Melanocyte Stimulating Hormone (MSH)

 

 

Melanocyte Stimulating Hormone (MSH)

•    Secreted by Pars inermedia (intermediate lobe) of pituitary

•    Occurs in three different forms (α, β, and γ MSH)

•    Biosynthesis:

– MSH is the cleavage product of a large precursor peptide called pro-opiomelano cortin (POMC)

•    Protease enzyme cleave POMC into ACTH and βlipotropin

•    ACTH is further cleaved to form MSH

 

•    Mechanism:

•    Act by binding the G-protein coupled melanocortin 1 receptor (MC1R) on melanocytes

– Increases cAMP levels through adenylyl cyclase

•    Functions of MSH:

•    Stimulate the production and release of melanin from melanocytes in skin & hair (melanogenesis)

–  Darkens the skin

•    α-MSH acts on hypothalamus

–  Suppresses appetite

–  Arouse sexually

 

 

Hormones of Posterior Pituitary

•    Two hormones are secreted from posterior pituitary

–  Antidiuretic Hormone (ADH) or Vasopressin

–  Oxytocin • Chemistry:

•    Both ADH and oxytocin are nonapeptides

–  Consist of 9 aminoacids

•    Oxytocin differs from vasopressin at 3^rd and 8^th amino acid residues

•    Disulfide linkage is present between 1^st and 6^th cysteine

 

•    Both hormones are synthesized in neurosecretory neurons in hypothalamus and stored in posterior pituitary

–  Released independent of each other

                                                    

•    Regulation of secretion:

•    ADH is regulated by osmoreceptor of hypothalamus and barroreceptors of heart

–  Any increase in plasma osmolarity stimulate its release

•    Oxytocin is regulated by neural impulses from nipples

•    Mechanism of Action:

•    ADH stimulates the production of cAMP through adenylyl cyclase

–  cAMP promotes water reabsorption

•    Inhibitors of adenylyl cyclase (Ca²⁺) inhibit ADH

•    Oxytocin act through G-protein coupled oxytocin receptors (OTR) that increase intracellular Ca²⁺ through phospholypase-C pathway

•    Biochemical Functions:

•    Antidiuretic Hormone (ADH)/ Vasopressin:

•    Primarily concerned with the water balance in the body

–  Water re-absorption leads to the formation of concentrated (hypertonic) urine

•    Low volume 

•    High specific gravity

•    High concentration of Na⁺, Cl^-, PO₄^3- and urea

•    Reabsorbed water increase blood volume and in turn increase blood pressure

–  It can also cause vasoconstriction as well as glycogenolysis by increasing intracellular Ca²⁺

•    The failure of secretion of ADH or defect in the receptors of target cells is described as diabetes insipidus

–  A situation in which patient urinates excessively just like in diabetes mellitis (Polyuria)

•    Diabetes insipidus is characterized by high volumes of very dilute urine (20-30L/day)

–  This may cause:

•    severe dehydration

•    Na⁺ depletion that may lead to electrolyte imbalance

–  The overall symptoms may include drowsiness, irritibility, nausea, vomiting, convulsions, stupor or coma

•    Oxytocin:

•    Primary function of oxytocin is the contraction of smooth muscles

–  Uterine smooth muscles to induce labour

–  Myoepithelial smooth muscles to cause milk ejection

•    Galactobolic effect

•    The number of receptors as well as secretion is increased by estrogen while decreased by progesterone