Effect of angiotensin II blockade on renal injury in mineralocorticoid-salt hypertension

Expression of NAD(P)H oxidase subunits and their contribution to cardiovascular damage in aldosterone/salt-induced hypertensive rat

NAD(P)H oxidase plays an important role in hypertension and its complication in aldosterone-salt rat. We questioned whether NAD(P)H oxidase subunit expression and activity are modulated by aldosterone and whether this is associated with target-organ damage. Rats were infused with aldosterone (0.75 microg/hr/day) for 6 weeks and were given 0.9% NaCl+/-losartan (30 mg/kg/day), spironolactone (200 mg/kg/day), and apocynin (1.5 mM/L). Aldosterone-salt hypertension was prevented completely by spironolactone and modestly by losartan and apocynin. Aldosterone increased aortic NAD(P)H oxidase activity by 34% and spironolactone and losartan inhibited the activity. Aortic expression of the subunits p47(phox), gp91(phox), and p22(phox) increased in aldosterone-infused rats by 5.5, 4.7, and 3.2-fold, respectively, which was decreased completely by spironolactone and partially by losartan and apocynin. Therefore, the increased expression of NAD(P)H oxidase may contribute to cardiovascular damage in aldosterone-salt hypertension through the increased expression of each subunit.

Augmentation of Intrarenal Angiotensin II Levels in Uninephrectomized Aldosterone/Salt-Treated Hypertensive Rats; Renoprotective Effects of an Ultrahigh Dose of Olmesartan

Recent studies have suggested that aldosterone plays a role in the pathogenesis of renal injury. In this study, we investigated whether local angiotensin II (Ang II) activity contributes to the progression of renal injury in aldosterone/salt-induced hypertensive rats. Uninephrectomized rats were treated with 1% NaCl in a drinking solution and one of the following combinations for 6 weeks: vehicle (2% ethanol, s.c.; n=9), aldosterone (0.75 mug/h, s.c.; n=8), aldosterone+Ang II type 1 receptor blocker olmesartan (10 mg/kg/day, p.o.; n=8), or aldosterone+olmesartan (100 mg/kg/day, p.o.; n=9). Aldosterone/salt-treated hypertensive rats exhibited severe proteinuria and renal injury characterized by glomerular sclerosis and tubulointerstitial fibrosis. Aldosterone/salt-induced renal injury was associated with augmented expression of angiotensin converting enzyme and Ang II levels in the renal cortex and medullary tissues. Renal cortical and medullary mRNA expression of transforming growth factor-beta (TGF-beta) and connective tissue growth factor (CTGF) as well as the collagen contents were increased in aldosterone/salt-treated hypertensive rats. Treatment with olmesartan (10 or 100 mg/kg/day) had no effect on blood pressure but attenuated proteinuria in a dose-dependent manner. Olmesartan at 10 mg/kg/day tended to decrease renal cortical and medullary Ang II levels, TGF-beta and CTGF expression, and collagen contents; however, these changes were not significant. On the other hand, an ultrahigh dose of olmesartan (100 mg/kg/day) significantly decreased these values and ameliorated renal injury. These data suggest that augmented local Ang II activity contributes, at least partially, to the progression of aldosterone/salt-dependent renal injury.

