TRH (Thyrotropin-releasing hormone).

• TRH stimulates anterior pituitary to release TSH (Thyroid stimulating hormone).

• TSH triggers thyroid gland to release thyroxine, which regulates metabolism and homeostasis.

• Negative Feedback

• Elevated TSH and thyroxine levels inhibit TRH release from hypothalamus, reducing further synthesis.

Insect Hormonal Regulation

• PTTH (Prothoracicotropic hormone): Released by insect brain, stimulates prothoracic glands to release ecdysteroids (like ecdysone) for molting.

• Juvenile Hormone: Regulates larval characteristics; its decrease signals the end of juvenile stages.

Human Endocrine System

• Major glands include:

• Hypothalamus: Master gland, regulates endocrine function via the pituitary gland.

• Pituitary Gland: Two parts - anterior (true endocrine) and posterior (extension of hypothalamus).

Anterior Pituitary

• Synthesizes hormones; regulates various physiological processes.

Posterior Pituitary

• Stores hormones produced in hypothalamus:

  • Oxytocin: Influences childbirth and lactation.

  • ADH (Antidiuretic hormone): Regulates water balance.