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Growth Hormone & Anterior Pituitary Hormone Regulation

Somatotrophic Cells & Growth Hormone (GH)

  • One of 5 anterior-pituitary cell types; most abundant.
  • Secrete Growth Hormone (GH) (a.k.a. somatotropin) — most abundant anterior-pituitary hormone.
  • GH acts on growth and metabolism.

Direct & Indirect Actions of GH

  • Indirect (growth) actions – via Insulin-Like Growth Factors (IGFs)
    • GH ➜ liver, bone, skeletal muscle, cartilage ➜ release of IGFs.
    • IGFs stimulate bone lengthening, soft-tissue hypertrophy, protein synthesis, wound healing.
  • Direct metabolic actions (energy-mobilising)
    1. Glucose-sparing effect
    • ↓ glucose uptake in most cells so neurons can continue ATP production during scarcity.
    1. Lipolysis
    • GH➜ adipocytes ➜ breakdown of triglycerides: (\text{TG}\rightarrow \text{Glycerol}+3\,\text{FA})
    • Released fatty acids oxidised for ATP.
    1. Hepatic glucose output
    • Stimulates glycogenolysis & gluconeogenesis ➜ ↑ blood glucose.
  • Net result: ample ATP for the energy-intensive process of growth.
  • Real-world illustration: Teenagers in growth spurts experience ↑ appetite to meet ATP/nutrient demand.

Stimuli That Increase GH Release

  • Hypoglycaemia (\downarrow\text{[Glucose]}).
  • ↓ blood fatty acids.
  • ↑ blood amino acids (signals protein availability).
  • Exercise, stress, deep sleep, puberty hormones, etc. (implied).

Hypothalamic–Pituitary Regulation Loop

  • Step 1 – Hypothalamus
    • Releases Growth Hormone-Releasing Hormone (GHRH) into the hypophyseal portal system.
  • Step 2 – Anterior Pituitary (Somatotrophs)
    • GHRH binds somatotrophs ➜ GH secretion.
  • Step 3 – Target Actions
    • Metabolic (direct) & Growth (via IGFs) as above.
  • Negative Feedback
    • ↑ GH & ↑ IGF levels ➜ inhibit GHRH release (hypothalamus).
    • IGFs also directly inhibit GH secretion at the pituitary.
  • Growth Hormone-Inhibiting Hormone (GHIH) / Somatostatin
    • Secreted by hypothalamus when nutrient/energy levels are adequate or GH/IGF are high.
    • Travels via the same portal system; suppresses GH release.

Key Principles & Clarifications

  • Feedback is hormone-based, not organ-based: the hormone concentrations act as the signals.
  • Typical endocrine pattern: stimulus ➜ hypothalamic releasing hormone ➜ anterior-pituitary hormone ➜ peripheral hormone ➜ negative feedback.

Quick Reference – Anterior Pituitary Hormones & Their Regulation

  • GH (Growth Hormone)
    • + by GHRH
    • – by GHIH (somatostatin), ↑GH, ↑IGF
  • TSH (Thyroid-Stimulating Hormone)
    • + by TRH (Thyrotropin-Releasing Hormone)
    • – by thyroid hormones (T₃/T₄)
  • ACTH (Adrenocorticotropic Hormone)
    • + by CRH (Corticotropin-Releasing Hormone)
    • – by glucocorticoids, chiefly cortisol
  • FSH (Follicle-Stimulating Hormone)
    • + by GnRH (Gonadotropin-Releasing Hormone)
    • – by estrogens (ovary) / inhibin (testis)
  • LH (Luteinizing Hormone)
    • + by GnRH
    • – by estrogens, progesterone, testosterone
  • PRL (Prolactin)
    • + by PRH (Prolactin-Releasing Hormone)
    • – by PIH (Dopamine) & ↑prolactin itself (autoinhibition)

Mnemonic – “TPFLAG” (Anterior-Pituitary Hormones)

  • T – TSH
  • P – Prolactin
  • F – FSH
  • L – LH
  • A – ACTH
  • G – GH

Practical/Clinical Connections

  • GH deficiency in children ➜ dwarfism; excess ➜ gigantism.
  • Adult excess ➜ acromegaly (enlarged hands, jaw, organs) owing to post-epiphyseal-plate closure.
  • Somatostatin analogues used therapeutically to suppress GH (acromegaly) or other hypersecretory states.
  • Exercise & sleep hygiene naturally augment physiologic GH spikes — exploited in paediatric endocrinology.