Aldosterone as a mediator of progressive renal disease: Pathogenetic and clinical implications

Aldosterone as a Mediator of Progressive Renal Disease: Pathogenetic and Clinical Implications Murray Epstein, MD ® End-stage renal disease is an enormous public health burden with an increasing incidence and prevalence. This escalating prevalence suggests that newer therapeutic interventions and strategies are needed to complement current antihypertensive approaches. Although much evidence shows that angiotensin II mediates progressive renal disease, recent evidence also implicates aldosterone as an important pathogenetic factor in progressive renal disease. Several lines of experimental evidence show that selective blockade of aldosterone, independent of renin-angiotensin blockade, reduces proteinuria and nephrosclerosis in the spontaneously hypertensive strokeprone rat model and reduces proteinuria and glomeruiosclerosis in the subtotally nephrectomized rat model (ie, remnant kidney). Although pharmacological blockade with angiotensin II-receptor blockers and angiotensinconverting enzyme inhibitors reduces proteinuria and nephrosclerosis and/or glomerulosclerosis, selective reinfusion of amdosterone restores these abnormalities despite continued renin-angiotensin blockade. Based on this theoretic construct, randomized clinical studies will be initiated to delineate the potential renal-protective effects of antihypertensive therapy using aldosterone-receptor blockade. This is a US government work. There are no restrictions on its use.

INDEX WORDS: Aldosterone; aldosterone-receptor antagonist; hypertensive renal disease; end-stage renal disease

(ESRD). ND-STAGE RENAL disease (ESRD) is an enormous public health burden with an incidence and prevalence that is increasing alarmingly in the United States. There were more than 79,000 new cases of ESRD in 1997, with a total prevalence in the United States of approximately 304,000. l Only 2 years before this, in 1995, the US prevalence was 257,266. 2 The prevalence of ESRD has increased every year since 1988, nearly doubling over the past decade. 1 The two most important causes of ESRD are diabetic nephropathy and hypertension, and the incidence attributable t o each continues to increase substantially each year. From 1995 to 1997, the prevalence of ESRD in patients with diabetes increased 25% (from 80,667 to 100,892 patients) and that attributable to hypertension increased 14% (from 63,891 to 72,961 patients). I,a This increase in hypertension-induced ESRD occurred even though other complications of hypertension, such as stroke and coronary heart disease, decreased substantively because of improved blood pressure awareness and control) This escalating prevalence of ESRD suggests that newer therapeutic interventions and strategies are needed to complement current approaches. To adequately address the treatment and prevention of ESRD, we must first determine and appropriately manage the causative mechanisms involved. The mediators of progressive renal disease have not been fully established, but they include a wide array of vasoactive hormones,

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growth factors, and cytokines, including the reninangiotensin-aldosterone system (RAAS), endothelin, transforming growth factor-~, and others. 4-6 The RAAS is an essential causative link between hypertension and ESRD. Clinical trials 7,s have repeatedly shown that interruption of this system dramatically reduces renal injury in patients with hypertension and diabetes. Although angiotensin II is often the primary mediator of the RAAS that is associated with renal disease progression, a number of studies 9-14raised the possibility that aldosterone per se is etiologic in mediating progressive renal disease. Consequently, clinical studies are ongoing to verifY the value of aldosterone-receptor blockade in ~ p r o v ing hypertension-related outcomes, with specific attention to improvements in end-organ protection, including the kidney. 15 This review focuses on the role of hypertension in the development of ESRD and considers emerging evidence that aldosterone, independent of renin:and angiotensin, may participate in meFrom the Department of Medicine, University of Miami School of Medicine, Miami, FL. Received June 8, 2000; accepted in revised form September 15, 2000. Address reprint requests to Murray Epstein, MD, Nephrology Section, VA Medical Center, 1201 N W 16th St, Miami, FL 33125. E-mail: murray, epstein @med. va.gov This is a US government work. There are no restrictions on its use. 02 72-6386/01/3704-000250.00/0 doi: l O.1053/ajkd.2001.22821

