Orris
Reading File
Finding Sources
Finding Sources
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Reading File
Reading File
Reading File
Loading Image
Loading Image
Finding Sources
Reading File
I now have all the core content needed. Let me compile the comprehensive answer.

Growth Hormone: Secretion, Actions, Regulation, and IGF-1

1. Source and Chemistry

Growth hormone (GH), also called somatotropin, is a 191-amino acid single-chain polypeptide with two internal disulfide bridges. It is synthesized and secreted by somatotrophs scattered throughout the anterior pituitary. The GH gene resides on chromosome 17 and is a member of a gene family that includes prolactin (16% homology) and human placental lactogen (80% homology). GH exists in at least three molecular forms; the predominant circulating form is the 22-kDa monomer.
  • Costanzo Physiology 7th ed., p. 411
  • Medical Physiology (Boron & Boulpaep), p. 1458

2. Pulsatile Secretion

GH is secreted in a pulsatile pattern, with secretory bursts occurring approximately every 2 hours throughout the day. The largest and most consistent burst occurs within the first hour of slow-wave sleep (stages III and IV). Between pulses, basal GH levels are very low or undetectable.
Graph - GH secretion over 24 hours:
GH secretion during sleep - pulsatile pattern with major peaks at night
The graph shows low, irregular GH pulses during daytime waking hours, with dramatically elevated, high-amplitude secretory bursts during the sleep period (shaded blue). The highest peaks occur around midnight to 2 AM (slow-wave sleep).
  • Medical Physiology, p. 1460

3. Regulation of Secretion

GH secretion is governed by the interplay of three hypothalamic/peripheral regulators and multiple feedback loops.

3a. GHRH (Growth Hormone-Releasing Hormone)

  • Produced exclusively in neurons of the arcuate nucleus of the hypothalamus
  • A 40- or 44-amino acid peptide (full biological activity in first 29 amino acids)
  • Binds specific receptors on somatotrophs; signals via Gs protein → adenylyl cyclase (↑cAMP) and phospholipase C (↑IP3/Ca²+)
  • Net effect: stimulates both synthesis and secretion of GH

3b. Somatostatin (SRIF - Somatotropin Release-Inhibiting Factor)

  • Also released from the hypothalamus (periventricular nucleus)
  • Binds its own receptor on somatotrophs, coupled to Gi protein → ↓cAMP
  • Blocks the action of GHRH on somatotrophs, inhibiting GH secretion
  • The pulsatile pattern of GH reflects the alternating dominance of GHRH surges vs. somatostatin withdrawal

3c. Ghrelin

  • A 28-amino acid peptide produced mainly by oxyntic (P/D1) cells of the stomach
  • The endogenous ligand for the GH secretagogue (GHS) receptor on pituitary somatotrophs and hypothalamic neurons
  • Potently stimulates GH release, stimulates appetite, and regulates glucose metabolism
  • Its physiological interaction with GHRH and somatostatin is complex; it acts synergistically with GHRH
StimulatoryInhibitory
Hypoglycemia (↓blood glucose)Hyperglycemia (↑blood glucose)
↑Amino acids (e.g., arginine)↑Free fatty acids
Fasting / starvationObesity
ExerciseSomatostatin
Stress (trauma, fever)Somatomedins (IGF-1)
Slow-wave sleep (III & IV)GH itself (short-loop)
Puberty (estrogen, testosterone)Senescence
α-Adrenergic agonistsβ-Adrenergic agonists
  • Costanzo Physiology 7th ed., p. 413; Basic Medical Biochemistry 6e, p. 1482

4. GH-IGF-1 Axis Flowchart

GH-IGF-I axis: hypothalamus → GHRH/somatostatin → anterior pituitary → GH → liver/cells → IGF-I, with feedback loops
The diagram shows: stimuli (↓blood glucose, ↑amino acids, sleep, stress, exercise) → brain → hypothalamus releases GHRH (+) and somatostatin (-) → anterior pituitary somatotrophs release GH → GH stimulates liver and other cells to produce IGF-I → IGF-I feeds back negatively to inhibit both GHRH (at hypothalamus) and GH release (at pituitary), and stimulates somatostatin release.

5. Feedback Regulation

Three feedback loops control GH secretion (all negative):
  1. Ultrashort loop: GHRH inhibits its own release from the hypothalamus
  2. Short loop: GH itself inhibits somatotrophs directly (short-loop feedback), and stimulates somatostatin secretion from the hypothalamus
  3. Long loop (IGF-1-mediated):
    • Circulating IGF-1 acts directly on somatotrophs to suppress GH secretion (via tyrosine kinase receptor, independent of cAMP)
    • IGF-1 suppresses GHRH release from the hypothalamus
    • IGF-1 increases somatostatin secretion from the hypothalamus
The net result: any rise in GH → rises in IGF-1 → shut down further GH secretion at both pituitary and hypothalamic levels.
  • Medical Physiology, p. 1641-1643; Costanzo Physiology, p. 415

6. Actions of Growth Hormone

GH has both direct (short-term, acute) and indirect (long-term, growth-promoting via IGF-1) actions.

