Endocrine System

5.1 Mechanisms of Hormone Action

Explains hormone types, mechanisms of action, second messenger systems, and classification by target tissues.

• Endocrine system
  • Glands secrete hormones into bloodstream
  • Hormones bind to receptors on target tissues

• Peptide Hormones
  • Made of amino acids, derived from larger precursors
  • Packaged in Golgi, released via exocytosis
  • Act as first messengers, triggering second messengers (e.g., cAMP, IP3)
  • GPCRs activate/inhibit adenylate cyclase → changes cAMP → activates PKA → phosphorylates CREB
  • Rapid, short-lived effects
  • Travel freely in blood, no carrier needed
• Steroid Hormones
  • Derived from cholesterol, produced by gonads & adrenal cortex
  • Nonpolar, cross membranes
  • Receptor-hormone complexes bind DNA → transcriptional changes
  • Dimerization: 2 complexes pair
  • Slower onset, longer duration
  • Require protein carriers (specific or nonspecific)
  • Most end in -one, -ol, -oid (e.g., testosterone, cortisol)
• Amino Acid Derivative Hormones
  • Examples: E, NE, T3, T4
  • Derived from 1–2 amino acids
  • Catecholamines (E, NE): fast acting, short-lived, GPCRs
  • Thyroid hormones: slow onset, longer duration, intracellular receptors
• Classification by target tissue
  • Direct hormones: act directly on target (e.g., insulin)
  • Tropic hormones: require intermediary (e.g., GnRH → LH/FSH → gonads)
  • Tropics often originate in brain/anterior pituitary

5.2 Endocrine Organs and Hormones

Details major endocrine organs, their hormones, mechanisms, feedback loops, and physiological effects.

• Hypothalamus
  • Bridge between nervous & endocrine systems
  • Regulates anterior pituitary via tropic hormones (via hypophyseal portal system)
  • Responds to light, osmolarity, appetite, satiety
  • Negative feedback maintains homeostasis
  • Tropic hormones:
    • GnRH → FSH/LH
    • GHRH → GH
    • TRH → TSH
    • CRF → ACTH
    • PIF (dopamine) → decreases prolactin
  • Axes example: HPA axis (CRF → ACTH → cortisol; cortisol inhibits hypothalamus & anterior pituitary)
• Posterior Pituitary
  • Hormones synthesized in hypothalamus, released via axons
  • Oxytocin: uterine contractions, milk letdown, bonding (positive feedback)
  • ADH (vasopressin): increases water reabsorption in kidneys
• Anterior Pituitary
  • Tropic: FSH, LH, ACTH, TSH
  • Direct: prolactin, endorphins, GH
  • GH promotes growth, requires glucose
    • Excess in children: gigantism
    • Excess in adults: acromegaly
  • Mnemonic: FLAT PEG (FSH, LH, ACTH, TSH - Prolactin, Endorphins, GH)
• Thyroid
  • Controlled by TSH
  • Produces T3/T4 (iodinated tyrosine): ↑ metabolism, protein & fat turnover
  • Calcitonin from C cells: ↓ plasma Ca²⁺ via kidney, gut, bone
  • Disorders:
    • Iodine deficiency → hypothyroidism (cretinism in children)
    • Excess hormone → hyperthyroidism
• Parathyroid
  • 4 glands, posterior thyroid
  • PTH: ↑ blood Ca²⁺, ↑ kidney reabsorption, ↑ bone resorption, ↑ gut absorption via Vitamin D
  • Antagonistic to calcitonin
• Adrenal Cortex
  • Corticosteroids (3 S's):
    • Salt → mineralocorticoids (aldosterone): ↑ Na⁺ reabsorption, ↑ water retention, RAAS regulated
    • Sugar → glucocorticoids (cortisol, cortisone): ↑ gluconeogenesis, ↓ protein synthesis, stress response, ACTH regulated
    • Sex → cortical sex hormones (androgens): minor in females, masculinization if excess
• Adrenal Medulla
  • NS-derived, secretes E & NE (catecholamines)
  • Rapid sympathetic effects
• Pancreas
  • Exocrine: digestive enzymes
  • Endocrine: islets of Langerhans
    • Alpha: glucagon → fasting, glycogenolysis, gluconeogenesis
    • Beta: insulin → fed state, glucose uptake/storage
    • Delta: somatostatin → inhibits insulin & glucagon, ↑ by high glucose/AA
• Gonads
  • Testes → testosterone (sexual differentiation)
  • Ovaries → estrogen, progesterone (secondary sex characteristics)
• Pineal Gland
  • Melatonin → circadian rhythm regulation
• Other organs
  • GI tract: endocrine peptides
  • Kidneys: ADH, erythropoietin
  • Heart: ANP → Na⁺ excretion, volume regulation
  • Thymus: thymosin → T cell development, atrophies in adulthood

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