American Joumal of Kidney Diseases, Vo137, No 4 (April), 2001: pp 677-688

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diating progressive renal disease. Additionally, the therapeutic implications of aldosteronereceptor blockade are discussed. ROLE OF HYPERTENSION IN PROGRESSIVE RENAL DISEASE

As detailed in a number of recent publications,16-21 hypertension is a major cause of ESRD. Arterial hypertension is not only a primary cause of chronic renal failure, but a risk factor for the progression of renal disease initiated by other mechanisms, such as diabetic nephropathy, chronic glomerulonephritis, and polycystic kidney disease. In these latter settings, the elevation in systemic blood pressure appears to enhance and amplify the progressive decline in renal function. As an example, the Hypertension Detection and Follow-Up Program 19 found a nearly threefold increase in the risk for developing clinically significant hypercreatinemia in patients with a diastolic blood pressure (DBP) of 115 into Hg or greater compared with those with a DBP between 90 and 104 mm Hg. Other well-designed clinical trials ~7,Is,2° have shown similar results; greater blood pressure levels are associated with a decline in renal function evidenced by an increase in serum creatinine level. TRADITIONAL CONCEPTS: RAAS

A number of experimental models, including the subtotal renal ablation model (remnant kidney model), have shown the effects of hypertension in progressive renal disease. Systemic hypertension promotes increased glomerular hydrostatic pressure and increased nephron plasma flow; these promote glomerular hyperfiltration and, ultimately, proteinuria and progressive renal failure. Several investigators using diverse experimental models have shown that not all antihypertensive agents that adequately control systemic pressure are able to reduce glomerular capillary pressure or attenuate renal injury. 22,23 Because increased pressure in the glomerular capillary is the major mediator of renal disease progression, investigators have focused their investigative efforts at this point to interrupt the cycle of progressive renal injury. It was proposed that such pharmacological intervention would prevent and even reverse disease progression. Studies using animal models have shown that

angiotensin-converting enzyme (ACE) inhibitors block the generation of angiotensin II with a consequent decrease in angiotensin II in the renal vasculature, attenuating efferent arteriolar constriction and thus glomerular capillary pressure. 22-24 Based on this theoretic construct, a number of clinical trials 7,s have established that interruption of the RAAS cascade with ACE inhibitors is beneficial in limiting renal disease progression. These trials have validated the concept that counteracting angiotensin II by pharmacological blockade results in attenuation of progressive renal disease. In the Diabetes Collaborative Study, Lewis et al 8 examined whether captopril has renal-protective effects in diabetic nephropathy independent of its capacity to reduce blood pressure. In this trial, patients with insulin-dependent diabetes mellitus, urinary protein excretion of 500 mg/d or greater, and serum creatinine level of 2.5 mg/dL or less were treated with captopril or placebo. The primary end point was a doubling of the baseline serum creatinine concentration. Compared with placebo, patients treated with captopril showed a 50% reduction in the risk for death, dialysis, and renal transplantation that was independent of the difference in blood pressure between groups. This study underscores the preferential renal-protective effect of ACE inhibitors that is independent of blood pressure reduction. Subsequently, the Ramipril Efficacy in Nephropathy (REIN) core study7 examined the use of an ACE inhibitor, ramipril, on the progression of nondiabetic nephropathy. In this study, ramipril was compared with conventional antihypertensire treatment (dosed to achieve DBP < 90 mm Hg) in reducing the rate of decline in glomerular filtration rate (GFR), reducing proteinuria, and preventing ESRD. Results showed that in patients with proteinuria of 3 g/d or greater of protein, the monthly decline in GFR with ramipril was significantly less than that with placebo (P = 0.03), and the risk for renal disease progression was significantly reduced even after adjustment for changes in blood pressure (P = 0.04). In the subsequent REIN follow-up trial, 25ramipril prevented the decline in GFR over time and eliminated the need for dialysis after treatment for 3 years or greater in patients with chronic nephropathy.

