Endocrine Physiology Basics

Basic Concepts of Endocrine Regulation

• Endocrine physiology studies homeostasis, using hormones as mediators
• Hormones, derived from the Greek word "horman" (meaning to set in motion), are soluble factors that regulate various aspects of homeostasis.
• The endocrine system is a distributed network comprised of glands and circulating messengers, often influenced by the central nervous system (CNS) and/or the autonomic nervous system.
• It isn't defined by anatomical boundaries.

Outline

• Introduction
• Evolution of Hormones & Their Actions on Target Cells
• Hormone Secretion, Synthesis & Processing
• Hormone Transport in the Blood
• Hormone Action
• Principles of Feedback Control

Introduction

• Endocrine physiology focuses on maintaining homeostasis.
• Hormones are soluble factors regulating homeostasis.
• The word "hormone" comes from the Greek word "horman," meaning "to set in motion."

Evolution of Hormones & Their Actions on Target Cells

• Hormones include steroids, amines, and peptides.
• Peptide hormones are the most prevalent.
• Many hormones are grouped into families based on structural similarities and receptor similarities.
• The diversity of hormones increases with increasing complexity of life forms, reflecting the increased challenges of providing homeostasis in complex organisms.
• This diversity is reflected in the evolution of hormone structure and receptors.
• Peptide hormones are often heterodimers, containing a common alpha chain with varying beta chains produced from homologous genes.

Hormone Action

• Steroids and thyroid hormones diffuse across cell membranes and act primarily within the cell.
• These hormones bind to cytoplasmic proteins known as nuclear receptors to form a dimer.
  • The receptor-ligand complex translocates to the nucleus, where it binds to DNA triggering either increased or decreased gene transcription in the target tissue.

Hormone Secretion (Synthesis & Processing)

• Hormone synthesis is regulated by their chemical nature.
• Peptide hormone synthesis is predominantly controlled at the transcriptional level.
• Amine and steroid hormone synthesis is indirectly controlled by regulating the production of key synthetic enzymes and substrate availability.
• Peptide hormones are initially synthesized as pre-pro-hormones, cleaved into pro-hormones, and ultimately into final hormones following proteolytic cleavage.
  • The specific cleavage site and resultant hormones vary depending on the specific proteases involved.
• Hormone precursors are usually inactive.
• Synthesis often occurs in specific vesicles designed to export the hormones and associated fragments from the cell.
• Peptide hormone synthesis is often under translational control within the cells.
  • Some hormones include binding sites on their regulatory genes for other hormones allowing other hormones to regulate their synthesis in a specific way.

Hormone Secretion

• Peptide hormones are stored in secretory vesicles (granules) within secreting cells.
• A signal activates the exocytosis of stored granules.
• Steroid hormones are secreted via diffusion, not stored beforehand.
• Secretion is regulated by altering activity of enzymes and carrier proteins in hormone synthesis.
  • Steroid synthesis from cholesterol occurs in mitochondria using a specific carrier (STAR) as a first step. The carrier expression is regulated by genes.
  • Trophic hormones and cytokines bind to receptors on steroid-secreting cells.
  • Activation of these receptors results in expression of steroid acute regulatory protein (StAR) that traffics cholesterol into the mitochondria for synthesis.

Hormone Transport in the Blood

• Hormone stability affects their half-life and has therapeutic implications, for hormone replacement therapy.
• Only free hormones can cross cell membranes to mediate feedback regulation.
• Plasma carriers provide a reservoir of inactive hormones and prevent degradation or uptake.

Hormone Action

• Catecholamines and most peptide hormones are soluble and transported dissolved directly in plasma or through membrane transport systems.

• Steroid hormones circulate bound to carrier proteins (e.g., sex hormone-binding globulin (SHBG)) which are produced by the liver.

  • Carrier proteins increase the solubility of lipid-based hormones and prevent loss of hormones in the kidneys. They serve as a reservoir from which free hormones are released in equilibrium,

Principles of Feedback Control

• Feedback regulation ensures target cell responsiveness to hormonal action controls the activating endocrine organ.
• Feedback loops can be either negative or positive.
• Positive feedback increases or continues the stimulus that initiated the release. This is seen in parturition.
• Negative feedback involves dampening of the stimulus.
• The hypothalamus often works with the pituitary gland to control release of hormones affecting various body systems.

Clinical Implications

• Measuring hormone levels during various stages (e.g., circadian rhythms, after meals) can help diagnose hormone deficiencies or excesses

Explore the fundamental concepts of endocrine regulation and physiology with this quiz. You'll learn about hormones, their evolution, actions, synthesis, transport, and the principles of feedback control. Test your understanding of how these soluble factors contribute to homeostasis.