Tissue-Specific Regulation of Type 1 Angiotensin II Receptor mRNA Levels in the Rat

Abstract

Most of the biological effects of the renin-angiotensin system are mediated by the binding of angiotensin II (Ang II) to the type 1 Ang II (AT ₁ ) receptor, the predominant receptor subtype present after fetal life. To study tissue-specific regulation of the expression of the AT ₁ receptor in the rat, we altered activity of the renin-angiotensin system by feeding rats a low (0.07% NaCl), normal (0.3% NaCl), or high (7.5% NaCl) salt chow for 14 days; infusing Ang II (200 ng/kg per minute IP) or vehicle for 7 days; and administering an angiotensin-converting enzyme inhibitor (captopril, 100 mg/dL in the drinking water) or vehicle for 7 days. Renin, angiotensinogen, and total AT ₁ receptor mRNA levels were measured by slot-blot hybridization with cRNA probes, and AT ₁ receptor subtypes (A and B) were measured by reverse transcription–polymerase chain reaction in the presence of a cRNA internal standard. Plasma renin concentration and renal renin, renal and hepatic angiotensinogen, and hepatic AT ₁ receptor mRNA levels were all inversely related to salt intake; in contrast, renal AT ₁ receptor mRNA levels were significantly lower in rats fed low salt, a difference that was exclusively due to a decrease in the AT _1A subtype. This difference did not appear to be mediated by a change in the circulating levels of Ang II, because Ang II infusion reduced plasma renin concentration and renal renin mRNA with no effect on either angiotensinogen or AT ₁ receptor mRNA levels in kidney or liver; renal Ang II receptor density (determined by in situ autoradiography) decreased, presumably via a posttranscriptional mechanism. Similarly, inhibition of Ang II generation with captopril increased plasma renin concentration and renal renin mRNA levels without altering renal or hepatic angiotensinogen mRNA or renal AT ₁ receptor mRNA levels. Thus, AT ₁ receptor gene expression is regulated in a tissue-specific manner that is distinct from other components of systemic and local renin-angiotensin systems and that appears to be mediated by a mechanism other than through changes in the circulating levels of Ang II.