ALDOSTERONE: MEDIATOR OF PROGRESSIVE RENAL INJURY

EMERGING CONCEPTS: ROLE OF ALDOSTERONE IN PROGRESSIVE RENAL DISEASE

Although much accumulated evidence implicates angiotensin II in mediating renal disease, recent evidence also suggests that aldosterone is an important factor in causing progressive renal disease through both hemodynamic effects and direct cellular actions (Table 1). 10,26,27Hyperaldosteronism and adrenal hypertrophy are common findings in the remnant kidney model, with plasma levels of aldosterone increased approximately 10-fold. 1° Clinical studies 26,27 have also shown a relationship between augmented levels of aldosterone and renal deterioration. Berl et a126 found that plasma aldosterone levels were elevated in five of eight normokalemic patients with renal failure and five of six patients with a creatinine clearance less than 15 mL/min. In a subsequent study by Hene et a l y plasma aldosterone levels of 28 patients with creatinine clearances less than 50% of normal increased despite normal serum potassium levels and plasma renin activity. It is likely that potassium and angiotensin It, bath !ncreased in patients with renal failure, act synergistically to promote the aldostetone excess that accompanies renal insufficiency and progressive renal disease. 5 A number of experimental models are consistent with the concept that aldosterone may have a Table 1.

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pathogenetic role in mediating renal injury. In a study by Quanet al, ~3 hypertension, proteinuria, and structural renal injury were less preva!ent in rats that underwent subtotal nephrectomy with adrenalectomy compared with rats that underwent partial nephrectomy but had intact adrenal glands. This occurred despite large doses of replacement glucocorticoid (aldosterone was not replaced) in the adrenalectomized rats. In the deoxycorticosterone acetate-salt hypertensive rat model, exogenous mineralocorticoid administration induced lesions of malignant nephrosclerosis and stroke. 9 Development of this lesion is likely caused by the intrinsic action of mineralocorticoids because these rats showed low levels of plasma renin activity and responded poorly to ACE inhibitor therapy. Horiuchi et al n found an increased concenu'ation of aldosterone receptors in the kidneys of a substrain of stroke-prone spontaneously hypertensive rats (SHRSPs), M-SHRSPs. in which the development of malignant nephroscterosis occurred without salt loading. Furthermore, WistarFurth rats, which are unresponsive to the actions of aldosterone, are resistmm to developing nephropathy in response to subtotal nephrectomy.~4 Although many studies bare shown a beneficial effect of ACE inhibition in retarding progressive renal disease, this intervention does not differentiate between the relative contribution of

Relationship Between Augmented Aidosterone Levels and Progressive Renal Disease

Study Description Remnant kidney model Untreated remnant rats had 10-fold greater elevations in aldosterone levels and greater degrees of proteinuria and glomerulosclerosis than sham-operated rats (P < 0.05) Proteinuria and glomerulosclerosis were reduced in remnant rats treated with Iosartan and enalapdl versus untreated remnant rats (P < 0.05) Selective infusion of aldosterone to remnant rats treated with Iosartan and enalapril restored proteinuria and glomerulosclerosis to an extent similar to that seen in untreated remnant rats Clinical studies of patients with chronic renal insufficiency Eight normokalemic patients with chronic renal failure (mean CLcR, 14.3 mL/min) had elevated aldoster0ne levels With normal sodium intake, 5 of 8 patients had elevated aldosterone levels With low sodium intake, all 7 patients had elevated aldosterone levels Atdosterone levels were assessed in 28 patients with varying degrees of renal insufficiency who had normN Serum potassium levels and plasma renin activity All 28 patients with renal insufficiency had increased aldosterone levels compared with 22 healthy volunteers Aldosterone levels were greatest among patients with the greatest impairment in renal function (CLcR, 3 to 10 mL/min) Abbreviation: CLcR , creatinine clearance.

