Chapter 18 PPT Endocrine_AR

Fundamentals of the Endocrine System

• Basic concept

  • One tissue (gland) influences another tissue (target) using chemical messengers (hormones).

  • Blood serves as the transport highway; only cells with the correct receptor ("target cells") respond.

• Endocrine vs. Exocrine

  • Endocrine glands secrete into blood; exocrine glands secrete into ducts.

  • Most hormones act system-wide, but a few have very specific local effects.

Major Endocrine Organs (anatomical list)

• Pituitary (anterior & posterior)

• Hypothalamus (neuro-endocrine “master”)

• Pineal gland

• Thyroid & Parathyroids

• Thymus

• Adrenal glands (cortex & medulla)

• Pancreas (islets)

• Gonads (testes, ovaries)

• Misc. endocrine tissue in kidney, GI tract, adipose, heart, etc.

Hormone Chemistry

• Water-soluble hormones

  • Composition: peptides (insulin, hGH, ADH, oxytocin, TSH, ACTH, FSH, LH, PRL, MSH), amines (epinephrine, norepinephrine, dopamine, serotonin, melatonin, histamine), eicosanoids (prostaglandins, leukotrienes).

  • Transport: dissolve freely in plasma, no carrier needed.

  • Cell entry: cannot cross lipid bilayer; bind surface receptors & use second messengers (e.g., cAMP).

  • Action: rapid, signal amplification.

• Lipid-soluble hormones

  • Composition: steroids (testosterone, progesterone, estrogen, cortisol, aldosterone), iodothyronines (T3, T4), calcitriol, nitric oxide (gas).

  • Transport: require plasma protein carriers.

  • Cell entry: diffuse through membrane → bind intracellular/nuclear receptors → gene transcription.

  • Action: slower onset, longer duration.

• Quick comparison

  • Water-soluble: protein-based, stored pre-formed, fast, membrane receptors.

  • Lipid-soluble: cholesterol/iodine based, synthesized on demand, slow, nuclear receptors.

Hormone Interaction Types

• Antagonistic – opposing effects (e.g., insulin vs. glucagon).

• Synergistic – combined effect greater than individual (epinephrine + thyroxine → amplified lipolysis).

• Permissive – one hormone primes tissue for another (other ovarian hormones prime LH to cause ovulation).

Receptor Regulation

• Down-regulation – chronic high hormone → ↓ receptor number → ↓ sensitivity.

• Up-regulation – chronic low hormone → ↑ receptor number → ↑ sensitivity.

Paracrine & Autocrine Signals

• Paracrine – act on neighboring cells (histamine ↑ capillary permeability).

• Autocrine – act on same cell (interleukins; prostaglandins; human pheromones even act between individuals).

Second-Messenger Cascade (water-soluble example)

• Hormone binds membrane receptor → activates G-protein → activates adenylate cyclase.

• ATP \xrightarrow{adenylate\,cyclase} cAMP (second messenger).

• cAMP activates protein kinases → phosphorylate many enzymes (signal amplification).

• Phosphodiesterase terminates signal by converting cAMP to AMP.

Hypothalamic–Pituitary (H-P) Axis

• Hypothalamus

  • Secretes releasing/inhibiting hormones into hypophyseal portal system (blood route to anterior pituitary).

  • Sends axons to posterior pituitary (neural route).

• Anterior Pituitary (Adenohypophysis) – 6 hormones

  • FSH