POSTERIOR PITUITARY GLAND POSTERIOR PITUITARY HORMONES Posterior pituitary

POSTERIOR PITUITARY GLAND

POSTERIOR PITUITARY HORMONES § Posterior pituitary gland releases 2 hormones: 1. Antidiuretic hormone (ADH) / arginine vasopressin (AVP). 2. Oxytocin § Produced in hypothalamic nuclei: Supraoptic nucleus (ADH) Paraventricular nucleus (Oxytocin)

POSTERIOR PITUITARY HORMONES • Both are transported slowly along the hypothalamohypophyseal tract in combination with carrier protein called neurophysin, to the nerve endings in the posterior pituitary gland where they are stored. • ADH is stored but not synthesized in posterior pitutary. • Half life- 16 -20 minutes

ANTIDIURETIC HORMONE

VASOPRESSIN- ACTION • V 1 A receptor Vascular smooth muscle Action: Vasoconstriction. • V 1 B receptor Location: Pituitary corticotrophs Action: Enhanced ACTH release • V 2 receptor Baso-lateral membrane of distal nephron. Action: Antidiuresis via production and action on acquaporin 2

VASOPRESSIN- ACTION • ADH is vasoconstrictor. ADH binds to V 1 receptors on vascular smooth muscle to cause vasoconstriction- hence, the name "vasopressin".

ANTIDIURETIC HORMONE- ACTION

REGULATION OF ADH • Increased osmotic pressure – stimulates osmoreceptors by shrinkage • Impulses travel via hypothalamo-hypophysial tract • Reflex secretion of ADH. • Hyposmolality- expansion of osmoreceptors- decreases ADH secretion

REGULATION OF ADH • Hypovolemia- most potent stimulus than hyperosmolality 10% decrease in volume- sufficient to release ADH. • Pain, post operative state, emotional stress Increases ADH secretion- direct action on supraoptic nucleus. • Liver and kidney diseases – Increased ADH secretion because ADH is mainly inactivated by liver and kidney.

APPLIED ASPECTS Diabetis insepidus • Complete/ partial failure of ADH secretion. • Decreased renal water reabsorption • Polyuria- 3 -20 L/day • Pure water loss- dilute urine • Urine osmolality < 300 mosm/L (DM- >1200 mosm/L) • Stimulate thirst- increased water intake- Polydypsia.

APPLIED ASPECTS • Types : Neurogenic (central, or cranial) Problem in Hypothalamus or Posterior pituitary gland. Head injury, pituitary tumors Treatment: ADH. Nephrogenic (Peripheral) Resistance of V 2 receptors in collecting ducts of the kidneys. Acquired- drugs like rifampicin, aminoglycoside, lithium Genetic- X linked recessive defect - V 2 receptor gene deficient Autosomal defect- acuaporin gene is deficient.

APPLIED ASPECTS Syndrome of inappropriate ADH secretion(SIADH) Excessive secretion of ADH Head injury Ectopic production of ADH- CA lung, pancreas , ovary and bladder. • Neurologic diseases: Multiple sclerosis, brain abscess, meningitis. • Drugs: desmopressin, chlorpropamide. • •

OXYTOCIN

OXYTOCIN • Synthesized in paraventricular nuclei of hypothalamus. • Transported to the posterior pituitary.

MILK EJECTION REFLEX • Neurohumoral reflex • Oxytocin contraction of myoepithelial cells- expelling milk into ducts. • Conditioned reflex- sight, sound and thought of child.

Remember: Oxytocin is concerned with ejection of milk, while prolactin is concerned with production of milk.

PARTURITION REFLEX • Stretching of cervix- stimulate paraventricular nucleus – secretes oxytocin • Contraction of uterus • Expelling of fetus.

APPLIED ASPECTS • Preparation of oxytocin (oxytocics) are used for induction of labor. • Oxytocin infusion facilitates progression of labor. • Injected immediately after delivery- prevents excessive postpartum haemorrhage.

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