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.
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