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)Since ADH and oxytocin exert their Post Date: Mon, 13 Oct 2008 10:36:53 +0000 Modifica-tions of structure other than at positions 3 and 8 alter biological activity to varying degrees. (See Walter et al, 1967; Sawyer and Manning, 1973.)Since ADH and oxytocin exert their primary effects in minute concentrations, an increase in po-tency would not be of great significance. Autor of the post: Undefined Another compound under investigation Post Date: Mon, 13 Oct 2008 10:24:29 +0000 Therefore, investigation of synthetic congeners has been di-rected toward increased duration of action, greater selectivity with respect to a single action, or the ability competitively to antagonize an action of the parent hormones. For example, 2-phenylalanine-8-lysine vasopressin, felypressin, which shows an in-creased ratio of pressor-to-antidiuretic activity, is in use in Europe as a vasoconstrictor agent in several applications for which vasopressin has been used (see below). Another compound under investigation in man is l-deamino-8-D-arginine vasopressin (DDAVP). Autor of the post: Undefined Secretory Interrelationship between ADH and Oxy-tocin Post Date: Mon, 13 Oct 2008 10:13:06 +0000 Both of the structural modifications in this congener reduce pressor activity and slow enzy-matic inactivation while enhancing antidiuretic ac-tivity. Thus, DDAVP is a more potent and signifi-cantly longer-acting antidiuretic agent than ADH and has negligible effects, in the doses used, on vascular and other smooth muscle (Vavra et al, 1968; Rudinger et al, 1972).Secretory Interrelationship between ADH and Oxy-tocin. Autor of the post: Undefined Physio-logical responses (antidiuresis, milk ejection Post Date: Mon, 13 Oct 2008 9:55:29 +0000 It is clear that oxytocin plays a physiological role very different from that of ADH (see Chapter 42). Nevertheless, the formation, storage, and release of the two hormones are closely interrelated. Physio-logical responses (antidiuresis, milk ejection, changes in uterine motility) indicate that their simultaneous release may occur whether the stimulus is appro-priate for ADH secretion (hyperosmolarity) or for oxytocin secretion (parturition, suckling, coitus), or is nonspecific (pain, apprehension). Autor of the post: Undefined The stimulus supplied by hemor-rhage Post Date: Mon, 13 Oct 2008 9:39:49 +0000 However, Holland and coworkers (1958) have observed re-sponses, in lactating rabbits, to the injection of hypertonic saline solution that indicate the releaseof oxytocin in excess of ADH. Greater depletion of oxytocin than of ADH in the posterior lobe of the dog during lactation was observed by van Dyke and coworkers (1955). The stimulus supplied by hemor-rhage appears to be quite specific in that very large amounts of ADH, but little oxytocin, are released. Autor of the post: Undefined As has already been pointed out Post Date: Mon, 13 Oct 2008 9:26:19 +0000 (For details, see Sawyer, 1961; Discussion on poly-peptide hormones, 1968.)PHYSIOLOGICAL AND PHARMACOLOGICAL PROPERTIES.As has already been pointed out, the pri-mary action of ADH is to modulate the renal tubular reabsorption of water. Autor of the post: Undefined The Formation of Hypo-tonic Urine Post Date: Mon, 13 Oct 2008 9:10:22 +0000 In considering the extrarenal responses to ADH, it should be kept in mind that most of these are elic-ited only by concentrations far higher than those required for antidiuresis and are gen-erally considered to be incidental actions subserving no physiological requirement in mammals.Renal Action. The Formation of Hypo-tonic Urine. Autor of the post: Undefined Thus, the fluid entering Post Date: Mon, 13 Oct 2008 8:52:35 +0000 Following the isosmotic re-absorption of the major portion of the glo-merular filtrate in the proximal tubule, which is freely permeable to water at all times, electrolyte reabsorption continues in the as-cending limb of Henle's loop as well as in the distal convolution and the collecting ducts. However, permeability to water de-creases in the ascending thin limb and be-comes extremely low in the thick ascending portion. Thus, the fluid entering the distal convolution is always hypotonic and, when circulating concentrations of ADH are low, remains so as electrolyte reabsorption con-tinues along the remainder of the tubule and collecting system. Autor of the post: Undefined The Formation of Isotonic and Hypertonic Post Date: Mon, 13 Oct 2008 8:41:27 +0000 When the secretion of the hormone is completely suppressed by over-hydration, 15% or more of the water of the glomerular filtrate may escape into the urine virtually free of electrolyte. The osmotically unobligated water lost during the excretion of hypotonic urine is designated as the posi-tive free-water clearance.The Formation of Isotonic and Hypertonic Urine. Autor of the post: Undefined In primates, it is believed Post Date: Mon, 13 Oct 2008 8:28:56 +0000 Increasing concentrations of circu-lating ADH result in increasing permeability of the distal segment and the collecting sys-tem to water. In some species, this response first appears early in the distal convolution. In primates, it is believed to begin in the cortical collecting tubules or in the terminalportion of the distal convolution, which is morphologically similar. 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