Endocrinology/Objectives/Lecture 3
From PhysioWiki
Somatotropic axis
NAME the hypothalamic hormones controlling growth hormone (GH) secretion. EXPLAIN where they are produced.
| Hormone | Origin |
|---|---|
| GHRH | Ventromedial and arcuate nuclei |
| Somatostatin | Periventricular nucleus |
DESCRIBE the types of feedback that control GH secretion.
| Hormone | Effect |
|---|---|
| GHRH |
|
| SS |
|
| GH |
|
| IGF-1 |
|
USLF: ultrashort-loop feedback
SLF: short-loop feedback
LLF: long-loop feedback
DESCRIBE the physiological actions of GH on bone, muscle, fat, and carbohydrate.
Bone (via IGF-1):
- Increase protein, RNA, and DNA synthesis
- Increase hypertrophy and hyperplasia
- Net result is increase in organ size and function
Muscle (via GH and IGF-1):
- Decrease gluclose uptake
- Increase amino acid uptake
- Increase protein synthesis
- Net result is to increase lean body mass
Fat (via GF):
- Decrease glucose uptake
- Increase lipolysis
- Net result is to decrease adiposity
Carbohydrate (via IGF-1):
- Increase gluconeogenesis (mainly in liver)
- Decrease glucose uptake by adipose and muscle tissue
- Net result as a hyperglycemic hormone is to increase plasma glucose levels
DEFINE acromegaly, gigantism (giantism), dwarfism, and Laron dwarfism.
Acromegaly is the increase in growth and deposition of circumferential bone and soft tissue as a result of increased postpubertal GH (such as from a postpubertal pituitary tumor).
Gigantism is an increase in long bone growth due to a prepubertal increase in GH (such as from a prepubertal pituitary tumor).
(Note that a pituitary tumor present pre- and postpubertally can lead to both acromegaly and gigantism in a single individual.)
Dwarfism is a decrease in stature and size. There are two types, pituitary and Laron dwarfism. Pituitary dwarfism results from inadequate production and secretion of GH. Laron dwarfism results from a lack of functional GH receptors. Individuals with Laron dwarfism may have normal or elevated GH levels; elevated GH results from a lack of long-loop negative feedback by IGF-1 on GH secretion.
DESCRIBE the role of a pituitary adenoma secreting GH in causing disease.
The effects of pituitary adenomas on growth depend on the age of the individual. Prepubertal pituitary adenomas result in gigantism while postpubertal pituitary adenomas result in acromegaly.
Since GH also has high sequence homology to prolactin (the difference is that GH has two disulfide bridges, while prolactin has three), high GH levels may cause milk production by binding prolactin receptors in mammary glands. (Hence bovine growth hormone injections into cows.)
(Read book)
DEFINE insulin-like growth factor (IGF-1, IGF-2). DESCRIBE where they are produced and their actions.
IGF-1, also called somatomedin C, is a peptide hormone functionally resembling insulin. It decreases the uptake of glucose in adipocytes and muscle, while increasing glucose uptake by other tissues. IGF-1 is produced in the liver.
IGF-2 is produced by the fetal liver and other tissues and has hematopoetic activity in addition to its insulin-like effects.
DESCRIBE the effects of stress on GH secretion.
Acute stress promotes GH secretion while chronic stress inhibits GH secretion.
DESCRIBE the relationship of estrogen and testosterone to GH secretion.
Testosterone maximally stimulates GH and SS secretion, resulting in high plasma peaks and low troughs in the pulsatile secretion of GH. Estrogen also stimulates GH and SS, but not as readily as testosterone. Thus, plasma GH peaks are not as high in response to estrogen as they are in response to testosterone; similarly, the troughs are higher. In addition, estrogen results in irregular pulsatile GH secretion compared to testosterone-mediated GH secretion.
