Two of the three somatotrophic hormones that exist are from the hypophysis: human growth hormone (HGH) and prolactin. The other hormone originates in the placenta and is known as human placental lactogen or chorionic somatomammotropin. The three hormones are simple peptide chains with disulfide bonds among the chains.
The HGH molecule, secreted by the anterior lobe of the hypophysis by somatotroph cells, is predominantly a simple polypeptide chain with a molecular weight of 20,000 and is produced in much lesser quantity through alternative overlapping of human growth hormone mRNA. Approximately 4 to 10% of the hypophysis gland's net weight is HGH (10 to 15 mg per gland. Less than 5% of the contents of the hypophysis is released each day and it is believed that a considerable amount is destroyed right there in the hypophysis before it is secreted.
Plasmatic levels in a fasting adult at rest are generally less than 1 mglml, and occasionally reach the lowest sensitivity levels. HGH levels are slightly higher in women than in men; this difference stands out even more during exercise, when increase in both sexes.
HGH levels are very high at the end of fetal life and during the first few days after birth, frequently reaching levels higher than 50ng/ml. These levels rapidly decrease during the following weeks, stabilizing in quantities slightly higher than those observed in the adult. It is secreted episodically or in a pulsating manner throughout a 2 to 4 hour period, its levels occasionally reaching up to 50 ng/ml especially in young men. The acute pulsating secretions of HGH are less common after the forth decade of life and rarely occur after the sixth.
The average life of human growth hormone is between 20 to 25 minutes. 12 HGH secretion in a normal male has been estimated to be aproximately 350 pm2/24 hr, and in the premenopausic female it has been found to be about 500 VLm2/24 hr.
Secretion is regulated primarily by two hypothalmic hormones: one of them is the releaser known as GHRH which stimulates HGH secretion by the somatotroph cells in the anterior lobe of the hypophysis; the other hormone is somatostatin, also known as SRIH ("Somatotroph Release Inhibiting Factor"), which inhibits the liberation of human growth hormone. Each pulse is generated by a combination of an increase in GHRH and a decrease in SRIH.
At the same time hypothalmic GHRH and SRIH release is regulated by biogenic amines that are derived from neurosecretory neurons in the Central Nervous System (CNS). Somatostatin secretion is regulated primarily by cholinergic mechanisms (for example, the agonists inhibit and the antagonists stimulate the production of SRIH). On the other hand, GHRH increases through the action of a-adrenergic neurons, dopaminergic, and is suppressed by the b-adrenergic tracts.
Under physiological conditions, the hypophysis secretes aproximately eight peaks or pulses per day and basal levels are maintained very low between pulses. In children and young adults, approximately 50 to 75 percent of the daily production of HGH occurs at night, especially in the first hours following the onset of deep sleep. Secretion is also stimulated by exercise, emotional stress, foods high in protein content, rapid beginning of hypoglycemia, as well as prolonged fasting.
On the other hand, it is known that in old age there is a decrease in pulse production. The preceeding is associated with the fact that, in advanced age, the number and size of somatotroph cells decreases" and also the release by GHRH stimulation is lessened. There are studies in animals that strongly suggest the concept that an increase in the secretion of somatostatin is responsible for the decrease in advanced age.
Continued Part 6: Metabolic Human Growth Hormone