Hormones

Hormones and Endocrinology

  • Definition: The study of homeostatic chemical adjustments and the activities accomplished by hormones.

Neural and Endocrine Regulation

  • Hormones influence ion movement down electrochemical gradients, affecting membrane potential.

  • Some chemicals function as both hormones and neurotransmitters.

  • For physiological regulation, a chemical must have target cells with specific receptor proteins.

Chemical Classification of Hormones

1. Amines

  • Derived From: Tyrosine and tryptophan

  • Examples: Hormones from thyroid, medulla, and pineal glands

2. Polypeptides and Proteins

  • Examples: Insulin, oxytocin, growth hormone

3. Glycoproteins

  • Long polypeptides bound to carbohydrates

  • Examples: Follicle-stimulating and luteinizing hormones

4. Steroids

  • Source: Lipids derived from cholesterol (secreted by adrenal glands and gonads)

  • Examples: Testosterone, cortisol

Effective Plasma Concentration

  • Influenced by:

    1. Rate of secretion

    2. Metabolic activation or conversion

    3. Binding to plasma proteins

    4. Rate of removal by inactivation and excretion

  • Hormones are inactivated within minutes to days.

  • Plasma concentrations can vary daily, regulated by light/dark cycles (e.g., cortisol, melatonin, testosterone).

Target Cell Responsiveness

  • Varies with the number of receptors.

  • Down Regulation: High concentrations of hormone over a long period decrease receptor count.

  • Synergistic Effects: Hormones can work together, where hormone A increases receptors for hormone B.

  • Antagonistic Effects: Hormone A can decrease receptors for hormone B.

Release of Hormones

  • Hormones are chemical signals released from ductless glands at distant sites.

  • Paracrine Effect: Hormones released at adjacent sites.

  • Types:

    • Hydrophilic hormones are carried by blood or exist in free form.

    • Hydrophobic hormones are bound to plasma proteins.

Endocrine Gland Characteristics

  1. Can produce multiple hormones and have various receptor types (e.g., anterior pituitary).

  2. The same hormone may be secreted by different glands.

  3. A single hormone can target multiple cell types with differing functions (e.g., insulin in fat, liver, and muscle).

  4. Target cells can possess receptors for multiple hormones.

  5. Some organs/tissues have both endocrine and non-endocrine functions (e.g., ovaries).