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Luteinising hormone

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Luteinising hormone (LH) is a hormone produced by gonadotroph cells of the anterior pituitary gland. Like other hormones of the pituitary gland, it is packaged in large secretory vesicles that are secreted into the blood by a process of calcium-dependent exocytosis. Gonadotrophs also produce another hormone, follicle-stimulating hormone (FSH); the synthesis and secretion of LH and FSH are regulated differentially. LH secretion is regulated by the actions of LH-releasing hormone (LHRH, also known as gonadotrophin-releasing hormone, GnRH). LHRH is produced in the hypothalamus by neuroendocrine neurones that are mainly located in the preoptic area. The LHRH neurones project axomns to the median eminence, where LHRH is secreted from nerve endings into the capillary vessels of the hypothalamo-hypophysial portal circulation. These vessels carry the LHRH to the anterior pituitary gland where it activates gonadotrophs by interacting with LH receptors.

Physiology

In both males and female mammals, LH is essential for reproduction.

In females, LH is secreted in pulses throughout the ovarian cycle, but the frequency and amplitude of the pulses vary with changing levels of estrogen and progesterone. At the begimnning of the cycle (at the time of menstruation in women), FSH is secreted in large amounts and initiates follicular growth. This developing follicles produce estrogen in increasing quantities, and this feeds back on the hypothalamus to progressively suppress the secretion of FSH and LH (negative feedback). However when estrogen concentrations have been high enough for long enough, this triggers a profound stimulation of LH secretion, mainly as a result of triggering a surge of LHRH release. This surge triggers ovulation and initiates the conversion of the residual follicle into a corpus luteum that, in turn, produces progesterone to prepare the endometrium for implantation.

After the menopause, LH secretion is elevated as a result of the loss of negative feedback from estrogens; its effects on bloodfolw are responsible for the "hot flushes" that some women experience at this time.


In males, pulsatile secretion of LH acts upon the Leydig cells of the testis and is responsible for the production of testosterone, an androgen that exerts both endocrine activity and intratesticular activity such as [[spermatogenesis].

Structure

LH is a glycoprotein. Each monomeric unit is a sugar-like protein molecule; two of these make the full, functional protein. Its structure is similar to those of (FSH), thyroid-stimulating hormone (TSH), and human chorionic gonadotropin (hCG). The protein dimer contains 2 polypeptide units, (alpha and beta subunits) connected by two disulfide bridges. The alpha subunits of LH, FSH, TSH, and hCG are identical, and contain 92 amino acids, but the beta subunits vary. LH has a beta subunit of 121 amino acids that is responsible for interaction with the LH receptor. This beta subunit contains the same amino acids in sequence as that of hCG, but the hCG beta subunit contains an additional 24 amino acids.

Genes

The gene for the alpha subunit is on chromosome 6q12.21. The beta subunit gene is in the LHB/CGB gene cluster on chromosome 19q13.32.


References