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
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