DIFFERENTIAL REGULATION OF ANGIOTENSIN-II RECEPTOR SUBTYPES IN RAT-KIDNEY BY LOW DIETARY-SODIUM

This study was designed to determine whether expression of renal messenger RNA (mRNA) encoding the two known angiotensin II type 1 (AT(1)) receptor subtypes (AT(1A) and AT(1B)) can be regulated by dietary sodium. Seven-week-old male Wistar rats were fed a low-sodium diet (0.07%, n=9) or a normal-sodium diet (0.5%, n=9 [control]) for 14 days. A rat AT(1) complementary DNA (cDNA) probe, which hybridizes to mRNA encoding both the AT(1A) and AT(1B) receptor subtypes, and cDNA probes, which are selective for AT(1A) or AT(1B) mRNA, were used in Northern blot or in situ hybridization analysis. By use of Northern blot analysis, renal mRNA levels for the AT(1) and AT(1A) receptors in rats fed a low-sodium diet were found to be increased twofold (P<.05) compared with control. Because renal AT(1B) mRNA content was not detected by Northern blot analysis, quantitative image analysis of in situ hybridization with a digoxigenin-labeled cRNA probe made from AT(1B) cDNA was used. In situ hybridization analysis indicated that AT(1B) mRNA was expressed in the proximal and collecting tubules of the kidney in rats fed a normal-sodium diet. The low-sodium diet significantly decreased the percent positive staining area of AT(1B) mRNA in the renal cortex (5.51 +/- 0.77% versus 2.73 +/- 0.35%, P<.05) and medulla (4.7 +/- 0.70% versus 2.01 +/- 0.43%, P<.05) compared with the control diet. These results indicate that the increase in AT(1) mRNA levels in the kidney induced by low sodium intake is the result of a selective increase in AT(1A) mRNA and suggest that AT(1A) is the predominant receptor subtype of AT(1) in the kidney. The data also suggest that dietary sodium differentially modulates the expression of genes encoding AT(1) receptor subtypes, because there is an inverse relationship between the expression of the AT(1A) and AT(1B) subtypes in response to a low-sodium diet. The functional implications are discussed.