POSTTRANSCRIPTIONAL REGULATION OF ANGIOTENSIN RECEPTORS

在大鼠中，两种不同的血管紧张素Ia型受体（AT）mRNA由单个ATlaR合成。 基因这些转录物由外显子1和3（El，3）以及外显子1、2和3（El，2，3）组成。这两份成绩单 编码相同的受体蛋白，仅5'前导序列（5' LS）的长度不同。在我们 在前一个资助期，我们确定E1，2，3转录物在外显子2内具有RNA顺式元件， 降低翻译效率。这些剪接变体在翻译效率上的差异导致更高的翻译效率。 与表达E1，3转录物的细胞相比，表达E1，3转录物的细胞中的ATlaR密度和信号传导活性 El，2，3.我们的新研究表明，ATlaR剪接变体的翻译效率在老年人中降低， 大鼠我们还发现，响应时间-即，调节肾上腺皮质的时间 与年轻大鼠相比，老年大鼠对钠摄入量改变的ATjR密度显著延长 动物老年动物的这种迟缓的反应时间也与肾上腺反应减弱有关;即， 老年大鼠钠摄入量改变引起的血浆醛固酮变化幅度明显较小 与年轻的动物相比。这些观察结果支持了临床和实验研究，表明衰老是 与组织对Ang II的反应性降低相关，并使我们得出以下一般假设： 与衰老相关的组织对Ang 11的反应性降低是由于AT^R的翻译调节受损 通过外显子2内的RNA顺式作用元件转录;这种失调导致转录的速度、幅度 和AT 2 R对Ang 11反应的阈值敏感性，从而有助于众所周知的与年龄相关的 体液和电解质平衡受损。我们计划在四岁以下的年轻和老年大鼠中测试这一假设 目的1：对低钠（LS）饮食和血管紧张素（Ang）的反应 II输注降压剂量，模拟维持在NS大鼠中的钠限制所达到的水平 目标2：在钠限制后重新平衡至正常钠（NS）饮食期间和在重新平衡期间， 大鼠血管紧张素转换酶抑制剂激发平衡至降低水平的血管紧张素II 维持LS饮食2周。在目标3中，我们将研究RNA顺式作用元件的作用机制 外显子2内有助于肾上腺小球和血管平滑肌中Ang II介导的ATjR调节 肌肉细胞钠摄入量改变对ATjR表达和活性的调节在不同组织中存在差异。 钠限制上调肾上腺肾小球细胞上ATjR的密度， 介导的醛固酮分泌。相反，限钠可下调血管AT：Rs， 血管紧张素II的血管收缩性。ATxR在肾脏的特定区域也受到差异调节， 改变钠的摄入量。在这个建议中，我们计划将重点放在肾上腺和肠系膜阻力 这是因为ATjR在这些组织中通过改变钠摄入量而受到调节。这些研究将 确定AT：R在这些受调控组织中的转录后调控机制， 这些机制是如何在老年大鼠中因钠摄入量的改变而受损的。

In the rat,two distinct angiotensin type la receptor (AT]aR) mRNAs are synthesized from a single ATlaR gene. These transcripts are comprised of exons 1 and 3 (El,3) and exons 1, 2 and 3 (El,2,3). These 2 transcripts code for identical receptor proteins and differ only in the lengths of their 5' leader sequence (5'LS). During our previous funding period, we established that the El,2,3 transcript possesses RNA ciselements within exon 2 that inhibit the efficiency of translation. These splice variant differences in translational efficiency result in higher ATlaR densities and signaling activity in cells expressing the El,3 transcript compared to those expressing the El,2,3. Our new studies suggest that the translational efficiency of ATlaR splice variants is diminished in aged rats. We have also found that the response time - i.e., the time it takes to up and down regulate adrenal cortical ATjR densities in response to altered sodium intake is significantly longer in aged rats compared to young animals. This sluggish response time in aged animals is also associated with diminished adrenal responses; i.e., the magnitude ofchange in plasma aldosterone induced by altered sodium intake is significantly less in aged rats compared to young animals. These observations support clinical and experimental studies that indicate aging is associated with reduced tissue responsiveness to Ang II and have led us to the following general hypothesis: Reduced tissue responsiveness to Ang 11that is associated with aging is due to impaired translational regulation of AT^R transcripts via RNA cis acting elements within exon 2; this dysregulation leads to attenuation in the rapidity, magnitude and threshold sensitivity of the AT2R response to Ang 11 and thereby contributes to the well known age-associated impairments influid and electrolyte homeostasis. We plan to test this hypothesis in young and aged rats under four manipulations of the renin angiotensin system including in Aim 1: response to a low sodium (LS)diet and Ang II infusion at a subpressor dose that mimics the levels achieved by sodium restriction in rats maintained on a NS diet; and in Aim 2: during re-equilibration to a normal sodium (NS) diet after sodium restriction and during re- equilibration to reduced levels of Ang II by challenge with an angiotensin converting enzyme inhibitor in rats maintained on a LS diet for 2 weeks. In Aim 3, we will investigate the mechanisms of RNA cis acting elements within exon 2 that contribute to Ang II-mediated ATjR regulation in adrenal glomerulosa and vascular smooth muscle cells. The regulation of ATjR expression and activityby altered sodium intake varies in different tissues. Sodium restriction up-regulates the density of ATjRs on adrenal glomerulosa cells and increases adrenal ATjR- mediated aldosterone secretion. In contrast, sodium restriction down-regulates vascular AT:Rs and reduces vascular contractility to Ang II. ATxRs are also differentially regulated in specific regions of the kidney and brain by altered sodium intake. In this proposal, we plan to focus on the adrenal and mesenteric resistance arteries because the ATjR is reciprocally regulated in these tissues by altered sodium intake. These studies will determine the post-transcriptional mechanisms of AT:R regulation in these reciprocally regulated tissues and how these mechanisms are impaired in aged rats in response to altered sodium intake.