Reference

Greene et al1°

Bed et a126

Hene et a127

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renin and angiotensin versus aldosterone. To evaluate the possible contribution of aldosterone per se, Rocha et a128implanted time-release pellets of spironolactone, an aldosterone-receptor antagonist, or placebo in saline-drinking (1% NaC1) SHRSPs ingesting a Stroke-Prone Rodent diet. This model is known to induce severe hypertension and glomerular and vascular lesions characteristic of the thrombotic microangiopathy observed in malignant nephrosclerosis. Blood pressure and urinary protein excretion, assessed for 3 to 4 weeks, showed that mineralocorticoid-receptor blockade with spironolactone markedly attenuated urinary protein excretion (placebo group, 150 mg/d versus spironolactone group, 39 mg/d; P < 0.0001). Proteinuria remained at baseline levels in the spironolactone group approximately 12 weeks later, although urinary protein excretion remained elevated in placebo-implanted animals (136 mg/d versus spironolactone group, 39 mg/d; P < 0.0001; Fig 1).28 Histological examination showed fewer nephrosclerotic and cerebrovascular lesions in the spironolactone group than the placebo group (P < 0.01 and P < 0.001, respectively). Notably, systolic blood pressure did not differ between the two groups at any time during the study (Fig 1). In a subsequent study, Rocha et a112evaluated whether an aldosterone infusion would reverse the renal-protective effects of captopril therapy in SHRSPs. The study divided SHRSPs into five groups: vehicle (control), captopril (50 mg/kg/ d), aldosterone infusion (40/xg/kg/d), or captopril (50 mg/kg/d) with aldosterone infusion (at 20 and 40/xg/kg/d). Data from this analysis are shown in Fig 2.12 Animals in the control and aldosterone-infusion groups experienced marked proteinuria and similar degrees of renal injury (21% and 29%, respectively). Conversely, captopril treatment reduced endogenous aldosterone levels and prevented the development of proteinuria and glomerular and renal vascular lesions. However, subsequent aldosterone infusion reversed the ability of captopril to confer this protection. The aldosterone-infused captopriltreated rats showed proteinuria and renal vascular and glomerular lesions despite ACE inhibitor treatment. Systolic blood pressure in captopriltreated SHSRPs administered aldosterone infusion was not significantly different than in

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SHSRPs treated with captopril alone. Thus, the renal injury induced by aldosterone was independent of blood pressure increases, suggesting a more direct tissue effect of aldosterone. This concept has been extended to other experimental models, including the remnant kidney model. In the following experiments, as in the previously mentioned studies, the role of aldosterone has been dissociated from that of angiotensin II in the progression of renal disease. Greene et all0 evaluated four treatment groups (shamoperated rats; untreated remnant rats, remnant rats treated with losartan and enalapril, and remnant rats treated with losartan and enalapril followed by aldosterone infusion) to distinguish the relative importance of aldosterone in the progression of renal injury. They observed that remnant rats had a 10-fold elevation in aldosterone levels in comparison with sham-operated rats, Conversely, remnant rats undergoing treatment with losartan and enalapril manifested suppressed al-

ALDOSTERONE: MEDIATOR OF PROGRESSIVE RENAL INJURY

681

["-1 Vehicle (N=8) B B Capt. (N=I 0) Capt. + ALDO 20 (N=6) Capt. + ALDO 40 (N=7) ~ 1 ALDO 40 (N=7)

Fig 2. Effect of aidostetone blockade on urinary protein excretion and glomerulopathy in SHRSPs. Bar graphs showing (A) plasma aldosterone, (B) urinary protein excretion, (C) glomerular lesions, and (D) renal vascular lesions in captopriltreated, saline-drinking SHRSPs at the end of 2 weeks of treatment with aldosterone or vehicle° *P < 0.01 compared with captopril. **P < 0.001 compared with captopril. #P < 0.01 compared with vehicle. ##P < 0.001 compared with vehicle. Values are mean _+ SEM. Abbreviations: capt, captopril; ALDO, aldosterone. (Adapted and reprinted with permission. TM)

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dosterone levels and a decrease in proteinuria, hypertension, and glomerulosclerosis compared with the remnant rats not administered these agents. In the final group, remnant rats administered losartan and enalapril followed by aldosterone infusion, degrees of proteinuria, hypertension, and glomeruloscterosis were similar to those of untreated remnant rats. These results further support an independent pathogenetic role for aldosterone as a mediator of progressive renal disease. EXTRARENAL ADVERSE EFFECTS OF ALDOSTERONE: IMPLICATIONS FOR PROGRESSIVE RENAL INJURY

