Antidiuretic hormone . See Vasopressin

Aquaporins help water to pass through biological membranes. They form hydrophilic pores that allow H2O molecules, but not hydrated ions or larger molecules, to pass through. Aquaporins are particularly important in the kidney, where they promote the reuptake of water (see p. 328). Aquaporin-2 in the renal collecting ducts is regulated by antidiuretic hormone (ADH, vasopressin), which via cAMP leads to shifting of the channels from the ER into the plasma membrane. 

Alcohol inhibits the release of vasopressin (antidiuretic hormone see Chapter 29) from the posterior pituitary gland, resulting in enhanced diuresis. The volume loading that accompanies imbibing complements the diuresis that occurs as a result of reduced vasopressin secretion. Alcoholics have less urine output than do control subjects in response to a challenge dose with ethanol, suggesting that tolerance develops to the diuretic effects of ethanol. Alcoholics withdrawing from alcohol exhibit increased vasopressin release and a consequent retention of water, as well as dilutional hyponatremia. 

The collecting duct system (connecting tubule, initial collecting tubule, cortical collecting duct, and outer and inner medullary collecting ducts) is an area of fine control of ultrafiltrate composition and volume and is where final adjustments in electrolyte composition are ttuide. In addition, vasopressin (also called antidiuretic hormone see Chapter 29) modulates water permeability of this part of the nephron. 

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). 

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. 

Water. Water resorption in the proximal tubule is a passive process in which water follows the osmotically active particles, particularly the Na" ions. Fine regulation of water excretion (diuresis) takes place in the collecting ducts, where the peptide hormone vasopressin (antidiuretic hormone, ADH) operates. This promotes recovery of water by stimulating the transfer of aquaporins (see p. 220) into the plasma membrane of the tubule cells via V2 receptors. A lack of ADH leads to the disease picture of diabetes insipidus, in which up to 30 L of final urine is produced per day. 

Historically vasopressin and oxytocin, two nonapep-tides, were the first peptide neurohormones to be considered they are stored in the neurohypophysis and released into the bloodstream upon an appropriate stimulus. In the periphery, oxytocin stimulates the contraction of epididymal and uterine smooth muscle (see Chapter 62) and vasopressin (antidiuretic hormone) facilitates the reabsorption of water from the kidney tubules. In addition to these well-accepted roles as neurohormones, there is convincing evidence that these compounds function as neurotransmitters they both possess potent inhibitory actions on neurohypophyseal neurons. The significance of their neurotransmitter function is not yet clear. 

Water and electrolyte metabohsm is regulated by aldosterone from the adrenal gland, renin from the kidney, and vasopressin (antidiuretic hormone [ADH]) from the posterior pituitary gland (see Chapters 45 and 46). 

The adenohypophysis secretes growth hormone (GH), prolactin (PRL), thyrotropin (TSH), adrenocorticotropin (ACTH), foUicle-stimulating hormone (FSH), and luteinizing hormone (LH), all of which are proteins or peptides (see Table 28-1 in Chapter 28). It also secretes P-lipotropin (P-LPH) and a number of smaller peptides of undetermined significance Vasopressin (also known as antidiuretic hormone [ADH]) and oxytocin are produced in the hypothalamus and are carried through the neurohypophyseal nerve axons to the neurohypophysis. Thus the neurohypophysis is not a discrete endocrine organ, but rather functions as a reservoir for these two hormones. 

In certain cases such as toad urinary bladder and mammalian kidney, the movement of small polar molecules, especially urea, are significantly increased by antidiuretic hormones [52], Recently, it has been found that urea and water transport across the toad bladder can be separately activated by low concentration of vasopressin or 8 Br-cAMP [57], Based on these studies it was concluded that membrane channels for water and small polar nonelectrolytes differ significantly in both their dimensions and densities. The solute channels are limited in number, have relatively large radii and carry only a small fraction of water flow. On the other hand, the water channels have small radii. These findings provide strong experimental support for the concept of membrane pores which we have been advocating (see Fig. 5 in [4]). In this respect it is not unreasonable to expect that PCMBS and phloretin would also inhibit the ADH-sensitive increase in the permeability of these systems to urea and other small polar nonelectrolytes. 

Vasopressin is a peptide hormone released by the posterior pituitary in response to rising plasma tonicity or falling blood pressure. Vasopressin possesses antidiuretic and vasopressor properties. A deficiency of this hormone results in diabetes insipidus (see Chapters 15 and 17). 

The posterior pituitary gland produces two honnones vasopressin (antidiuretic honnone) and oxjrtncin (see (3iap. 53). Ibsterior ixtuitary honnones are summarized in die Summary Dmg Table Anterior and Ibsterior Rtuitary Hormones. 

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