Effect of Aldosterone and MR Blockade on the Brain and the Kidney

The renin-angiotensin-aldosterone system (RAAS) plays a central role in the development of hypertension and the progression of end-organ damage. Although angiotensin-I converting enzyme (ACE) inhibitors and angiotensin II (Ang II) subtype-1 (AT(1)) receptor antagonists can initially suppress plasma aldosterone, it is now well established that aldosterone escape may occur whereby aldosterone levels return to, or exceed, baseline levels. The classical effects of aldosterone relate mainly to its action on epithelial cells to regulate water and electrolyte balance. However, the presence of mineralocorticoid receptors (MR) at nonepithelial sites in the brain, heart and vasculature, is consonant with the fact that aldosterone also has direct effects in these tissues. Substantial evidence now exists that supports the action of aldosterone at non-epithelial sites which in turn provokes a number of deleterious effects on the cardiovascular system including necrosis and fibrosis of the vasculature and the heart, vascular stiffening and injury, reduced fibrinolysis, endothelial dysfunction, catecholamine release and production of cardiac arrhythmias. Several studies have now shown that vascular and target-organ protective effects of MR antagonism occurs in the absence of significant blood pressure lowering or fluid loss, which is consistent with a major role for endogenous mineralocorticoids as direct mediators of cardiovascular injury. Adverse cardiovascular effects may occur in response to aldosterone alone, activation of the RAAS or aldosterone escape during chronic ACE inhibition or AT(1) receptor antagonism. The specific blockade of aldosterone action should prove to be of great therapeutic value in the prevention of cerebral and renal vascular disease and associated end-organ damage.

Aldosterone plays a pivotal role in the pathogenesis of thrombotic microangiopathy in SHRSP

Angiotensin-converting enzyme inhibitors and aldosterone receptor antagonists ameliorate malignant nephrosclerotic lesions of thrombotic microangiopathy in salt-loaded, stroke-prone, spontaneously hypertensive rats (SHRSP) without controlling hypertension. This suggests that angiotensin II (Ang II) and/or aldosterone (ALDO) plays a critical role in renal injury in this model. For evaluating their relative roles in the pathogenesis of thrombotic microangiopathy, SHRSP were adrenalectomized and infused with vehicle, Ang II, or ALDO or were sham-operated for adrenalectomy (SHAM). Saline-drinking rats were assigned to one of four groups: SHAM, adrenalectomy, adrenalectomy + Ang II (25 ng/min, subcutaneously), or adrenalectomy + ALDO (40 micro g/kg per d, subcutaneously). All SHRSP received dexamethasone (12 micro g/kg per d, subcutaneously). Adrenalectomy did not show changes in body weight, plasma creatinine, sodium and potassium, and daily urinary sodium and potassium excretion; did not prevent hypertension but prevented proteinuria (12 +/- 1 versus 49 +/- 3 mg/d; P < 0.01); and abrogated thrombotic microangiopathy and decreased plasma aldosterone (<16 versus 710 +/- 91 pg/ml; P < 0.001) compared with SHAM. Systolic BP in adrenalectomy + Ang II and adrenalectomy + ALDO (238 +/- 8 and 241 +/- 9 mmHg, respectively) was similar to SHAM. Despite Ang II infusion, proteinuria (17 +/- 9 mg/d) and thrombotic microangiopathy and plasma aldosterone (18 +/- 18 pg/ml) remained low but daily urinary excretion of sodium and potassium were not different from adrenalectomy + ALDO. Adrenalectomy + ALDO showed plasma aldosterone levels of 735 +/- 147 pg/ml; plasma potassium was lower; plasma creatinine and proteinuria (78 +/- 7 mg/d) were greater and thrombotic microangiopathy lesions were comparable to SHAM. These results demonstrate a pivotal role for aldosterone in the development of thrombotic microangiopathy, independent of hypertension.

Aldosterone in renal disease

PURPOSE OF REVIEW

Interruption of the renin-angiotensin-aldosterone system, chiefly with angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers, has yielded beneficial results in retarding injury and progression in numerous intrinsic renal diseases. The renoprotection offered by these agents is incomplete and far from optimal. Studying mediators of progression other than angiotensin II is therefore extremely important. The emerging role of aldosterone in progression of renal disease and the utility of its antagonism is discussed here.

RECENT FINDINGS

The experimental evidence linking aldosterone to renal disease is discussed. The exciting results from clinical studies employing mineralocorticoid receptor blockers are also described.

SUMMARY

Aldosterone antagonism offers additional antiproteinuric benefits to those achieved with angiotensin-converting enzyme inhibition. Long-term trials addressing effectiveness and safety, especially in regards to hyperkalemia, are greatly needed.