It should be emphasized that data supporting a pathogenetic role for aldosterone are not limited to the kidney, but are generalized phenomena. For several years, mineralocorticoids, including aldosterone, have been implicated in the injury to extrarenal components of the vascular system. A review of this evidence provides additional insight into the potential mechanisms involved in aldosterone-mediated renal injury. Numerous studies over the past 10 years suggest that the nonepithelial actions of mineralocorticoids are responsible for their vascular and myocardial fibrotic and trophic effects.29-31 Although the primary site of pharmacological ac-

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tion of aldosterone is at mineralocorticoid receptors in the epithelium of the distal nephron, colon, and rectum, receptors also have been located on nonepithelial sites in blood vessels, brain, and heart. 32-35 In addition, sites of aldosterone formation outside the adrenal gland have been discovered, including human endothelial cells and vascular smooth muscle cells (VSMCs) 36 and myocardial cells in animal studies .37 it was recently hypothesized that aldosterone exerts a direct cellular effect to induce fibrosis and hypertrophy in VSMCs and myocardial cells, effects that previously may have been inaccurately attributed solely to the systemic hypertension caused by mineralocorticoids (Table 2 ) . 30'3841 Several studies 29,31 have linked rpdneralocorticolds with myocardial fibrosis through stimulation of collagen formation in myocardial cells. Circulating ald0sterone may mediate vascular fibrosis by the direct interaction of this steroid hormone with high affinity low-capacity corticold receptors located inthe cytosol of vascular fibroblaSts. When activated, the receptor loses its heat-shock protein and its monomeric form reaches the cell nucleus, where it binds to DNA with its binding region to initiate the expression of messenger RNA for type I collagen (or other protein) synthesis. 42

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Table 2. Potential Mechanisms by Which Aldosterone Mediates Fibrosis and Collagen Formation Upregulation of angiotensin II receptorsa° Potentiation of the pressor responses of angiotensin II3° Increases in sodium influx in vascular smooth muscle cells (VSMCs) 39 Inhibition of norepinephrine uptake in VSMCs and myocardial cells 3s,4° Participation in VSMC hypertrophy41 Data from Ullian et al, 3° Barr et al, 38 Kornel and SmosznaKonaszewska, 39 Weber and Purdy, 4° and Stier et al. 41

Ullian et aP ° showed that aldosterone may promote VSMC hypertrophy by inducing upregulation of angiotensin II receptors, thus potentiating the pressor responses of angiotensin II, an effect largely inhibited by spironolactone. Other potential mechanisms of pathogenetic nonepithelial effects of aldosterone include increased sodium influx in cultured VSMCs 39 and potentiation of the effects of catecholamines through inhibition of norepinephrine uptake in VSMCs 4° and myocardial cells. 38 Finally, recent in vivo data indicate that aldosterone activity, assessed by serum aldosterone levels, correlates with plasminogen activator inhibitor-1 antigen levels, supporting an interaction between aldosterone and the fibrinolytic system. 43 Finally, aldosterone produces endothelial dysfunction, perhaps by inhibiting nitric oxide release. 44

Aldosterone-Receptor Blockade: Results From Experimental Models Animal models of aldosterone-receptor blockade have shown the ability of mineralocorticoid antagonism to retard the progression of vascular and myocardial fibrosis, even in the absence of reduced blood pressure. In a trial to discern the individual abilities of angiotensin II antagonism and aldosterone blockade to reverse the pathological accumulation of types I and III collagen in the myocardium, the effects of losartan and spironolactone were compared in the two-kidney, oneclip, hypertensive rat model (Goldblatt model). 45 In untreated Goldblatt model rats, collagen content increased mainly around coronary arteries, and the number and surface area of fibrotic microscars increased. In rats subsequently treated with losartan, both systolic blood pressure and type I collagen fiber content decreased. Spirono-

