6:58 ص
Unknown
Growth Hormone and Prolactin Family 004
 |
| water fasting weight loss |
GR, a hormone with structural similarity to
prolactin, is released from thes omatotrophs, an abundant (50%) cell type in
the anterior pituitary. GR is released in pulsatile bursts, with the majority
of secretion occurring hocturnally in association with slow-wave sleep. (40)
stimulated release of GH is inhibited by
somatostatin, a peptide synthesized in most brain regions, predominantly in the
periventricular nucleus, arcuate nucleus, and ventromedial nucleus of the
hypothalamus. Somostatin is also produced in peripheral organs, including the
endocrine pancreas, where it also plays a role in the inhibition of hormone
release. Axons from somatostatin neurons run caudally through the hypothalamus
to form a discrete pathway toward the midline that enters the median eminence.
GH induces physiologic effects on its target cells
directly by activating the GH receptor and indirectly by stimulating the
synthesis and secretion of IGF-I. IGF-I is a small peptide (about 7.5 kD)
structurally related to proinsulin that mediates several of the anabolic and
mitogenic effects of GH in peripheral tissues. GH stimulates longitudinal
growth by increasing the formation of new bone and cartilage. The growth
effects of GH are not critical during the gestational period, but begin
gradually during the first and second years of life and peak at the time of
puberty. Before the epiphyses in long bones have fused, GH stimulates
chondrogenesis and widening of the cartilaginous epiphysial plates, followed by
bone matrix deposition .(40)
1.
Prolactin :
Prolactin is a polypeptide hormone
synthesized and secreted by lactotrophs in the anterior pituitary gland. The
lactotrophs account for approximately 15-20% of the cell population of the
anterior pituitary gland. However, this percentage increases dramatically in
response to elevated estrogen levels, particularly during pregnancy. Prolactin
levels are
higher in females than in males, and the role of prolactin in male physiology
is not completely understood. Plasma concentrations of prolactin are highest
during sleep and lowest during the
waking hours in humans(41).
The physiologic effects of prolactin are
mediated by the prolactin receptors which are found in the mammary gland. The
main physiologic effects of prolactin are stimulation of growth and development
of the mammary gland, synthesis of milk, and maintenance of milk secretion.
Prolactin stimulates glucose and amino acid uptake and synthesis of the milk
proteins casein and -lactalbumin, the milk sugar lactose, and milk fats by the
mammary epithelial cells. Prolactin modulates immune cell function through endocrine
mechanisms. Finally, prolactin can act as a cytokine because it is released by
cells of the immune system and regulates the lymphocyte responses by both
paracrine and autocrine mechanisms.(41)
Posted in: 
0 التعليقات:
إرسال تعليق