In this article we will discuss about the actions of antidiuretic hormone and actions of oxytocin hormones that are secreted by posterior pituitary hormones.
Posterior pituitor, gland is also known as neurohypophysis. The hormones of posterior pituitary gland are synthesized in the neurons of the hypothalamus. The two different groups of neurons in the hypothalamus, which synthesize these hormones are, supraoptic and paraventricular nuclei (they are also known as magno cellular neurons in general).
The hormones are transported from the hypothalamus to the posterior pituitary gland through the axoplasmic flow (Fig. 6.14). Supraoptic and para- ventricular nuclei of the hypothalamus are connected to the posterior pituitary gland through hypothalamo- hypophyseal tract. When impulses come through this tract to the posterior pituitary gland, the stored hormones in the gland are released into the circulation (Fig. 6.15).
Supraoptic nucleus predominantly secretes antidiuretic hormone (ADH/vasopressin) and para ventricular nucleus predominantly secretes oxytocin hormone. Both are peptide hormones.
Type # 1. Actions of Antidiuretic Hormone:
As the name indicates, the hormone decreases the volume of water lost in the urine and thereby helps to maintain body water content by conservation of water. The sites of action are kidney and smooth muscle of blood vessels.
1. Water reabsorption (Fig. 6.16):
i. It acts on the epithelial cells lining the collecting duct and also distal convoluted tubule through V2 receptors.
ii. This brings about the insertion of water channels (aquaporins) in the epithelial cells.
iii. Through these channels, water gets reabsorbed.
iv. Water reabsorbed in these parts of the nephrons is under the influence of this hormone.
2. Urea movement:
It facilitates the rate of urea recycled from the collecting duct. Because of reabsorption of water from DCT and CD, the concentration of urea increases in fluid present in tubular lumen.
Due to the concentration gradient between the tubular lumen and interstitium in renal medulla, urea diffuses into the interstitium from the lumen. From the interstitial region of renal medulla along the concentration gradient, urea diffuses back into the fluid present in the ascending limb of loop of Henle.
3. Rate of blood flow through vasa recta:
It reduces the rate of blood flow through the vasa recta and increases the time available for the operation of countercurrent system. Decreased rate of blood flow through vasa recta is essential concentration of urine.
All the three aforesaid actions help in concentration of urine.
4. Vascular smooth muscle:
In addition to the above actions, ADH in large doses acts on the smooth muscle of blood vessels through the V, receptors and brings about vasoconstriction. Because of this, there will be increase in peripheral resistance and hence increase in diastolic blood pressure.
Regulation of Secretion:
Regulation of secretion is brought about by two important mechanisms. There is involvement of osmoreceptors and volume receptors in regulation of secretion (Fig. 6.17).
They are present in the hypothalamus near the supraoptic nucleus. When the plasma osmolality increases (normal is 300 mOsm/L water), by exosmosis water moves out of osmoreceptors into the interstitium. This leads to shrinkage of the osmoreceptors and consequent stimulation of the receptors. This in turn stimulates the supraoptic nucleus.
From the supraoptic nucleus, more impulses are sent to posterior pituitary through the hypothalamohypophyseal tract. Because of this, more antidiuretic hormone will be released into the circulation. This helps for increased retention of water through the kidneys and normalizes the osmolality of the plasma.
2. Volume receptors:
Volume receptors are present in the walls of the great veins and in the walls of the right side of heart. These receptors are also known as low pressure receptors. Increase in blood volume stretches the walls of great veins and the right side of heart and stimulates these receptors. Afferent impulses from the receptors reach the hypothalamus and inhibit the activity of supraoptic nucleus.
This decreases the frequency of impulses along the hypothalamohypophyseal tract to posterior pituitary gland. So less amount of antidiuretic hormone is released from the posterior pituitary gland into circulation. This decreases the volume of water reabsorbed from the renal tubules. Increased urine output will restore the blood volume.
Some of the other factors that can alter secretion of the hormone are:
It occurs due to deficiency of antidiuretic hormone secretion.
Some of the features are:
Increased volume of urine excretion due to loss of concentrating ability in kidney.
Increased thirst center activity due to decrease in body water content. Person drinks more water.
iii. Glucose will be absent in urine and blood glucose level will be normal.
iv. Specific gravity of the urine will be less due to excretion of more dilute urine most of the times.
Type # 2. Actions of Oxytocin:
On the breasts:
It acts on the myoepithelial cells of the breasts. This causes the ejection of milk from the alveolar ducts of the breasts.
i. It acts on the myometrium of the uterus and causes severe contraction of the myometrium. This leads to the parturition or delivery of the fetus due to increase in the intrauterine pressure.
ii. In the non-pregnant woman, oxytocin is believed to bring about minute myometrial contractions which facilitate movement of sperm along the uterus to reach fallopian tubes.
iii. In males, it is believed to bring about contraction of smooth muscles in vas deferens and facilitate sperm movement in the genital system.
Regulation of Secretion:
Regulation of secretion is brought about by neuroendocrine reflex. In this reflex, part of the reflex pathway is neural and part is hormonal. Milk ejection and parturition reflexes are classical examples of neuroendocrine reflex.
Suckling of the breasts brings about the stimulation of the touch receptors present on the nipple and areola. Afferent impulses are carried to the hypothalamus through the ascending tracts in spinal cord and stimulate the paraventricular nucleus.
The nucleus stimulation leads to more impulses along the hypothalamohypophyseal tract to posterior pituitary gland. This brings about the release of the hormone from the posterior pituitary gland into the circulation. The hormone on reaching breasts brings about milk ejection by acting on the myoepithelial cells.
When head of the fetus presses on the cervix, the stretch receptors present in the walls of cervix are stimulated and the afferent impulses from these receptors reach the hypothalamus along the ascending tracts in spinal cord. This brings about the stimulation of the paraventricular nucleus in the hypothalamus.
More impulses conveyed along the hypothalamohypophyseal tract to the posterior pituitary gland. This leads to the release of the hormone into circulation. The hormone acts on the myometrium present in the walls of uterus to bring about contractions of the uterus resulting in parturition (delivery of fetus).
Milk ejection reflex (Fig. 6.18):