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Anatomical and Post Date: Mon, 13 Oct 2008 15:50:19 +0000 In addition, other drugs may modify thevolume and osmotic pressure of the urine by inducing alterations in solute excretion or renal hemodynamics {see introduction to Section VIII). However, the latter drugs usu-ally do not seriously interfere with the main-tenance of water balance since compensatory readjustments of the rate of ADH release readily occur.Anatomical and Physiological Considerations. Autor of the post: Undefined Thus, the posterior pituitary can Post Date: Mon, 13 Oct 2008 15:30:39 +0000 The neurohypophysis is richly innervated with non-myelinated nerve fibers that originate, for the most part, in the supraoptic and paraventricular nuclei of the hypothalamus. The hypothalamic nuclei are, in turn, linked with subthalamic and thalamic nuclei and through these with the sensory and motor sys-tems and all parts of the cerebral cortex. Thus, the posterior pituitary can be subjected to nerve impulses of diverse origin. Autor of the post: Undefined The neurons originating Post Date: Mon, 13 Oct 2008 15:17:10 +0000 ADH and also oxytocin {see Chapter 42) are formed in cell bodies located principally in the para-ventricular and supraoptic nuclei and then migrate along the axons of these cells to perivascular nerve endings in the posterior lobe {see Scharrer and Scharrer, 1954). The hormones accumulate in the nerve endings, largely in "neurosecretory granules," within which they are bound to protein {see below). The neurons originating in the supraoptic nucleus are believed to release ADH almost exclusively, whereas the other hypothalamiconeurohypophyseal tracts contain both hormones in varying ratios (Bisset et al, 1966). Autor of the post: Undefined Douglas and coworkers have shown Post Date: Mon, 13 Oct 2008 15:00:32 +0000 It is generally accepted that the stimuli for the release of hormone are transmitted to or arise in the hypothalamic nuclei and that axonal impulses tra-versing the hypothalamiconeurohypophyseal tract effect the release of hormone at the nerve endings in the posterior lobe. Electrical stimulation of the supraoptic or paraventricular nuclei results in secre-tion of hormone, as does the injection into these areas of acetylcholine or physostigmine. Douglas and coworkers have shown that electrical stimulation of the axons leading into the isolated posterior lobe results in ADH release, as does depolarization of these axons and their neurosecretory terminals by high concentrations of potassium ion. Autor of the post: Undefined After this "pool" has been Post Date: Mon, 13 Oct 2008 14:45:11 +0000 They have also shown that calcium ion plays an essential role in linking depolarization to secretion {see Douglas and Poisner, 1964; Mikiten and Douglas, 1965). Less than 20% of the hormone content of the posterior lobe is readily releasable. After this "pool" has been expended, the rate ofrelease falls to very much lowerlevels, even in the presence of continued intensive stimulation. Autor of the post: Undefined There has been intensive investigation Post Date: Mon, 13 Oct 2008 14:25:16 +0000 Little is known of the factors that govern or limit the rates of formation and migration of the hormones. However, severe depletion of the hor-mone content of the gland has been demonstrated after prolonged physiological stimulation in vivo.There has been intensive investigation of possible mechanisms for the release of ADH and oxytocin into the blood stream. Autor of the post: Undefined The latter seems likely Post Date: Mon, 13 Oct 2008 14:12:23 +0000 It has been found that a portion of the hormones in the gland is present in protein-bound form in cytoplasm and that calcium ion inhibits this binding. It has been suggested that the influx of calcium associated with the arrival of the nerve impulse thus makes some hormone avail-able for diffusion out of the nerve terminal It has also been suggested that, although calcium does not cause the release of hormone from isolated neuro-secretory granules in vitro, the influx of this ion might facilitate the exocytosis of the contents of granules located near the nerve membrane (see Ginsberg, 1968). The latter seems likely to constitute the principal mechanism for the release of the hor-mone. Autor of the post: Undefined In well-hydrated sub-jects the plasma Post Date: Mon, 13 Oct 2008 13:55:56 +0000 Together with the mechanisms in the central nerv-ous system (CNS) that govern thirst, the hypo-thalamiconeurohypophyseal system for the secretion of ADH provides the homeostatic control that main-tains the osmotic pressure of body fluids within extremely narrow limits; the rate of secretion of ADH is directly influenced by the degree of body hydration. A large body of evidence indicates that there are osmoreceptors, specifically sensitive to changes in extracellular electrolyte concentration, located in the CNS, probably within the hypothala-mic nuclei mentioned above. In well-hydrated sub-jects the plasma concentration of ADH is generally less than 1 microunit per milliliter (1 /tU/ml). Autor of the post: Undefined There is also evidence for volume Post Date: Mon, 13 Oct 2008 13:40:01 +0000 Mod-erate dehydration results in elevation of plasma con-centrations to 5 to 10 /Ø/ml, associated with the excretion of markedly hypertonic urine. Prolonged severe dehydration or the infusion of hypertonic solutions has resulted in the elevation of plasma concentrations to as high as 20 to 100 /xU/ml (see Lauson, 1967).There is also evidence for volume receptors that influence the activity of the hypothalamiconeuro-hypophyseal system (see Smith, 1957; Share, 1962; Moore, 1971). Autor of the post: Undefined For example, hemorrhage has been Post Date: Mon, 13 Oct 2008 13:28:18 +0000 It has been suggested that such re-ceptors exist in the left atrium and the pulmonary veins. According to this concept, a decrease or an increase in the circulating blood volume results, respectively, in the stimulation or the suppression of the secretion of ADH. For example, hemorrhage has been shown to be a very powerful stimulus for the release of ADH. 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