lactone treatment decreased type lII collagen content without reducing systolic blood pressure. Thus, these results support a humoral effect of aldosterone in addition to hemodynamic effects in stimulating myocardial fibrosis. Other experimental models (including spontaneously hypertensive rats, the Goldblatt model of renovascular hypertension, and aldosterone models of acquired arterial hypertension) have further shown the ability of spironolactone to prevent myocardial and aortic fibrosis independent of blood pressure levels.e6,47 In examining these cellular-toxic effects of aldosterone at sites outside the kidney, it seems possible that such extrarenal actions that promote fibrosis and hypertrophy in vascular and myocardial cells may be at least partly responsible for aldosterone's adverse effects at the level of the glomerulus. As noted previously, hyperaldosteronism is a consistent finding in the remnant kidney model. 10These investigators propose that although sodium retention and vasoconstrictive effects of aldosterone may have a partial role in renal injury, these mechanisms are insufficient explanations for the pathogenetic findings. Other vascular actions of aldosterone, such as enhanced ion permeability in VSMCs, changes in baroreceptor function, and amplification of local vasoconstrictor systems, also may be involved in disease progression.

Aldosterone-Receptor Blockade: Results From Clinical Trials In recent years, clinical trials have begun to validate the ability of aldosterone-receptor antagonism to favorably affect myocardial fibrosis in humans. Using serum measurements of procollagen type HI amino terminal peptide, a serum marker of myocardial collagen turnover, MacFadyen et a148 showed that patients with congestive heart failure treated with a diuretic, ACE inhibitor, and spironolactone had significantly reduced myocardial collagen turnover in comparison with patients treated with a diuretic, ACE inhibitor, and placebo. This trial showed the ability of endogenous aldosterone, which is present despite ACE inhibitor therapy, to stimulate myocardial fibrosis in human cardiac failure. Farquiharson and Struthers 49 indirectly showed that aldosterone could have a role in endothelial

ALDOSTERONE: MEDEATOROF PROGRESSIVE RENAL INJURY

dysfunction in chronic heart failure. They performed a randomized, placebo-controlled, doubleblind, crossover study of 10 patients with New York Heart Association classes II and III chronic heart failure on standard diuretic and ACE inhibitor therapy, comparing 50 mg/d of spironolactone for 1 month versus placebo. Forearm vascular endothelial function was assessed by bilateral forearm venous occlusion plethysmography using acetylcholine and N-monoethyl-L-arginine (L-NMMA), with sodium nitroprusside as a control vasodilator. The aldosterone antagonist, spironolactone, substantively increased forearm blood flow response to acetylcholine compared with placebo, with an associated increase in vasoconstriction caused by L-NMMA. They concluded that antagonizing the aldosterone receptor improves endothelial dysfunction and increases nitric oxide bioactivity in chronic heart failure. Recently, the Randomized Aldactone Evaluation Study (RALES) 15 examined the effect of spironolactone on overall morbidity and mortality in patients with severe heart failure treated with standard therapy with an ACE inhibitor, a loop diuretic, !and digoxin combined with either a nonhemodynamic dose of spironolactone (25 rag/d) or placebo. This seminal trial was discontinued early by the data safety monitoring board after a mean follow-up of 24 months and showed that spironolactone-treated patients had a 30% reduction in the risk for death from all causes compared with the placebo groupl. This reduction in mortality was largely attributed to a reduction in death from progressive heart failure and sudden cardiac death. With respect to improvements in morbidity, patients: in the spironolactone group had a 35% decreased frequency of hospitalization for worsening heart failure compared with patients in the placebo group and showed significant improvement in the symptoms of heart failure (P ~ 0~001). The RALES investigators attributed the beneficial actions of spironolactone to the drug's favorable effects on myocardial and vascular fibrosis and its ability to increase myocardial uptake of norepinephrine, in addition to its anticipated ability to prevent sodium retention and potassium loss.

