BIOL5_Ch11_Endocrine

Endocrine System Overview

• The endocrine system is a system of ductless glands that secrete hormones into the bloodstream.

• Hormones are messenger molecules that travel to distant body cells, initiating physiological responses:

  • Control enzymatic reactions (metabolism)

  • Transport ions or molecules across cell membranes

  • Influence gene expression and protein synthesis.

Components of Feedback Loops

• Homeostasis is maintained by negative feedback loops consisting of:

  • Sensor: Detects deviations from a set point.

  • Set point: Normal physiological values.

  • Integrating center: Receives information from sensors and determines responses.

  • Effectors: Produce the response based on the sensor inputs.

Major Endocrine Glands

• Ductless glands that release hormones directly into the bloodstream.

• Hormones target cells that have receptors for those hormones, affecting cell metabolism.

• Neurohormones: Secreted by specialized neurons into the blood.

Endocrine Glands and Hormones Table

Hormone Activity: Target Cell Sensitivity

• Hormones affect only target cells with specific protein receptors.

• Receptors display:

  • Specificity: Unique to particular hormones

  • High affinity: Strong attraction to hormones

  • Low capacity: Limited number of receptors present.

• Receptors are continuously synthesized and broken down to maintain hormone sensitivity.

Regulation of Protein Receptors

• Down-regulation: Occurs in high hormone concentration; decreases receptor sensitivity.

• Up-regulation: Occurs in low hormone concentration; increases receptor sensitivity.

Types of Hormones

1. Circulating Hormones: Travel through blood to distant target cells.

2. Local Hormones: Act on nearby cells (autocrine) or on adjacent cells (paracrine).

Hormonal Chemical Classification

• Lipid-soluble Hormones (hydrophobic):

  • Includes steroid hormones, thyroid hormones, and eicosanoid hormones.

  • Circulate bound to transport proteins to prevent degradation.

• Water-soluble Hormones (hydrophilic):

  • Includes amine hormones and peptide hormones.

  • Circulate freely in plasma and are not bound to proteins.

Hormone Mechanisms of Action

Lipophilic (Steroid) Hormones

• Have receptors inside target cells; affect genes directly (genomic action).

• Slow action (at least 30 minutes).

Hydrophilic Hormones

• Bind to receptors on target cell surfaces and act quickly through second messengers.

• May involve both cell surface and cytoplasmic receptors for steroids.

Mechanism of Action

• For Water-Soluble Hormones:

  • Bind to surface receptors, activating G proteins that, in turn, stimulate adenylate cyclase.

  • Converts ATP to cAMP (second messenger) which activates protein kinases.

  • Results in physiological responses via phosphorylated proteins.

Steroid Hormone Action Pathway

• Travel in blood bound to carrier proteins, dissociate to enter target cells.

• Hormone-receptor complex binds to DNA at specific sites to stimulate transcription of target genes.

Thyroid Hormone Action

• Primarily secretes T4, which converts to T3 in target cells.

• T3 binds to specific nuclear receptors that form a heterodimer influencing transcription of target genes.

Summary of Hormonal Actions

• Hormonal effects are influenced by:

  • Concentration of hormone in blood

  • Number of receptors on target cells

  • Interactions with other hormones (permissive, synergistic, antagonistic effects).