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In 1957, Jacobson et al performed bilateral nephrectomies on rats and showed that these animals no longer responsed to phlebotomy or cobalt administration with increased Ep production. Rats that only had their ureters ligated and that developed similar elevations of the blood urea nitrogen still responded to cobalt or  phlebotomy in the usual  fashion .Similiar results were found in mice , rabbits , baboons , and dogs .In humans , Ep levels were found to be low in the serum of anemic patients with severe uremia , but after successful  renal transplantation increases in blood Ep levels and hematocrit were observed. Furthermore , erythropoietic activity was detected in the perfusates from isolated rabbit kidneys and was produced by renal cell cultures in vitro . Although the evidence to support the renal origin of Ep was substantial , Ep had not been extracted from normal kidneys or from various normal kidney fractions until very recently. To explain this difficulty , three different hypotheses have been proposed .
  Current evidence indicates that the stimulation of Ep production by hypoxia or ischemia may be mediated via increases in renal prostaglandin and cyclic AMP  levels . The administration of cyclic AMP to plethoric mice increased Ep production and led to and increased red cell mass in normal mice . prostaglandin raised the renal cyclic AMP concentration in isolated perfused dog kidneys as well as the amount of Ep in the perfusates ;the administration of prostaglandin  to polycythemic mice increased both prostaglandin E and Ep blood levels in dogs . The effect on both these parameters as well as on hypoxia-induced Ep production was blocked by indomethacin, a known inhibitor of prostaglandin synthesis .Since prostaglandins are located mainly in the renal medulla , while Ep has been identified by fluorescent antibodies in the renal cortex , these observations are most consistent with the hypothesis that renal hypoxia or ischemia produces a medullary release of prostaglandin which in turn elevates renal corticla cyclic AMP levels. The newly formed cyclic AMP THEN may activate a protein kinase that leads to increased Ep production , possibly through phosphorylation of a precursor renal erythropoietic factor.
   Although the kidney is the major site controlling Ep formation , after bilateral nephrectomy the production of hormone and erythropoiesis continues in most species , but at a considerably lower level .Anephric mice and rats retain about 10%of their original capacity to increase Ep production in the presence of hypoxia , while anephric patients who become very anemic have Ep in their plasma .The extrarenal Ep of rats appears to be similar to Ep of renal origin since it stimulates heme synthesis by marrow cells and is neutralized by antiserum to urinary Ep . Several lines of study point to the liver as the primary site of extrarenal  Ep production. Ep is produced primarily in the liver during fetal life .Hepatectomy abolishes hypoxia-stimulated hormone production in anephric rats , but not in animals with intact kidneys . In partially hepatectomized rats , the extrarenal Ep response to hypoxia correlates directly with  liver regeneration and is most increased during the period of greatest proliferation . Administration of colloidal carbon or zymosan, which induces liver reticuloendothelial cell hyperplasia, enhances Ep production in nephrectomized rats exposed to hypoxia .This effect, which is also blocked by hepatectomy , suggests that the liver reticuloendothelial system may be and important factor in extrarenal Ep production .

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ringzhu(金币+10): 辛苦欢迎常来~~ 2011-03-17 15:21:07

1957年，Jacobson等人对大鼠做了双侧肾切除，发现这些动物不再对放血或钴处理产生Ep升高的反应。而只是结扎了输尿管并出现类似血液尿素氮水平升高现象的大鼠仍能够对放血和钴处理产生正常反应。在小鼠、兔、狒狒和狗中也发现了类似的结果。在人类中，有严重尿毒症的贫血症患者血清Ep水平较低，但是成功肾移植后，血液Ep浓度和血细胞比容升高。此外，切除的兔肾的灌注液具有促红细胞生成活性，体外培养的肾细胞也产生红细胞生成活性。尽管大量证据支持Ep产生自肾，但直到最近才从正常肾或各种正常肾组分中提取到Ep。为解释这种现象，人们提出了三种假说。目前证据显示低氧或缺血刺激引起的Ep生成可能是通过肾前列腺素和cAMP升高调节的。给予多血症小鼠cAMP能够增加Ep生成，而在正常小鼠中会导致红细胞数量增加。前列腺素能够提高分离灌注的狗肾的肾cAMP浓度，以及灌注液中的Ep含量。在多血症小鼠中，前列腺素能够提高前列腺素E水平，在狗中，前列腺素能够增加血液Ep含量。前列腺素对这些参数以及低氧引起的Ep生成的影响受到消炎痛的阻断，而消炎痛是已知的前列腺素合成抑制剂。前列腺素主要存在于神髓质，通过免疫荧光抗体发现Ep存在于肾皮质，这与一个假说高度一致，这个假说认为肾低氧或缺血导致髓质的前列腺素释放，这又提高了皮质cAMP的水平。新形成的cAMP可能激活一个蛋白激酶，通过磷酸化一个肾红细胞生成因子的前体，导致Ep生成增加，尽管肾是控制Ep形成的主要位点，但多数物种在双侧肾切除后继续有激素和红细胞生成，只是含量非常低。无肾小鼠和大鼠保留了大约10％的低氧时Ep生成增加的能力，而极度贫血的无肾病人血浆中含有Ep。大鼠肾以外Ep与肾来源Ep类似，因为肾外Ep刺激骨髓细胞合成亚铁血红素，并能够被尿Ep抗血清中和。有几条线索提示肝脏是肾外Ep生成的主要位置。在胚胎时期，Ep主要是在肝脏中生成的。肝切除后，低氧引起的激素生成在无肾大鼠消失，但在肾完整动物中不存在消失的情况。在部分肝切除的大鼠中，肾外Ep对低氧的反应能力直接与肝重生相关，并且在肝增生最强时反应能力提升最快。胶态炭或酵母聚糖能够引起肝网状内皮细胞增生，给予肾切除大鼠胶态炭或酵母聚糖后能够增强低氧暴露的大鼠中Ep的生成。这一效应也被肝切除阻断，说明肝网状内皮细胞系统可能是肾外Ep生成的一个重要因素。

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