Aldosterone as a mediator in cardiovascular injury

The renin-angiotensin-aldosterone system plays a central role in the development of hypertension and the progression of end-organ damage. Although angiotensin-converting enzyme inhibitors and angiotensin II receptor antagonists can initially suppress plasma aldosterone, it is now well established that aldosterone escape may occur, whereby aldosterone levels return to or exceed baseline levels. The classic effects of aldosterone relate mainly to its action on epithelial cells to regulate water and electrolyte balance. However, blood pressure reduction or fluid loss could not account for the results of the Randomized Aldactone Evaluation Study, which showed that a low dose of spironolactone in addition to conventional therapy could decrease the overall risk of mortality by 30% among patients with severe congestive heart failure. The action of aldosterone at nonepithelial sites in the brain, heart, and vasculature is consistent with the presence of mineralocorticoid receptors in these tissues. Aldosterone has a number of deleterious effects on the cardiovascular system, including myocardial necrosis and fibrosis, vascular stiffening and injury, reduced fibrinolysis, endothelial dysfunction, catecholamine release, and production of cardiac arrhythmias. Several studies have now shown vascular and target-organ protective effects of aldosterone receptor antagonism in the absence of significant blood pressure lowering, consistent with a major role for endogenous mineralocorticoids as mediators of cardiovascular injury. The advent of selective aldosterone receptor antagonists such as eplerenone should prove of great therapeutic value in the prevention of cardiovascular disease and associated end-organ damage.

Glomerular osteopontin expression and macrophage infiltration in glomerulosclerosis of DOCA-salt rats

Expression of the chemoattractant osteopontin (OPN) may contribute to macrophage infiltration in many types of tubulointerstitial kidney disease, but the role of OPN in chronic glomerulosclerosis is unknown. We hypothesized that glomerular OPN expression and macrophage infiltration occur in deoxycorticosterone acetate (DOCA)-salt glomerulosclerosis in rats. Uninephrectomized rats receiving DOCA pellets and 1% saline were compared with control rats. OPN mRNA was determined by Northern blot, and OPN protein was determined by Western blot. The localization of OPN was studied by in situ hybridization and double immunohistochemistry with glomerular cell markers. Macrophage infiltration was quantified by counting ED-1-positive cells, and semiquantitative glomerulosclerosis scores were obtained. In DOCA-salt rats, OPN mRNA in the kidney was increased 2-fold over control after 9 days and 3 weeks and 20-fold after 6 weeks. Tubulointerstitial OPN staining was apparent after 21 days of DOCA treatment. Glomerular OPN mRNA and protein was detected after 42 days in parietal and visceral epithelial cells, activated myofibroblasts, and occasionally mesangial cells. Progressive glomerular macrophage infiltration occurred during the development of DOCA hypertension, paralleling the degree of glomerulosclerosis. Glomeruli staining positive for osteopontin contained more macrophages (18.4 +/- 3.4 per cross-section) than osteopontin-negative glomeruli (3.6 +/- 0.5; P < 0.05). Glomerular OPN expression occurs in chronic hypertensive glomerulosclerosis and is associated with macrophage infiltration. The data suggest a role for OPN as a chemoattractant in hypertensive glomerulosclerosis.

Aldosterone in renal disease

Blockade of the renin-angiotensin-aldosterone system has proved effective in retarding the progression of renal disease in the remnant kidney model, as well as other experimental diseases, and most importantly, in a range of progressive human renal diseases. Attention has focused on the role of angiotensin II in propagating progression both by its hemodynamic and non-hemodynamic actions. Recent evidence, predominantly in the remnant kidney model, indicates that the drugs used to block this hormone system, angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers, also lower aldosterone levels. Aldosterone as well as angiotensin II thus appears to be instrumental in sustaining the hypertension and fibroproliferative destruction of the residual kidney.