Posterior pituitary

Anterior pituitary hormones, Anterior pituitary-like hormones. Posterior pituitary hormones. Human growth hormone. 

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). 

The principal hormones of the human posterior pituitary include the two nonapeptides, oxytocin [50-56-6] and arginine vasopressin [11000-17-2] (antidiuretic hormone, ADH). Many other hormones, including opioid peptides (see Opioids, endogenous), cholecystokinin [9011-97-6] (CCK) (see Hormones, BRAIN oligopeptides), and gastrointestinal peptides, also have been located in mammalian neurohypophysis (6), but are usually found in much lower concentrations (7). Studies have demonstrated that oxytocin and vasopressin are synthesized in other human organs, both centrally and peripherally, and there is considerable evidence for their role as neurotransmitters (see Neuroregulators) (8). 

Synthesis of Posterior Pituitary Hormones and Their Analogues... 

In humans, the hypothalamic-derived protein and the hormone noncovalent complexes are packaged in neurosecretory granules, then migrate along axons at a rate of 1 4 mm/h until they reach the posterior pituitary where they are stored prior to release into the bloodstream by exocytosis (67). Considerable evidence suggests that posterior pituitary hormones function as neurotransmitters (68) vasopressin acts on the anterior pituitary to release adrenocorticotropic hormone [9002-60-2] (ACTH) (69) as well as on traditional target tissues such as kidneys. Both hormones promote other important central nervous system (CNS) effects (9,70). 

Logically, ADH receptor antagonists, and ADH synthesis and release inhibitors can be effective aquaretics. ADH, 8-arginine vasopressin [113-79-17, is synthesized in the hypothalamus of the brain, and is transported through the supraopticohypophyseal tract to the posterior pituitary where it is stored. Upon sensing an increase of plasma osmolaUty by brain osmoreceptors or a decrease of blood volume or blood pressure detected by the baroreceptors and volume receptors, ADH is released into the blood circulation it activates vasopressin receptors in blood vessels to raise blood pressure, and vasopressin V2 receptors of the nephrons of the kidney to retain water and electrolytes to expand the blood volume. 

SUM MARY DRUG TABLE ANTERIOR AND POSTERIOR PITUITARY HORMONES... 

There may be a decreased resistance and inability to The posterior pituitary gland produces two hormones localize infection. The nurse observes the skin daily for vasopressin (antidiuretic hormone) and oxytocin (see localized signs of infection, especially at injection sites Chap. 53). Posterior pituitary hormones are summarized or IV access sites. Visitors are monitored to protect the in the Summary Drag Table Anterior and Posterior patient against those witii infectious illness. Pituitary Hormones. 

Vasopressin (Rtressin Synthetic) and its derivatives, namely lypressin (Diapid) and desmopressin (DDAVP), regulate the reabsorption of water by the kidneys. Vasopressin is secreted by the pituitary when body fluids must be conserved. An example of this mechanism may be seen when an individual has severe vomiting and diarrhea with little or no fluid intake. When this and similar conditions are present, die posterior pituitary releases the hormone vasopressin, water in die kidneys is reabsorbed into die blood (ie, conserved), and die urine becomes concentrated. Vasopressin exhibits its greatest activity on die renal tubular epithelium, where it promotes water resoqition and smooth muscle contraction throughout die vascular bed. Vasopressin has some vasopressor activity. 

Zhang, SJ and Jackson, MB (1995) GABAa receptor activation and the excitability of nerve terminals in the rat posterior pituitary. /. Physiol. (Lond.) 483 583-595. 

The posterior pituitary is innervated by direct nervous stimulation from the hypothalamus, resulting in the release of specific hormones. The hypothalamus synthesizes two hormones, oxytocin and vasopressin. These hormones are stored in and released from the posterior pituitary lobe. Oxytocin exerts two actions (1) it promotes uterine contractions during labor, and (2) it contracts the smooth muscles in the breast to stimulate the release of milk from the mammary gland during lactation. Vasopressin is an antidiuretic hormone (ADH) essential for proper fluid and electrolyte balance in the body. Specifically, vasopressin increases the permeability of the distal convoluted tubules and collecting ducts of the nephrons to water. This causes the kidney to excrete less water in the urine. Consequently, the urine becomes more concentrated as water is conserved. 

In contrast to the posterior pituitary lobe, the anterior pituitary lobe is under the control of several releasing and inhibiting hormones secreted from the hypothalamus via a portal vein system. The anterior pituitary lobe, in turn, synthesizes and secretes six major hormones. Figure 43-1 summarizes the physiologic mediators and effects of each of these hormones. 

Vasopressin is a peptide hormone produced by the hypothalamus and secreted by the posterior pituitary in response to stimulation. Normal stimuli for vasopressin release are hyperosmolarity and hypovolemia, with thresholds for secretion of greater than 280 mOsm/kg and greater than 20% plasma volume depletion. A number of other stimuli, such as pain, nausea, epinephrine, and numerous drugs, induce release of vasopressin. Vasopressin release is inhibited by volume expansion, ethanol, and norepinephrine. The physiological effect of vasopressin is to promote free water clearence by altering the permeability of the renal collecting duct to water. In addition, it has a direct vasoconstrictor effect. Consequently, vasopressin results in water retention and volume restoration. In patients with septic shock, vasopressin is appropriately secreted in response to hypovolemia and to elevated serum osmolarity (R14). 

As noted above, MDA is a potent stimulator of monoamine release (see Table 7.1), and recent reports indicate that a number of MDMA metabolites are bioactive. For example, Forsling et al.61 showed that the metabolite 4-hydroxy-3-methoxymethamphetamine (HMMA) is more potent than MDMA as a stimulator of vasopressin secretion from rat posterior pituitaries in vitro. The neuroendocrine effects produced by in vivo administration of MDMA metabolites have not been examined. Monks et al.62 demonstrated that catechol metabolites of MDMA and MDA, namely, 3,4-dihydroxymethamphetamine (HHMA) and 3,4-dihydroxyamphetamine (HHA), exhibit neurotoxic properties when oxidized and conjugated with glutathione. Further characterization of the biological effects of MDMA metabolites is an important area of research. 

The hypothalamic releasing factors regulate release of the anterior pituitary trophic hormones. As summarized in Figure 52-1, the releasing factors are produced in various neuronal groups within the hypothalamus and are transported to the median eminence for release into the portal circulation to the anterior pituitary. Neurons in the hypothalamus also produce the hormones oxytocin and vasopressin, which are released by the posterior pituitary into the blood. Therefore, it is not surprising that behavior and experience, which influence the hypothalamus, sometimes alter the secretion of these hypothalamic releasing factors and hormones. 

The effects of both oxytocin and vasopressin are particularly responsive to reproductive steroids (Tables 7 and 8). Oxytocin and vasopressin are best known as neurohypophyseal peptides, released by the posterior pituitary. However, both peptides also are released within the nervous system. Receptors for oxytocin and vasopressin are distributed in areas of the CNS that have been implicated in reproduction, emotion and autonomic functions. 

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