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What is Posterior Pituitary?
Before discussion about posterior pituitary hormones we should have some idea about posterior pituitary. The posterior pituitary is a whitish lobe, comprising about l/4th of the pituitary. Histologically, it consists of nerve fibers and neuroglial cells. The nerve fibers come from the hypothalamus via the pituitary branch. These nerve fibers are collected in a tract called hypothalamic hypophysical tract-The neuro-hypophysis, which has no glandular cells is associated with the release of two hormones, vasopressin (antidiuretic hormone or ADH) and oxytocin, into blood. It is now known that both these hormones are not produced in pituitary at all; actually hypothalamus synthesizes these posterior pituitary hormones in the para-ventricular and supraoptic group of its nuclei. Later they are transported to the posterior pituitary via the hypothalamico hypophysial tract. These hormones are stored within the posterior pituitary.
What is Hypophysectomy?
The pituitary gland or hypophysis is commonly called the master endocrine gland because it secretes several hormones that control other endocrine glands. The pituitary gland itself has a master-the hypothalamus-Painful, stressful, and emotional experiences all cause changes in hypothalamic activity. Surgical treatment of pituitary adenomas via the trans-sphenoidal way is called hypophysectomy.
Hypophysectomy is usually done to relieve from pain. The mechanism by which pain relief is accomplished is unknown, but it is not related directly to the expected fall in the posterior pituitary hormones levels. Hormone replacement therapy is needed to replace pituitary secretion. Potential complications include endocrine deficits, damage to the optic nerves or oculomotor apparatus, and cerebrospinal (CSF) leakage.
Posterior Pituitary Hormones
Vusopressin (antidiuretic hormone, ADH) and oxytocin are two important neurohypophysial posterior pituitary hormones.
Vasopressin
Vasopressin is biosynthesized mainly in the supraoptic nuclei of the hypothalamus. It is a peptide hormone containing nine amino acids and has a molecular weight of 1,100. The osmolality of blood usually controls the secretion of vasopressin. Its secretion is stimulated under conditions of severe dehydration and water deprivation.
The elementary function of vasopressin is to preserve body water. It promotes reabsorption of water from the distal convoluted tubules of nephrons, reducing excretion of water in urine. Besides, it increases blood pressure by contracting blood vessels (vasoconstriction) in several tissues. It also stimulates contraction of intestinal smooth muscles.
Failure of secretion of vasopressin leads to a reduced renal reabsorption of water and a consequent elimination of a large volume of very dilute urine. This disorder is known as diabetes insipidus. It usually develops due to hypothalamic destruction or neurohypophysial destruction. To reduce thirst, the patient takes large quantity of water so that osmolality of the plasma is kept normal. The urine is insipid in contrast to the sweet urine of diabetes mellitus. Moreover, when the concentration of the hormone is very high in blood, it causes arteriolar smooth muscle contraction, leading to vasoconstriction and raise of arterial blood pressure.
Oxytocin
It is also one of the posterior pituitary hormones. . It is biosynthesized mainly in paraventricular nuclei of the hypothalamus. Like vasopressin, it is also a peptide hormone, containing nine amino acids and has a molecular weight of 1,000. Oxytocin is secreted into blood when hypothalamic neurons are stimulated either due to the distension of uterus by the full-term foetus or due to sucking of breasts by the infant.
The major functions of oxytocin are to cause contraction of the uterine muscles during childbirth. This helps in the delivery of the foetus. Besides, oxytocin is very important for the mechanism of discharge of milk from the mammary gland through nipple. The act of coitus can cause oxytocin release resulting in increased uterine wall contractions which help in passage of sperms, thereby increasing the chances of fertilization. So as a posterior pituitary hormone it has some greet activities.