Growth Hormone Physiology
Please see separate tutorial for Growth Hormone Pathologies.
Overview
Secretion:
Growth hormone, aka, somatotropin. (somatotrophin) is secreted by anterior
pituitary cells called somatotrophs.
Growth hormone secretion is pulsatile, and is highest during sleep.
Growth hormone levels vary throughout life; they begin to fall after adolescence.
Negative feedback loops regulate growth hormone secretion.
Somatotrophs make up more than half of the anterior pituitary, and growth hormone is the most abundantly secreted anterior pituitary hormone.
Direct & Indirect Effects:
Growth hormone is responsible for growth of almost all tissues, whether directly or indirectly via insulin-like growth factor 1 (IGF-1).
Hypothalamus and
Pituitary gland.
Neurosecretory cells originate in the arcuate nucleus of the hypothalamus, and their axons terminate on capillaries of the hypothalamic-pituitary portal system.
Within the anterior pituitary, there are various collections of endocrine cells;
– Somatotrophs produce and release GH; the nearby capillaries deliver hormone products to the blood supply.
Growth Hormone Release:
The hypothalamus secretes growth hormone releasing hormone (GHRH) into the neurosecretory cells.
When it reaches the anterior pituitary, GHRH stimulates somatotropin release of Growth Hormone (GH), which then travels in the blood to tissues throughout the body.
Growth effects: it increases cell growth, proliferation, and differentiation throughout the body.
Direct effects on growth:
– Increases bone length and muscle mass (growth hormone promotes protein deposition).
Indirect effects on growth:
– Diabetogenic effects: growth hormone increases glycogen breakdown in the liver to increase blood glucose, which can be used as fuel by growing body tissues.
– Growth hormone also increases insulin-like growth factor 1 release from the liver, which in turn promotes growth and differentiation of various tissues. IGF-1 is sometimes referred to as somatomedin-C
– Glucose-sparing effects: it increases lipolysis, and decreases glucose uptake by skeletal muscle and adipose tissue, which frees up energy for growth of tissues.
Growth Hormone Regulation
At the hypothalamus:
Sleep, hypoglycemia, and stress trigger the release of growth hormone releasing hormone.
Age and obesity are associated with reduced GHRH release, and, via negative feedback loops, glucose, IGF-1, and growth hormone also reduce GHRH release.
At the anterior pituitary:
Release of growth hormone is inhibited by growth hormone inhibitory hormone (aka, somatostatin, which is released by the hypothalamus), and via negative feedback signals from IGF-1 and Growth hormone (notice that IGF-1 and growth hormone provide negative feedback at both the hypothalamus and the anterior pituitary).
Additional factors that promote and inhibit growth hormone secretion:
Stimulates Secretion:
Growth hormone releasing hormone
Decreased blood glucose
Decreased blood free fatty acids
Increased blood arginine (an amino acid)
Protein deficiency and starvation
Stress and excitement (including a variety of experiences, including exercise and trauma)
Testosterone and estrogen
Deep sleep
Ghrelin, which is a growth hormone secretagogue (GHS) that is produced mainly in the stomach.
Inhibits Secretion:
Growth hormone inhibiting hormone (aka, somatostatin)
Increased blood glucose
Increased free fatty acids
Exogenous growth hormone
Insulin-like growth factor 1 (via negative feedback)
Aging
Obesity
Pause to recognize that low blood glucose, low levels of free fatty acids, and increased levels of amino acids in the blood stimulate growth hormone release, which ultimately reverses each of these states to maintain homeostasis.
Pathologies:
Growth Hormone Defects
Growth Hormone Excess
For full references, please see our tutorial on growth hormone physiology and pathology.
