Anterior Lobe Hormones Flashcards
How/when is growth hormone secreted?
It is synthesized in somatotrophs when stimulated by GHRH and is very structurally similar to prolactin and human placental lactogen
Describe the secretion of growth hormone
It is secreted in a pulsatile pattern with bursts of secretion occurring approximately every 2 hrs and the largest bursts occuring within 1 hr of falling asleep (stage III and IV sleep)
What factors can help stimulate GH release?
sleep
hypoglycemia and reduced free fatty acid concentrations, as well as fasting
Exercise, stress, and puberty
ghrelin
a-adrenergic agonists and arginine
What factors can inhibit GH release?
Obesity and aging, somatostatin, GH itself
B-adrenergic agonists
pregnancy
increased glucose and free fatty acid conc
When are GH release rates highest during life?
Steadily increases from birth into puberty where there is an enormous burst induced by estrogen in females and testosterone in males followed by gradual declines throughout the rest of life
Regulation of GH release
GHRH acts directly on somatotrophs in the anterior lobe to induce transcription via Gs protein to both adenlyl cyclase AND phospholipase C (this is unique!). Meanwhile, somatostain is also secreted by the hypothalamus and inhibits somaotropin action by blocking the action of GHRH via Gi binding.
In terms of regulation, GHRH can inhibit its own secretion via an ultrashort-loop; Somatomedins (by-products, aka insulin-like growth factors) of GH action on target tissue can inhibit secretion of GH from the anterior lobe; and Both GH and somatomedins can induce somatostatin secretion from the hypothalamus
Note that the effects of GH can be due to either direct GH binding OR a result of somatomedins (particularly IGF-1) binding to insulin receptors that have intrinsic tyrosine kinase activity
How would GH affect diabetes?
GH causes insulin resistance and decreases glucose uptake by muscle and adipose (aka diabetogenic effects because they result in increased BG) leading to an increase in blood insulin levels
Growth hormone excess causes acromegaly but the effects differ if it is present before or after puberty.
If present before puberty, gigantism occurs (CHF the most common cause of death).
After puberty, more localized manifestations are seen such as increased organ size, hand and foot growth, glucose intolerance, etc.
How is prolactin made? Stimulation?
Prolactin is made from lactotrophs in the anterior lobe of the pituitary especially during pregnancy (high estrogen stimulates) and lactation upon stimulation by TRH and is inhibited by tonic levels of dopamine
NOTE: Prolactin is structurally homolgous to GH
Regulation of Prolactin
Dopamine (aka PIF) tonically inhibits prolactin in people that are not pregnant or lactating (this dopamine can come from the median eminence, posterior lobe or by nonlactotroph cells in the anterior). Prolactin also inhibits its own production by promoting synthesis of dopamine
During suckling, affarent information transmitted to the hypothalamus inhibits dopamine production
What are the roles of prolactin?
1) Breast development
2) Lactogenesis by stimulating synthesis of lactose, casein, and lipids
3) Inhibition of ovulation (females) and spermatogenesis (males) by inhibiting synthesis of GnRH (which accounts for the decreased fertility during breast-feeding)
Why does lactation not occur during pregnancy?
Although prolactin levels are high during pregnancy, high levels of estrogen and progesterone down-regulate prolactin receptors in the the breast
What could be an effect of high prolactin levels in males?
parallel inhibition of GnRH secretion can result in infertility and decreased libido
What else could increase the amount of prolactin in the body?
renal failure via reduced elimination
