A Podcast on hCG

Transcript:

hCG, or the human chorionic gonadotropin is a hormone that is produced by the syncytiotrophoblast. The pituitary analog of hCG is produced in the pituitary gland. This is a part of a fertilized egg after conception. hCG is a glycoprotein that is made up of 237 amino acids. It is heterodimeric, with an alpha subunit identical to the LH (luteinizing hormone) and a beta structure unique to hCG. It behaves much like FSH and LH except for the fact it in not inhibited by rising levels of progesterone.

The hCG location of synthesis occurs in the egg/placenta. It occurs through a process that is taken by a simple neurohormone pathway. This is where a stimulus triggers a sensory neuron on either the hypothalamus or posterior pituitary and a neurosecretory cell sends signals to the blood vessel, which then targets effectors and creates a response.  In a simple endocrine pathway, a stimulus is received by a receptor protein in an endocrine cell which then goes through the blood vessel, to the target effectors, and then produces the response. In simple neuroendocrine pathways, the process contains aspects of both neurohormone and endocrine pathways. A stimulus goes through the first part of the neurohormone pathway by triggering the sensory neuron, which then goes through its process to reach the blood vessel. Once it is in the blood vessel, instead of going to the target effectors, it goes to an endocrine cell, where it then takes the normal route of the endocrine pathway.

It is peptide and water soluble, opposed to something like cortisol, which is lipid-soluble. Lipid soluble substances can enter a cell by dissolving in the lipid portion of the membrane and diffusing through it. Water soluble, or lipid-insoluble, substances cannot pass through membranes by dissolving in the lipids. There has to be water-filled channels or pores spanning the membrane through which these substances can be allowed to diffuse.  Since hCG is a water soluble hormone, its receptors are embedded in the plasma membrane, opposted to lipid-soluble hormones which have intracellular receptors.

The luteinizing hormone/choriogonadotropin receptor (LHCGR) is a transmembrane receptor found mostly in ovaries and testis, but may also by found in organs such as the uterus and breasts. The receptor interacts with LH and hCG and represents a G protein-coupled receptor. Its activation is necessary for the hormonal functioning during reproduction.

In males, hCG helps restore and maintain testosterone production in the testes by mimicking LH and causing a trigger for production and release of testosterone. If hCG is used too long and in high doses, like some male athletes might along with steroids, the rise in natural testosterone will inhibits its own production through negative feedback. Negative feedback occurs when the rate of the process decreases as the concentration of the product increases. Negative feedback controls the rate of a process to avoid accumulation of a product. Positive feedback occurs when the rate of a process increases as the concentration of the product increases. The rate of a process will continuously accelerate under positive feedback as long as substrate is available and the product is not consumed by some other process.

This hormone, however, is more related to pregnancy and the process of reproduction. The hCG interacts with LHCG receptors of the blastocyst, which is a fertilized ova, and helps in maintaining the corpus luteum, a temporary endocrine structure in female mammals, during the beginning of pregnancy. It allows the corpus luteum to secrete progesterone during the first trimester, which would be a positive feedback loop. Because of its very negative charge, hCG may repel immunce cells of the mother, which protects the fetus. Also, because it is similar to LH, hCG can also be utilized to induce ovulation in women or testosterone production in men.

Sources:

http://www.course-notes.org/Biology/Outlines/Chapter_45_Hormones_and_the_Endocrine_System

http://freezingblue.com/iphone/flashcards/printPreview.cgi?cardsetID=144760

http://en.wikipedia.org/wiki/Gonadotropin-releasing_hormone

http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter46/positive_and_negative_feedback.html

http://en.wikipedia.org/wiki/Human_chorionic_gonadotropin

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexHormones.html

http://www.jbc.org/content/270/34/20020.full

http://www.dls.ym.edu.tw/bio2/05082007Endocrine%20system.pdf

http://www.vusd.org/webpages/rnordell/ap.cfm?subpage=24381