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, 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 SI 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.
