Pituitary Gland - L3 Endocrine PDF

Pituitary gland Location : in a pocket of the sphenoid bone at the base of the brain Function: coordinating center for control of many endocrine glands Parts: 2 separate endocrine organs (three in some species) Anterior pituitary: secretes TSH “Thyrotropin”; ACTH; LH and FSH “Gonadotropins”; PRL a...

Pituitary gland Location : in a pocket of the sphenoid bone at the base of the brain Function: coordinating center for control of many endocrine glands Parts: 2 separate endocrine organs (three in some species) Anterior pituitary: secretes TSH “Thyrotropin”; ACTH; LH and FSH “Gonadotropins”; PRL and GH “Somatotropin”. Proopiomelanocortin (POMC) derivatives that regulate skin pigmentation. Posterior pituitary: secretes AVP and oxytocin Rudimentary Intermediate pituitary (not in adult human): secretes melanocyte stimulating hormones α- MSH and β-MSH that originally derived from POMC. Cell Types in the Anterior Pituitary  Five types of secretory cells have been identified in the anterior pituitary by immunocytochemistry and electron microscopy.  Some cells may contain two or more hormones.  FSH, LH, and TSH are made up of α and β subunits.  The α subunit must be combined with a β subunit characteristic of each hormone for maximal physiologic activity.  The α subunit has the same amino acid composition but different carbohydrate residues in all (i.e.; produced by MCQ: Some cells may contain two or more hormones. It is notable a single gene) that some hormones are glycoproteins with α and β subunits.  The β subunits has differ structure in all that is confer Which of the following hormones does not such configuration? hormonal specificity (i.e.; produced by separate genes) A.FSH  Human chorionic gonadotropin is a glycoprotein B.GH hormone secreted by placenta also has α and β C.TSH subunits D.hCG E.LH MCQ: In many ways, pituitary gland consists of at least two separate endocrine organs that contain a plethora of hormonally active MCQ Five types of secretory cells have been identified in the substances. Which of the followings is not a hormone secreted anterior pituitary by immunocytochemistry and electron by anterior pituitary? microscopy. Which cells have the highest percentage of total A.Thyrotropin secretory cells? B.Corticotropin A. Thyrotropes C.Vasopressin B. Lactotropes D.Gonadotropin C. Corticotropes E.Somatotropin D. Somatotropes E. Gonadotropes First: hGH-N, codes for the most abundant (“normal”) form of GH; Second: hGH-V, codes for the variant form of GH Third and fourth: two code for human chorionic somatomammotropin (hCS) Fifth: is probably an hCS pseudogene.  GH (GH) that is secreted into the circulation by the pituitary gland consists of a complex mixture of 1. hGH-N 2. hGH-N derived peptides with varying degrees of posttranslational modifications, such as glycosylation 3. A splice variant of hGH-N that lacks amino acids 32–46  These various peptides share a broad range of functions and may occasionally exert actions in opposition to one another.  hGH-V and hCS are primarily products of the placenta, and as a consequence are only found in appreciable quantities in the circulation during pregnancy  Plasma protein binding: 50%  Normal plasma level: 3ng/ml (bound + free)  Metabolism: rapidly (partly in the liver)  Half-life: 6-20min  Secretion rate: 0.2-1.0 mg/day in adult The GH receptor:  It is a 620-amino-acid protein with a large extracellular portion, a transmembrane domain, and a large cytoplasmic portion.  It is a member of the cytokine receptor superfamily.  GH has two domains that can bind to its receptor (GHR) when it binds to one receptor, the second binding site attracts another, producing a homodimer (dimerization is essential for receptor activation).  GH has widespread effects in the body. Like insulin, GH activates many different intracellular signaling cascades. Mechanism of action:  When GH bind its receptors it activates the JAK2–STAT pathway.  Binding of GH to GHR  activate JAK2  phosphorylate STATs  STATs migrate to nucleus  gene transcription  JAK2 is a Janus cytoplasmic tyrosine kinases.  STATs (signal transducers and activators of transcription) are cytoplasmic transcription factors.  JAK–STAT pathways are known also to mediate the effects of prolactin and various other growth factors Growth effect  Protein deposition by chondrocytes and osteogenic cells  Enhance the reproduction rate of chondrocytes and osteogenic cells  Convert chondrocytes into new osteocytes and causing deposition of new bone  Increase body protein contents and decrease body fat contents The protein content of the body is increased, and the fat content is decreased Effect on Protein – anabolic action  Fall in blood urea nitrogen (i.e.; decrease amino acid catabolism; +ve nitrogen balance)  It enhances amino acid transport through the cell membranes for protein synthesis  Stimulate soluble collagen synthesis (indexed by increasing excretion of 4-hydroxyproline) Effect on fats – increase lean body mass  It increases the release of fatty acids from adipose tissue (lipolysis) and hence decrease body fat  It enhances the use of fatty acids for energy production “increase metabolic rate”  Decrease plasma cholesterol Effect on electrolytes  Increase plasma level of phosphorus  Increase GI absorption of Ca2+  Reduce Na+ and K+ excretion by kidney (independent on adrenal gland) Effect on carbohydrates  GH is a diabetogenic hormone (it increase hepatic gluconeogenesis)  Decrease ability of pancreas to respond to insulinogenic stimuli such as glucose and arginine “inhibits β cell function”  It has anti-insulin action in muscle “on carbohydrate” but with similar insulin effects on protein “anabolic effects” Somatomedins (IGF-I and IGF-II)  Stimulated the incorporation of sulfate into cartilage and stimulated collagen formation.  IGF-I also called somatomedin C Structure:  Both are closely related to insulin; EXCEPT that their C chains are not separated and they have an extension of the A chain called the D domain. Synthesis:  Both are synthesized in liver, cartilage and many other tissues Plasma protein binding  Both are tightly bound to plasma proteins (this prolong the half-life of IGF-I)  IGF-I-binding protein-3 (IGFBP-3) accounting for 95% of the binding in the circulation (5 different binding proteins are found)  This prolong the half-life of IGF-I Receptors  IGF-I receptors are very similar to the insulin receptor and probably uses similar or identical intracellular signaling pathways.  IGF-II receptors has a distinct targeting of acid hydrolases and other proteins to intercellular organelles. Secretion  Secretion of IGF-I is independent of GH before birth BUT is stimulated by GH after birth.  IGF-II is largely independent of GH. Plasma conc.  IGF-I concentration in plasma rises during childhood and peaks at the time of puberty, THEN declines to low levels in old age. Effect  IGF-I has pronounced growth stimulating activity from childhood to adulthood (i.e.; after birth)  IGF-II plays a role in the growth of the fetus before birth  If IGF-II is overexpressed in human fetuses, several organs (tongue, other muscles, kidneys, heart, and liver), develop out of proportion to the rest of the body.  In adults, the gene for IGF-II is expressed only in the choroid plexus and meninges