Growth Hormone
Variations in secretion: Circadian Rhythm
GH secretion varies within the 24 hour sleep-wake cycle. Peak secretion ~ 1 hr after onset of deep sleep
is GH the magic pill, the fountain of youth?
GH supplements to middle aged males did not enhance muscle mass and strength gains over placebo effect when both groups preformed weight training exercises. (so no.)
GH stimulated by:
GHRH, Ghrelin (secreted in fasting situations by cells in stomach. tells hypothalamus to release GHRH), Protein Nutritional Deficiency, Decrease in FFA and glucose, fasting or caloric restriction, puberty, exercise, stage IV sleep, physiologic stress, estrogens and androgens
Growth Promoting Hormones
Growth Hormone, Insulin, Thyroid Hormones, Testosterone, Estrogen
Secretion of hGH
Growth hormone releasing hormone (GHRH) induces synthesis and stims secretion of GH. Somatostatin inhibits synthesis and secretion. Pulsatile release of GHRH causes similar pulsatile release of GH. Somatostatin inhibits effect of GHRH on somatotrophic cells.
Gigantism
abnormally large growth due to an excess of growth hormone during childhood, before the bone growth plates have closed. (excess IGF-1 and GH in blood)
Acromegaly
disease characterized by excess growth hormone and IGF-1 levels in blood. (means large extremities)
GH inhibited by:
somatostatin, increased blood glucose, increased blood FFA, administration of exogenous GH, obesity, senescence, somatomedins
Synthesis of hGH
synthesized in anterior pituitary cells called somatotrophs (make up 40-50% of cells in anterior pituitary). Hormone is stored in large granules within somatotrphic cells.
Actions of GH & Somatomedins (IGF): Growth Promoting Effects
- require normal insulin levels and adequate carbo and protein 1. GH-IGF stim linear growth in bone by stim all aspects of chondrocyte function - increased amino acid uptake, protein synthesis, collage, chondroitin sulfate, cell size and number total bone mass and mineral content increased by GH, GH-IFG is potent stim of osteoblastic activity 2. hypertrophy and hyperplasia of visceral organs - see increased growth of hearth, lung, liver, pancreas, GI tract, adrenal glands -GH sensitizes the gonads to LH and FSH during puberty
Actions of GH & Somatomedins (IGF): Metabolic Effects
1. Increase protein synthesis (synergistic action with insulin): - increased AA transport into cells - stim of transcription and translation - inhibit protein catabolism 2. mobilization of fatty acids and utilization of fatty acids: -stims release of fatty acids from adipose cells and therefore see increase in plasma FFA and ketoacids -increases oxidation of fatty acids by body cells 3. Overall effect on carbo metabolism is to decrease utilization of glucose, therefore elevate blood glucose - elevated GH causes decreased entry of glucose into cells - GH is a diabetogenic hormone because it increases insulin resistance, excess GH can cause metabolic disturbances similar to what is observed in type 2 diabetes mellitus - stimulates glucose production in liver
Hormones that Inhibit Growth
Cortisol (glucocorticoid)
Somatomedin
Insulin-like-growth-factor (IGF). stimulates GH. secreted by liver
Variations in secretion: life span
plasma levels GH greatest in children and peak in puberty, lowestest in senescence
What does human growth hormone (hGH) do?
plays pivotal role in postnatal growth and development. Helps maintain lean body mass and bone density. Helps raise blood glucose and stimulate lipolysis and fat utilization during fasting.
Variations in secretion: sex differences
secretion greater in females, estradiol stims GH secretion with a peak just before ovulation, testosterone stims GH secretion in males