683 ANIMAL MODELS SUPPORTING A RENAL-PROTECTIVE EFFECT OF ALDOSTERONE-RECEPTOR BLOCKADE

Although it may seem speculative to extrapolate these experimental and clinical data of myocardial and vascular effects of aldosteronereceptor antagonists to potential benefit in the kidney, this information provides insight into the possible beneficial mechanisms of these agents in progressive renal disease. Recent experimental data have shown the ability of aldosteronereceptor antagonists to attenuate proteinuria and renal damage. As described previously, Rocha et a128 showed that aldosterone-receptor blockade with spironolactone markedly attenuated urinary protein excretion, as well as prevented the development of nephrosclerotic and cerebrovascular lesions independent of changes in arterial blood pressure in saline-drinking SHRSPs. Spironolactone. although an effective aldosterone-receptor antagonist, also has affinity for other steroid receptors, thus producing unwanted adverse effects. Consequently, the same investigators recently conducted a second study using eplerenone, a selective aldosterone-receptor antagonist. 5° Using the same saline-drinking SHRSP model, the investigators performed two experiments to evaluate whether eplerenone would provide the same renal-protective effects as spironolactone. In the first experiment, the impact of eplerenone on urinary protein excretion and blood pressure was compared with a control group after infusions of aldosterone or angiotensin II. Eplerenone prevented proteinuria (eplerenone. 15 mg/d of protein versus controls, 92 mg/d of protein; P < 0.001) and renal lesions (eplerenone, 2% versus controls~ 40%: P < 0.0005). However, there were no statistically significant differences between the eplerenone and control groups in blood pressure (226 versus 234 mm Hg, respectively). In the second experiment, five groups were studied: vehicle (control), captopril, captopril followed by an aldosterene infusion, captopril followed by an angiotensin iI infusion, and the combination of captopril and eplerenone followed by an angiotensin II infusion~ After 2 weeks, proteinuria (protein, t58. t21, and 96 mg/d versus 16 mg/d, respectively; P < 0.001) and glomerular (18%, 15%. and 16% versus 0%,

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respectively; P < 0.001) and renal vascular lesions (24%, 26%, and 17% versus 0%, respectively; P < 0.001) were significantly greater in the groups of controls, captopril plus aldosterone, and captopril plus angiotensin II versus the captopril-only group. Thus, both aldosterone and angiotensin II infusions reversed the renal-protective effects of captopril. However, when the group administered captopril plus angiotensin II was compared with the group administered a combination of captopril, eplerenone, and angiotensin II, the latter combination group had substantially less proteinuria (96 versus 28 mg/d of protein, respectively; P < 0.001) and significantly fewer glomerular (16% versus 4%, respectively; P < 0.001) and renal vascular lesions (17% versus 4%, respectively; P < 0.001). In contrast to the reversal of renal protection seen when angiotensin II was added to captopril treatment, the addition of eplerenone to the aforementioned regimen attenuated proteinuria and renal damage in SHRSPs. After 2 weeks of treatment, mean systolic blood pressures were elevated but not statistically different among the five treatment groups. In analogy with previous findings with spironolactone, eplerenone conferred renal protection independently of its effect on blood pressure. Of note, in a recent preliminary communication, Hostetter et al 5~ reported that aldosterone antagonists did not provide nephroprotection in the remnant kidney model despite a significant decrease in left ventricular hypertrophy. The reasons for the discrepancy between the SHRSP and remnant kidney models are not readily apparent. However, several formulations may explain these differences. For example, in the SHRSP studies, the diet was relatively low in potassium and the subsequent increase in aldosterone levels would tend to further worsen a tendency toward hypokalemia. Low potassium levels would tend to magnify the injury, thereby countervailing the ameliorative effects of the blockade. Second, it is conceivable that the renin-angiotensin system may have been altered differentially in the two models and the interplay of renin and aldostetone likely constitutes the final determinant of renal injury. Dosing of the mineralocorticoid antagonists also may account for some of the observed differences. As an example, in their original

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study, Greene et al 1° used a spironolactone dose of 400 mg/kg of body weight daily. In the more recent study, Hostetter et at 51 used 80 to 1O0 mg/100 g of body weight of eplerenone, which is relatively less than the previously used spironolactone dose. THERAPEUTIC IMPLICATIONS

Initially, one might anticipate that the adverse effects of aldosterone could be mitigated merely by blocking aldosterone synthesis by means of ACE inhibitors or angiotensin II-receptor blockers. This unfortunately is not the case. Several investigators have shown that use of ACE inhibitors initially causes an acute decrease in aldosterone concentration, but with continued use, this suppression is not sustained. 5a45 Thus, although it previously was assumed that the use of ACE inhibitors would suffice to suppress aldosterone in addition to the harmful effects of angiotensin II, this is not the case. It has been proposed that the use of aldosterone-receptor antagonists in addition to ACE inhibitors will have additional benefit in the prevention of end-organ damage.52,53