6a. Direct Metabolic Actions (Anti-Insulin / "Diabetogenic")

GH acts via a tyrosine kinase-associated receptor (JAK2 family) - a type I cytokine receptor. One GH molecule bridges two GHR monomers, causing dimerization and JAK2 activation → STAT transcription factors + MAP kinase/AKT pathways.
Target TissueDirect Effect
Muscle↓ Glucose uptake; ↑ amino acid transport; ↑ lean mass; ↑ N₂ retention
Adipose↑ Lipolysis; ↑ hormone-sensitive lipase; ↓ lipogenesis; ↓ glucose transport
Liver↑ Gluconeogenesis; insulin resistance
OverallInsulin resistance ("diabetogenic effect")
Chronic GH excess (e.g., acromegaly) → frank diabetes mellitus via persistent insulin resistance.

6b. Growth-Promoting Actions via IGF-1

Long-term growth-promoting effects of GH are largely indirect, mediated by IGF-1 (somatomedin C). GH stimulates hepatocytes and other tissues to synthesize and secrete IGF-1.
IGF-1 then acts on target tissues (bone, cartilage, muscle, viscera) through IGF-1 receptors (structurally similar to the insulin receptor; intrinsic tyrosine kinase activity with autophosphorylation).
Key growth-promoting effects:
  • Stimulates chondrocyte proliferation at epiphyseal growth plates → linear bone growth (only before epiphyseal closure)
  • After epiphyseal closure → periosteal bone thickening (as in acromegaly)
  • Stimulates protein synthesis and positive nitrogen balance
  • Promotes organ growth (muscle, heart, liver, kidneys, GI tract)
  • Cell proliferation and differentiation (autocrine/paracrine)
  • Costanzo Physiology, p. 415-416; Medical Physiology, p. 1658

7. Age-Related Changes in GH Secretion

Life StageGH Secretion
BirthNormal size; GH not essential in utero
Infancy-childhoodGradually increases; required for linear growth
PubertyEnormous surge (estrogen in females, testosterone in males → increased pulse frequency + amplitude) → pubertal growth spurt
AdulthoodDeclines to stable lower level
SenescenceFurther decline; lowest levels; ↓ pulse frequency and magnitude (somatopause)
GH deficiency from birth → pituitary dwarfism. GH excess from childhood → gigantism. GH excess after epiphyseal closure → acromegaly (no lengthening of long bones, but coarsening of features, enlargement of hands/feet, joint deformity, hypertension, cardiomegaly).

8. Note on IGF-1 (Insulin-Like Growth Factor-1)

Structure: IGF-1 (somatomedin C) is a 70-amino acid single-chain polypeptide structurally homologous to proinsulin, with three domains (A, B, C) - unlike insulin, the C-domain is not cleaved from IGF-1. It also has a unique D-domain.
Source: The liver is the primary source of circulating IGF-1 (endocrine actions). IGF-1 is also produced locally in bone, muscle, kidney, and cartilage (autocrine/paracrine actions).
Regulation of synthesis: GH is the principal driver of IGF-1 synthesis. Because GH is secreted pulsatilely, IGF-1 integrates and smooths GH secretion over time - plasma IGF-1 levels are therefore more stable than GH and serve as a better marker of overall GH status. IGF-1 levels also depend on nutritional state (fall in malnutrition even with normal GH).
Transport: >95% of circulating IGF-1 is bound to IGF-binding proteins (IGFBPs), especially IGFBP-3. IGFBP-3 itself is GH-dependent. This greatly prolongs the half-life of IGF-1 (hours vs. minutes for free GH).
Actions of IGF-1:
  • Growth promotion: chondrocyte and osteoblast proliferation, linear growth
  • Protein anabolism: ↑ amino acid uptake and protein synthesis
  • Metabolic: acute insulin-like effects (glucose uptake, anti-lipolysis) - can cause hypoglycemia at high doses
  • Feedback: inhibits GH secretion (both at pituitary and hypothalamus, as above)
  • Stimulates organ growth throughout the body
Clinical relevance: IGF-1 measurement is used clinically as a screening test for acromegaly (elevated) and GH deficiency (low). The IGF-1 level reflects mean 24-hour GH secretion far more accurately than a single random GH level (which could be at a nadir).
  • Medical Physiology (Boron & Boulpaep), p. 1658-1675
  • Costanzo Physiology 7th ed., p. 414-416
  • Basic Medical Biochemistry 6e, p. 1481-1485
  • Henry's Clinical Diagnosis, p. 610-612
This is a shared conversation. Sign in to Orris to start your own chat.