Growth Hormone

GROWTH HORMONE (GH)/SOMATOTROPIN

• Polypeptide.
• Synthesized & secreted by somatotropes.
• Subtype of acidophilic cells in adenohypophysis.

PHYSIOLOGICAL EFFECTS:

• Two major action –
• Stimulation of skeletal growth.
• Regulation of metabolism.

1. STIMULATION OF SKELETAL GROWTH:

• Mediated by “Somatomedins”/“Insulin-like Growth Factors” (IGF).
• Synthesized in liver.
• Somatomedin (IGF-1) stimulates proliferation of chondrocytes & osteocytes.
• Increases chondroitin sulfate deposition in cartilage.
• Increases ossification of newly formed cartilage.

CONDITIONS ASSOCIATED:

1. GH deficiency in early life –

• Causes dwarfism (reduced height).
• Yet, associated with Normal body proportion.

2. GH excess in early life –

• Leads to gigantism.

3. GH excess in adulthood –

• Results in Acromegaly.

2. REGULATION OF METABOLISM:

2a. On Protein metabolism:

• Predominantly anabolic effects on skeletal & cardiac muscle.
• By promoting amino acid transport into cells & Increasing protein synthesis.

2b. On Carbohydrate & Fat metabolism:

• Effects complicated by,
• Anti-insulin effect of GH.
• Insulin-like effects of somatomedins.

2c. Anti-insulin effects of GH – 

• Decreased peripheral glucose utilization.
• Increased gluconeogenesis.
• Hyperglycemia.
• Lipolysis.

Conditions associated:

• “Insulin-Resistant Diabetes mellitus” –
• Caused by GH excess, (mainly by anti-insulin effects of GH).

2d. Insulin-like effects of somatomedins/IGF – 

• Anti-lipolytic activity.

3. GH on kidneys:

• Promotes renal reabsorption of Ca2+, Na2+, K+& Cl–. 

REGULATION:

• Regulated by hypothalamic GHRH.
• GH secreted in pulsatile fashion throughout life.
• Elevated secretion rate immediately after birth & at puberty.
• Large bursts of secretion at night during deep sleep onset.

FACTORS INFLUENCING GH SECRETION –

1. Stimuli increasing GH secretion –

• Hypoglycemia.
• Exercise.
• Fasting.
• Protein meals.
• Aminoacids (like arginine).
• Stress.
• Glucagon.
• Pyrogen.
• Lysin vasopressin.
• L-dopa & α-adrenergics.
• Estrogen & androgens.
• 2-deoxyglucose.

2. Stimuli decreasing GH secretion:

• REM sleep.
• Glucose.
• Somatostatin.
• Cortisol.
• FFA.
• GH itself decreases
• IGF- 1.
• Medroxyprogesterone.

Exam Question

GROWTH HORMONE

• Effect on skeletal growth mediated by “Somatomedins”/”Insulin-like Growth Factors” (IGF)
• GH, through somatomedin (IGF-1), stimulates proliferation of chondrocytes & osteocytes, resulting in increased chondroitin sulfate deposition in cartilage.
• Growth hormone excess in adulthood results in acromegaly.
• Anti-insulin effects due to the direct effect of GH include decreased peripheral utilization of glucose, increased gluconeogenesis & hyperglycemia. 
• Insulin-like effects due to somatomedins(IGF) include “Anti-lipolytic” activity.
• GH is secreted in a pulsatile fashion.
• Large bursts of secretions occur at night during the onset of deep sleep.
• Stimuli increasing GH secretion – Exercise, hypoglycemia, fasting & stress.
• Stimuli decreasing GH secretion – REM sleep, Somatostatin & Cortisol.
• GH deficiency leads to “delayed fusion of epiphysis” – Ie., Proportionate dwarfism.
• Short stature, secondary to growth hormone deficiency is associated with “Normal body proportion”.