Activation of the renin-angiotensin system by a low-salt diet does not augment intratubular angiotensinogen and angiotensin II in rats

Abstract

In angiotensin II (ANG II) infusion hypertension, there is an augmentation of intratubular angiotensinogen (AGT) and ANG II leading to increased urinary AGT and ANG II excretion rates associated with tissue injury. However, the changes in urinary AGT and ANG II excretion rates and markers of renal injury during physiologically induced stimulation of the renin-angiotensin system (RAS) by a low-salt diet remain unclear. Male Sprague-Dawley rats received a low-salt diet (0.03% NaCl; n = 6) and normal-salt diet (0.3% NaCl, n = 6) for 13 days. Low-salt diet rats had markedly higher plasma renin activity and plasma ANG II levels. Kidney cortex renin mRNA, kidney AGT mRNA, and AGT immunoreactivity were not different; however, medullary renin mRNA, kidney renin content, and kidney ANG II levels were significantly elevated by the low-salt diet. Kidney renin immunoreactivity was also markedly increased in juxtaglomerular apparati and in cortical and medullary collecting ducts. Urinary AGT excretion rates and urinary ANG II excretion rates were not augmented by the low-salt diet. The low-salt diet caused mild renal fibrosis in glomeruli and the tubulointerstitium, but no other signs of kidney injury were evident. These results indicate that, in contrast to the response in ANG II infusion hypertension, the elevated plasma and intrarenal ANG II levels caused by physiological stimulation of RAS are not reflected by increased urinary AGT or ANG II excretion rates or the development of renal injury.

Fig. 1.

A: comparison of systolic blood pressures (SBPs) between low-salt diet (n = 6) and normal-salt diet rats (n = 6). B: comparison of urinary sodium excretion rates in low-salt diet (n = 6) and normal-salt diet rats (n = 6). Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.

Fig. 2.

A: comparison of plasma renin activity in low-salt diet (n = 6) and normal-salt diet rats (n = 6). B: comparison of plasma ANG II in low-salt diet (n = 6) and normal-salt diet rats (n = 6). Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.

Fig. 3.

A: comparison of kidney renin content in low-salt diet (n = 6) and normal-salt diet rats (n = 6). B: comparison of kidney ANG II in low-salt diet (n = 6) and normal-salt diet rats (n = 6). Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.

Fig. 4.

A: comparison of kidney cortex and medulla renin/β actin mRNA (% control) in low-salt diet (n = 6) and normal-salt diet rats (n = 6). B: quantification of the intensity of kidney renin immunoreactivity in normal-salt diet (a and c; n = 6) and low-salt diet (b and d; n = 6) rats. b: increased renin immunoreactivity in juxtaglomerular (JG) renin and cortical collecting duct cells in low-salt diet compared with normal-salt diet rats (a). d: increased renin immunoreactivity in medullary collecting duct cells in low-salt diet compared with normal-salt diet rats (c). e–g: densitometric analyses of the renin intensity immunoreactivity in JG renin, cortical collecting duct (CD) renin and medullary collecting duct renin. IHC, immunohistochemistry. Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.

Fig. 5.

A: comparison of urinary angiotensinogen (AGT) excretion rates in low-salt diet (n = 6) and normal-salt diet rats (n = 6). B: comparison of urinary ANG II excretion rates in low-salt diet (n = 6) and normal-salt diet rats (n = 6). Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.

Fig. 6.

Representative photomicrographs of glomerular expansion in normal-salt diet (A) and low-salt diet (B) rats and image analysis (C). Proliferative cell nuclei antigen (PCNA) was photomicrographed in glomeruli [D: normal salt (NS); E: low salt (LS); F: image analysis] and also in the tubulointerstitium (G: NS; H: LS; I: image analysis). Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.

Fig. 7.

Kidney immunoexpression of CD-68 as a marker of macrophages in cortical glomeruli (A: NS; B: LS; C: image analysis), juxtamedullary glomeruli (D: NS; E: LS; F: image analysis) and the tubulointerstitium (G: NS; H: LS; I: image analysis). Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.

Fig. 8.

Representative photomicrographs of collagen deposition in glomeruli (A: NS; B: LS; C: image analysis) and the tubulointerstitium (D: NS; E: LS; F: image analysis). Values are means ± SE. *P < 0.05 vs. normal-salt diet rats.