Pituitary gland neurohypophysis hormones

The adenohypophysis is derived embryonically from glandular tissue, specifically, Rathke s pouch. This tissue originates from the oropharynx, or the roof of the mouth. It then migrates toward the embryonic nervous tissue destined to form the neurohypophysis. When these two tissues come into contact, Rathke s pouch loses its connection with the roof of the mouth and the pituitary gland is formed. Unlike the neurohypophysis, which releases hormones originally synthesized in the hypothalamus, the adenohypophysis synthesizes its own hormones in specialized groups of cells. Similar to the neurohypophysis, however, the release of these hormones into the blood is regulated by the hypothalamus. 

Vasopressin (antidiuretic hormone) is a peptide synthesized in the hypothalamus and secreted from the neurohypophysis of the pituitary gland. This substance plays an important role in the long-term regulation of blood pressure through its action on the kidney to increase reabsorption of water. The major stimulus for release of vasopressin is an increase in plasma osmolarity. The resulting reabsorption of water dilutes the plasma toward its normal value of 290 mOsM. This activity is discussed in more detail in Chapter 10 (the endocrine system) and Chapter 19 (the renal system). 

In order to make adjustments in the water load, the reabsorption of the remaining 20% of the filtered water from the distal tubule and the collecting duct is physiologically controlled by antidiuretic hormone (ADH), also referred to as vasopressin. Antidiuretic hormone, synthesized in the hypothalamus and released from the neurohypophysis of the pituitary gland, promotes the... 

Connected to the brain by a stalk (Fig. 30-1), the pituitary gland releases at least ten peptide or protein hormones that regulate the activity of other endocrine (hormone-producing) glands in distant parts of the body. The pituitary is composed of several distinct parts the anterior lobe (adenohypophysis), a thin intermediate portion (pars intermedia), and a posterior lobe (neurohypophysis). Each has its own characteristic endocrine functions. 

The posterior lobe of the pituitary gland, the neurohypophysis, generally secretes two hormones of a family of nine found interspecifically. The same hormones are usually found in a zoological class, and the structure varies by only one or two residues among classes. These neurohypophyseal hormones are nonapeptides with a 1,6-disulfide bridge. In humans these hormones are oxytocin, which plays a role in lactation and parturition by causing smooth muscle contraction, and vasopressin, which has a homeostatic role in controlling water balance. 

In the course of the isolation of oxytocin, the second principal hormone of the pituitary gland, vasopressin, a peptide with pressor and antidiuretic effects, was similarly secured in pure form. Porcine glands yielded a vasopressin that contained lysine, while an arginine containing peptide was obtained from the neurohypophysis of cattle. Structure elucidation [63,64] revealed that the two vasopressins are quite analogous to oxytocin, except that isoleucin in position 3 of oxytocin is replaced by phenylalanine and position 8 is occupied by one of the basic amino acids, lysine or arginine, rather than by leucine ... 

A similar mechanism of secretion is thought to occur in the anterior and posterior pituitary gland and in the parafollicular cells of the thyroid (Trifaro, 1977). The process of exocytosis occurs in the neurohypophysis, with the release of oxytocin, vasopressin, and neurophysins I and II (Uttenthal et al., 1971 Nordmann et al., 1971) in the anterior pituitary, with the release of thyrotropin (Farquhar, 1969), adrenocor-ticotropin (Rennels and Shiino, 1968), and growth hormone (Farquhar, 1961) and in the alpha (Gomez-Acebo et al., 1968), beta (Lacy, 1961), and delta (Gomez-Acebo et al., 1968) cells of the pancreas. 

The posterior lobe of the pituitary, ie, the neurohypophysis, is under direct nervous control (1), unlike most other endocrine organs. The hormones stored in this gland are formed in hypothalamic nerve cells but pass through nerve stalks into the posterior pituitary. As early as 1895 it was found that pituitrin [50-57-7] an extract of the posterior lobe, raises blood pressure when injected (2), and that Pitocin [50-56-6] (Parke-Davis) causes contractions of smooth muscle, especially in the utems (3). Isolation of the active materials involved in these extracts is the result of work from several laboratories. Several highly active posterior pituitary extracts have been discovered (4), and it has been deterrnined that their biological activities result from peptide hormones, ie, low molecular weight substances not covalendy linked to proteins (qv) (5). 

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