Harmones of Posterior Pituitary

HORMONES OF POSTERIOR PITUITARY

Posterior pituitary hormones are:

1. Antidiuretic hormone (ADH) or vasopressin

2. Oxytocin.

Source of Secretion of Posterior Pituitary Hormones

Actually, the posterior pituitary does not secrete any hormone. ADH and oxytocin are synthesized in the hypothalamus. From hypothalamus, these two hormones are transported to the posterior pituitary through the nerve fibers of hypothalamo-hypophyseal tract, by means of axonic flow. Proteins involved in transport of these hormones are called neurophysins.

In the posterior pituitary, these hormones are stored at the nerve endings. Whenever, the impulses

from hypothalamus reach the posterior pituitary, these hormones are released from the nerve endings into the circulation. Hence, these two hormones are called neurohormones.

Experimental Evidence

Secretion of posterior pituitary hormones in hypothalamus and their transport to posterior pituitary are proved by experimental evidences. When the pituitary stalk is cut above the pituitary gland, by leaving the entire hypothalamus intact, the hormones drip through the cut end of the nerves in the pituitary stalk. This proves the fact that the hormones are secreted by hypothalamus.

Neurophysins

Neurophysins are the binding proteins which transport ADH and oxytocin from hypothalamus to posterior

pituitary via hypothalamo-hypophyseal tract and storage of these hormones in posterior pituitary. Neurophysin I or oxytocin-neurophysin is the binding protein for oxytocin and neurophysin II or ADH-neurophysin is the binding protein for ADH.

ANTIDIURETIC HORMONE

Source of Secretion

Antidiuretic hormone (ADH) is secreted mainly by supraoptic nucleus of hypothalamus. It is also secreted

by paraventricular nucleus in small quantity. From here, this hormone is transported to posterior pituitary

through the nerve fibers of hypothalamo-hypophyseal tract, by means of axonic flow.

Chemistry and Half-life

Antidiuretic hormone is a polypeptide containing 9 amino acids. Its half-life is 18 to 20 minutes.

Actions

Antidiuretic hormone has two actions:

1. Retention of water

2. Vasopressor action.

1. Retention of water

Major function of ADH is retention of water by acting on kidneys. It increases the facultative reabsorption of water from distal convoluted tubule and collecting duct in the kidneys.

In the absence of ADH, the distal convoluted tubule and collecting duct are totally impermeable to water. So, reabsorption of water does not occur in the renal tubules and dilute urine is excreted. This leads to loss of large amount of water through urine. This condition is called diabetes insipidus and the excretion of large amount of water is called diuresis.

Mode of action on renal tubules

ADH increases water reabsorption in tubular epithelial membrane by regulating the water channel proteins called aquaporins through V2 receptors.

2. Vasopressor action

In large amount, ADH shows vasoconstrictor action. Particularly, causes constriction of the arteries in all

parts of the body. Due to vasoconstriction, the blood pressure increases. ADH acts on blood vessels through V1A receptors. However, the amount of ADH required to cause the vasopressor effect is greater than the amount required to cause the antidiuretic effect.

Regulation of Secretion

ADH secretion depends upon the volume of body fluid and the osmolarity of the body fluids.

Potent stimulants for ADH secretion are:

1. Decrease in the extracellular fluid (ECF) volume

2. Increase in osmolar concentration in the ECF.

Role of osmoreceptors

Osmoreceptors are the receptors which give response to change in the osmolar concentration of the blood. These receptors are situated in the hypothalamus near supraoptic and paraventricular nuclei. When osmolar concentration of blood increases, the osmoreceptors are activated. In turn, the osmoreceptors stimulate the supraoptic and paraventricular nuclei which send motor

impulses to posterior pituitary through the nerve fibers and cause release of ADH. ADH causes reabsorption of water from the renal tubules. This increases ECF volume and restores the normal osmolarity.

OXYTOCIN

Source of Secretion

Oxytocin is secreted mainly by paraventricular nucleus of hypothalamus. It is also secreted by supraoptic nucleus in small quantity and it is transported from hypothalamus to posterior pituitary through the nerve fibers of hypothalamo-hypophyseal tract. In the posterior pituitary, the oxytocin is stored in

the nerve endings of hypothalamo-hypophyseal tract. When suitable stimuli reach the posterior pituitary from hypothalamus, oxytocin is released into the blood. Oxytocin is secreted in both males and females.

Chemistry and Half-life

Oxytocin is a polypeptide having 9 amino acids. It has a half-life of about 6 minutes.

Actions in Females

In females, oxytocin acts on mammary glands and uterus.

Action of oxytocin on mammary glands

Oxytocin causes ejection of milk from the mammary glands. Ducts of the mammary glands are lined by myoepithelial cells. Oxytocin causes contraction of the myoepithelial cells and flow of milk from alveoli of mammary glands to the exterior through duct system and nipple. The process by which the milk is ejected from alveoli of mammary glands is called milk ejection reflex or milk letdown reflex. It is one of the neuroendocrine reflexes.

Milk ejection reflex

Plenty of touch receptors are present on the mammary glands, particularly around the nipple. infant suckles mother nipple, the touch receptors are stimulated. The impulses discharged from touch

receptors are carried by the somatic afferent nerve fibers to paraventricular and supraoptic nuclei of

hypothalamus.

Action on uterus

Oxytocin acts on pregnant uterus and also non-pregnant uterus.

On pregnant uterus

Throughout the period of pregnancy, oxytocin secretion is inhibited by estrogen and progesterone. At the end of pregnancy, the secretion of these two hormones decreases suddenly and the secretion of oxytocin

increases. Oxytocin causes contraction of uterus and helps in the expulsion of fetus. During the later stages of pregnancy, the number of receptors for oxytocin increases in the wall of the uterus.

Because of this, the uterus becomes more sensitive to oxytocin.

Action in Males

In males, the release of oxytocin increases during ejaculation. It facilitates release of sperm into urethra

by causing contraction of smooth muscle fibers in reproductive tract, particularly vas deferens.

Mode of Action of Oxytocin

Oxytocin acts on mammary glands and uterus by activating G-protein coupled oxytocin receptor