Endocrine System: Hormone Coordination, Cascades, and Mechanisms

Hormone Basics and Target‐Cell Interactions

• Each cell type can express zero, one, or many hormone receptors.
• Hormones excel at coordinating responses that span multiple physiological systems.
• Example: epinephrine (adrenaline) and norepinephrine (nor-adrenaline)
  • Synthesised in the adrenal medulla (inner layer of the adrenal gland).
  • Adrenal glands are pyramid-shaped structures perched atop each kidney (two in total).
  • Classic “adrenaline rush” effects:
  • Increased heart rate (targets cardiac myocytes).
  • Faster breathing (acts on respiratory centers & bronchial smooth muscle).
  • Cutaneous vasoconstriction → facial flushing.
  • Vasodilation of vessels supplying skeletal muscle (more blood, O₂, glucose).
  • Liver stimulated to convert glycogen → glucose and release it (glycogenolysis).
  • One hormone pair can synchronise cardiovascular, respiratory, muscular and metabolic systems for the fight-or-flight response.

Hormone Pairs & Negative Feedback Examples

• Many hormones occur in antagonistic pairs that keep a variable near a set-point.
• Blood-glucose control
  • Insulin: lowers blood glucose by promoting cellular uptake.
  • Glucagon: raises blood glucose by stimulating glycogen breakdown and gluconeogenesis.
• Blood-calcium control (Calcitonin vs Parathyroid Hormone, PTH)
  • Calcitonin (thyroid-derived) released when $[Ca^{2+}]_{blood}$ is high.
  • Bone: stimulates osteoblasts ⇒ Ca²⁺ deposition.
  • Large intestine: decreases Ca²⁺ absorption.
  • Kidneys: decreases Ca²⁺ reabsorption ⇒ more urinary loss.
  • PTH (from four parathyroid glands on thyroid’s posterior surface) released when $[Ca^{2+}]_{blood}$ is low.
  • Bone: osteoclast activation ⇒ Ca²⁺ release.
  • Intestine: increases Ca²⁺ absorption (via vitamin D activation).
  • Kidneys: increases Ca²⁺ reabsorption.

Exocrine, Endocrine, Paracrine Quick Check

• Exocrine secretion: released into ducts or the gut lumen → technically “outside” the body.
• Neurotransmitters (e.g.
  acetylcholine) = paracrine signals; act on adjacent neurons, not distant targets.
• Endocrine secretion: hormone enters interstitial fluid → bloodstream → distant targets.

Structural Classes of Vertebrate Hormones

• Water-soluble (hydrophilic)
  • Amino-acid derivatives (e.g.
    epinephrine, norepinephrine, dopamine).
  • Peptide hormones (few AAs) & protein hormones (longer chains): insulin, growth hormone, ADH, oxytocin.
  • Freely dissolve in plasma; no carrier needed.
• Lipid-soluble (hydrophobic)
  • Steroid hormones: cortisol, aldosterone, testosterone, estradiol, progesterone.
  • Derived from cholesterol; require carrier proteins to travel in blood.

Mechanisms of Action

• Water-soluble hormones
  • Receptors embedded in plasma membrane.
  • Binding → second-messenger cascades (e.g.
    cAMP, IP₃) that alter existing proteins.
  • Rapid onset, short duration.
  • Example: ADH binds collecting-duct cells → cascade inserts aquaporins → water reabsorption rises.
• Lipid-soluble hormones
  • Diffuse through lipid bilayer; cytoplasmic or nuclear receptors.
  • Hormone–receptor complex acts as a transcription factor → up- or down-regulates gene expression.
  • Slower onset, longer-lasting effects.

Determinants of Hormone Effect Magnitude

1. Quantity released (secretion rate).
2. Carrier-protein availability for steroids.
3. Receptor density & sensitivity on target cells.
   • Case study: Prairie vs Montane voles
     • Both secrete similar oxytocin levels.
     • Prairie voles possess dense oxytocin (and vasopressin) receptors in brain areas governing affiliation ⇒ monogamy & paternal care.
     • Montane voles have sparse receptors ⇒ promiscuity & minimal care.
4. Hormone persistence / clearance rate (half-life).
   • Metabolism by target cells, liver, kidneys.
   • Excreted unchanged or as metabolites in urine; pregnancy tests detect urinary hCG metabolites.

