Hormones & Signal Transduction 5 Sections28 LessonsLifetime
The Growth Hormone
Hormones of the Anterior Pituitary
Hypothalamus produces 2 types of endocrine factors
- Hypothalamic neuropeptides
- Hypothalamic releasing factors
- The releasing factors are neurosecretions synthesized in the hypothalamus and released through the hypothalamic pituitary portal circulation.
- They have their effect on the secretion of pituitary tropic hormones.
- Secretion of hormones by Adenohypophysis (Anterior Pituitary), is under control of peptides secreted by hypothalamus.
- These can be stimulants or inhibitors.
- Their secretion is also under the feedback control of anterior pituitary tropic hormones – short loop feedback as well as target glands hormones (long feedback loop).
* “A tropin or tropic hormone is the one which influences the activities of other endocrine glands”
- Principally those involved in stress and reproduction.
- These are carried by the blood to other target gland.
- The pituitary tropins are under the positive & negative control of peptide factors from hypothalamus.
Growth hormone
- Growth hormone is a polypeptide with 191 amino acids.
- It is also known as Somatotropin.
- It is synthesized by the acidophils (somatotropic cells) of the anterior pituitary.
- The GH secretion is regulated by the balance between GHRH & GHIH (somatostatin).
- Plasma concentration of GH is < 2 ng/ml during day.
- The Secretary peak of GH occurs 3 hours after meals.
- Maximum level of GH is seen during deep sleep.
- Hypoglycemia stimulates Growth Hormone secretion & hyperglycemia suppresses it.
- Certain amino acids in blood increase Growth Hormone.
- Stress also stimulates GHRH (Growth Hormone Releasing Hormone).
- The hypothalamic GHRH stimulates Growth Hormone synthesis & release.
- Ghrelin, a peptide derived from stomach induces GHRH, directly stimulates the release of Growth Hormone.
- Somatostatin (hypothalamus) inhibits the GH secretion.
- The metabolic effect of Growth Hormone is mainly mediated by somatomedin, also known as Insulin-like Growth Factor-1 (IGF-1).
- Growth Hormone acts on receptors present in liver/skeletal muscles.
- Liver/muscles produces IGF-1 in response.
- Receptors act through the JAK – STAT pathway (will be discussed separately in pathways).
- Growth Hormone is anabolic.
- Increases the uptake of amino acids by cells; enhances protein synthesis (produces positive nitrogen balance) – insulin like effect.
- The anti-insulin effect of Growth hormone causes lipolysis – increased FFA – hyperlipidemia (energy production) & hyperglycemia.
- Overall effect is to stimulate growth of soft tissues, cartilage and bone (liver – somatomedin).
- The metabolic effect of Growth Hormone is mainly mediated by somatomedin (IGF-1).
- The growth of long bones is stimulated by this factor (IGF-1).
- The peripheral target hormone of Growth Hormone exerts feed back inhibition.
- Growth hormone needs thyroxine (hypothyroidism causes growth retardation).
- Thyroxine is essential for working of Growth Hormone receptors.
Direct effects of Growth hormone
- Increases blood glucose.
- Decreases specific amino acids from the blood.
Indirect effects of Growth hormone
- Growth of bone, cartilage & muscle.
IN THE NEXT LESSON WE WILL DISCUSS ABNORMALITIES OF GROWTH HORMONE
- Excess secretion by Growth Hormone secreting tumor before adolescence, leads to Gigantism.
- After adolescence – Acromegaly.
- Deficiency in early childhood results in pituitary Dwarfism.
- Deficiency in adults causes –NOTHING.
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