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Oxytocin stimulates both electri-cal Post Date: Sat, 11 Oct 2008 11:08:12 +0000 (For detailed discussions, see Symposium, 1961; Harris and Donovan, 1966.)PHARMACOLOGICAL PROPERTIESUterus. Oxytocin stimulates both electri-cal and contractile activity in uterine smooth muscle. Autor of the post: Undefined When estrogen levels are low Post Date: Sat, 11 Oct 2008 10:57:46 +0000 With higher concentrations, sus-tained decreases in resting membrane poten-tial occur. These effects are highly dependent on the presence of estrogen. When estrogen levels are low, the effect of oxytocin is much reduced. Autor of the post: Undefined However, progestins have been widely Post Date: Sat, 11 Oct 2008 10:40:04 +0000 The immature uterus is quite resist-ant (see Csapo, 1959). Although progesterone antagonizes the stimulant effect of oxytocin in vitro, the corresponding effect in the preg-nant human uterus has been difficult to demonstrate. However, progestins have been widely used clinically to attempt to reduce uterine activity in cases of threatened or ha-bitual abortion (see Chapter 68). Autor of the post: Undefined The responsiveness of the uterus Post Date: Sat, 11 Oct 2008 10:29:24 +0000 A very low level of motor activity prevails in the human uterus during the first and second trimesters of pregnancy. During the third trimester, spontaneous motor activity increases progressively until the sharp rise that constitutes the initiation of labor and delivery. The responsiveness of the uterus to oxytocin roughly parallels the increase in spontaneous activity. Autor of the post: Undefined Most of this in-crease occurred Post Date: Sat, 11 Oct 2008 10:11:08 +0000 Oxytocin can initiate or enhance rhythmic contractions at any time, but in early pregnancy only very high doseselicit a response. In a study in which the effect of oxytocin infusions was carefully quantitated in terms of force, duration, and frequency of contractions, Caldeyro-Barcia and Posiero (1959) found an eightfold in-crease in responsiveness between the twen-tieth and thirty-ninth week. Most of this in-crease occurred during the last 9 weeks. Autor of the post: Undefined Effect of Vasopressin Post Date: Sat, 11 Oct 2008 9:52:24 +0000 Thus, slow intravenous infusion of a few units of oxytocin usually is effective in initi-ating labor at term. However, there is con-siderable variability among individuals and labor has been observed to occur after infu-sion of as little as 25 milliunits (0,05 jug) of oxytocin (see below).Effect of Vasopressin. Autor of the post: Undefined Vasopressin also differs from oxytocin Post Date: Sat, 11 Oct 2008 9:39:49 +0000 Paradoxically, the non-pregnant uterus is more responsive to vasopressin than to oxytocin, especially when uterine activity is elevated, as in primary dysmenorrhea (see Torpin et al, 1947). However, the effect of vasopressin on uterine smooth muscle shows only a modest increase during pregnancy and is therefore far less than that of oxytocin at term. Vasopressin also differs from oxytocin in that it causes irregular, higher-frequency contractions associated with greater elevation of resting tone. Autor of the post: Undefined Contraction of these cells forces Post Date: Sat, 11 Oct 2008 9:29:30 +0000 Mammary Gland. The alveolar ramifica-tions of the mammary gland are surrounded by a network of modified smooth muscle, the myoepithelium. Contraction of these cells forces milk from the alveolar channels into the large sinuses, where it is easily available to the suckling infant. Autor of the post: Undefined Although the cate-cholamines inhibit milk Post Date: Sat, 11 Oct 2008 9:14:46 +0000 This function is known as milk ejection (milk letdown, in domestic animals). The myoepithelium is highly re-sponsive to oxytocin. Although the cate-cholamines inhibit milk ejection, the con-traction of the myoepithelium is not believed to be dependent on autonomic innervation, but is considered to be under the control of oxytocin and the reflex pathways that initiate the release of the hormone. Autor of the post: Undefined Cardiovascular System Post Date: Sat, 11 Oct 2008 8:58:14 +0000 Oxytocin is occa-sionally employed to promote milk ejection when this component of lactation appears to be inefficient in nursing mothers. It is also of limited usefulness in the relief of post-partum breast engorgement.Cardiovascular System. 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