Endocrine System Organisation & Cascades

• Unlike the digestive “tube,” endocrine tissues are scattered; linked only by bloodstream-borne chemical messages.
• About 50 % of endocrine glands participate in multi-step cascades.
• General cascade architecture (negative feedback):
  1. Hypothalamus releases a Releasing Hormone (RH).
  2. RH travels via hypophyseal portal vessels to anterior pituitary.
  3. Anterior pituitary secretes a Tropic Hormone (TH).
  4. TH circulates to a peripheral endocrine gland.
  5. Peripheral gland produces an End-product Hormone (EH) that exerts physiological effects.
  6. Rising EH levels inhibit both pituitary & hypothalamus (classical negative feedback).

Hypothalamus–Pituitary–Adrenal (HPA) Axis

• Step-by-step
  1. Hypothalamus: Corticotropin-Releasing Hormone (CRH).
  2. Anterior Pituitary: Adrenocorticotropic Hormone (ACTH).
  3. Target: Adrenal cortex (outer adrenal layer).
• Adrenal-cortex outputs
  • Mineralocorticoids (e.g.
    aldosterone)
  • Kidney: adjust $Na^+$/water reabsorption.
  • Affect blood volume and pressure.
  • Glucocorticoids
  • Mammals: cortisol; birds/reptiles: corticosterone.
  • Sustained stress response:
    • ↑ Blood glucose via gluconeogenesis.
    • Suppresses digestion, immune activity, reproductive function.
    • Evolutionarily adaptive for acute threats ("tiger" scenario) but deleterious when chronic.
  • Sex steroids (androgens, estrogens, progesterone) produced in smaller quantities by adrenal cortex; major production is gonadal (HPG axis, covered later).
  • Influence growth, puberty, and secondary sexual characteristics.

Posterior Pituitary Neurohormones

• Hypothalamic neurons extend axons into posterior pituitary; hormones stored in axon terminals until release.
• Antidiuretic Hormone (ADH) a.k.a. vasopressin
  • Water-soluble peptide.
  • Kidney collecting ducts: inserts aquaporins → concentrates urine, conserves water.
  • Also acts as a CNS neuromodulator for social bonding in some species.
• Oxytocin
  • Triggers uterine contractions & milk ejection in mammals.
  • CNS roles: pair bonding, trust, prosocial behaviour ("love hormone").
  • Receptor density strongly correlates with social patterns (vole example).

Hormone Clearance & Diagnostic Applications

• Breakdown routes
  • Target-cell enzymatic degradation (esp. intracellular steroids).
  • Hepatic metabolism → bile or blood.
  • Renal filtration → urinary excretion.
• Half-life influences physiological impact.
• Clinical test: detection of human chorionic gonadotropin (hCG) metabolites in urine confirms early pregnancy.

Key Numerical / Structural Facts to Remember

• $2$ adrenal glands (each atop a kidney).
• $4$ parathyroid glands embedded posterior to the thyroid.
• Peptide / protein hormones are generally water-soluble; steroid hormones are lipid-soluble.
• Cascade shorthand: $\text{Hypothalamus}\xrightarrow{RH}\text{Pituitary}\xrightarrow{TH}\text{Peripheral Gland}\xrightarrow{EH}\text{Target Tissues}$.

Practical & Ethical Implications

• Chronic psychological stress can maintain elevated cortisol, predisposing to hypertension, immunosuppression, infertility, and metabolic disorders.
• Manipulating oxytocin pathways (e.g.
  intranasal sprays) raises ethical questions about altering social or emotional behaviour.
• Environmental endocrine disruptors (e.g.
  synthetic estrogens) may mimic or block steroid hormones, altering wildlife development and human health.

Concept Connections / Prior Knowledge

• Thermoregulation: hypothalamus also houses the body’s thermostat—shows multifunctionality of this brain region.
• Renal physiology: ADH mechanism reinforces earlier lessons on water balance and collecting-duct permeability.
• Signal transduction: membrane-bound receptor cascades mirror G-protein pathways discussed in cell-biology units.