Production of human growth hormones is modulated by many factors, including
stress, exercise, nutrition, sleep and the growth hormone itself. However,
its primary controllers are two hypothalamic hormones.
Growth Hormone Releasing Hormone (GHRH) is a peptide that stimulates both the synthesis and secretion of human growth hormone.
Somatostatin (SS) is a peptide produced by several tissues in the body, including the hypothalamus. Somatostatin inhibits human growth hormone release in response to GHRH and to other stimulatory factors, such as low blood glucose concentration.
Growth Hormone Secretion is also a part of a negative feedback loop involving IGF-1. High blood levels of IGF-1 lead to decreased secretion of human growth hormones not only by directly suppressing the lactotroph, but by stimulating release of somatostatin from the gypothalamus.
Hgh Human Growth Hormone also feeds back to inhibit GHRH secretion and probably has a direct inhibitory effect on secretion from the lactotroph.
Integration of all the factors that affect human growth hormone synthesis and secretion lead to hgh cycles that are release in a pulsating fashion. Basal concentrations of hgh growth hormone in blood is very low. In children and young adults, the most intense period the hgh cycle is shortly after the onset of sleep in stages III and IV.
Several neurohormonal processes also regulate human growth hormone and the hgh cycles. Central cholinergic stimulation increases the hormone's release by reducing secretion of somatostatin. B-Adrenergic agonists increase somatostatin release, thereby inhibiting growth hormone secretion. Under normal physiologic conditions, thyroid-releasing hormones inhibits release of growth hormone by increasing somatostatin activity, whereas hypothyroidism increases its secretion. These regulatory effects suggest that b-adregenic blockers and possibly, amiodarone, which can induce a hypothyroid state, may facilitate the secretion of human growth hormone.
Other mechanisms affecting human growth hormone deficiency and release are glucocorticoids and metabolic substrates. Exogenously administered gluco-corticoids inhibit secretion of hypothalamic levels. Cachectic states marked by hypoglycemia, low circulating free fatty acid concentrations, and high circulating amino acid concentrations stimulate release
The Hgh Human Growth Hormone exerts its effect by binding to a single-chain plasma transmembrane glycoprotien receptor. The highest concentration of hgh receptors are found in the liver, adipose tissue, heart, kidneys, intestine, lungs, pancreas, cartilage, and skeletal muscle. Human growth hormone circulates bound to plasma proteins known as growth hormone binding proteins GHBP) At physiologic concentrations, approximately 50% of Hgh Growth Hormone is protein bound. Clearance occurs rapidly through the kidneys, liver, or target tissue, with an average half-life of 10-20 minutes.
Part
I of this article - What is Hgh?
Part
II of this article - Understanding the Three theories of hgh research