HyperkaIemia as a Potential Limiting Factor to Aldosterone-Receptor Antagonist Therapy An important consideration regarding the feasibility of aldosterone-receptor antagonist therapy is the risk for provoking hyperkalemia. Many patients with chronic renal disease already are administered an ACE inhibitor or angiotensin II-receptor blocker, which would enhance the risk for hyperkalemia. Despite such concerns, the experience with the RALES trial is reassuring. In that study, patients already on ACE inhibitor therapy who were randomized to spironolactone therapy experienced a 0.3-mmol/L increase in median potassium concentrations. Although the difference between the spironolactone and placebo groups achieved statistical significance (P < 0.001), the increase was not considered clinically important.15 Only further investigation that includes rigorous clinical trials will definitively address these concerns. Limitations of Nonselective Aldosterone-Receptor Blockade Although it is an effective antialdosterone agent, widespread use of spironolactone in hu-

ALDOSTERONE: MEDIATOR OF PROGRESSIVE RENAL INJURY

roans is limited by its tendency to produce undesirable sexual side effects. At standard doses, impotence and gynecomastia can be induced in men, whereas premenopausal women may experience menstrual disturbances. These adverse effects, caused by the binding of spironolactone to progesterone and androgen receptors, are substantial causes of drug discontinuation. In a study of 43 patients treated with long-term spironolactone therapy for mineralocorticoid excess syndromes, 13 patients (30%) were switched to alternate therapy because of the occurrence of gynecomastia (6 of 20 men) and menstrual disturbances or breast pain (7 of 23 women). 56 The RALES trial 15 reported a 10% incidence of gynecomastia or breast pain in its male subjects (patients in this triai were administered 25 to 50 mg/d of spironolactone). This incidence was significantly greater than placebo (10% versus 1%, P < 0.001) and caused significantly more patients to discontinue treatment (2% versus 0.2%; P = 0.006). Although troublesome, these side effects are reversible and dose related. The incidence of gynecomastia is 6.9% at doses of 50 mg/d or less but increases to 52% as doses are increased to greater than 150 mg/d. 57 Moreover, this adverse effect has a faster onset at greater dosesl.58 Other studies of women administered spirot~otactone for dermatologic disorders reiterate the dose relationship of these sexual side effects, ie, that doses substantially greater than 100 rng/d are more frequently associated with menstrual disturbances and breast enlargement. ~9,6° The advent of selective aldosteronereceptor antagonists Should reduce these side effects and lead to improved patient compliance with antialdosterone therapy.

Selective Aldosterone-Receptor Antagonists Preliminary data on a selective aldosteronereceptor antagonist, eplerenone, appear promising for the effective blockade of aldosterone and its harmful effects without the sexual disturbances of spironolactone. This agent is currently in phase !II trials for the treatment of hypertension and heart failure. Eplerenone has shown a much :lower affinity for androgen and progesterone receptors than spironolactone, but has twice the pmency with respect to antimineralocorticoid activi!y. 6t As mentioned previously, animal models have shown that i eplerenone prevents the

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occurrence of proteinuria and renal lesions in saline-drinking SHRSPs. 5° Furthermore, in the rat model of ascending aortic stenosis, investigators examined the effect of eplerenone versus no drug on left ventricular (LV) hypertrophy in the presence of severe pressure overload. Despite similar elevations in LV pressure, rats treated with eplerenone had significantly lower heart weight, LV end-diastolic pressure, and LV mass compared with untreated rats (P < 0.05 for all comparisons). 62 Early clinical studies 63,64 support the concept that eplerenone has similar efficacy to spironolactone without showing antiandrogenic adverse effects. In patients with mild to moderate hypertension, eplerenone provided similar blood pressure reductions to spironolactone that were sustained over the 24-hour dosing period. The incidence of adverse effects with eplerenone was similar to that of placebo, with no reports of gynecomastia. 63 In patients with New York Heart Association classes II to IV heart failure, both eplerenone and spironolactone produced significant decreases in brain natriuretic peptide levels and increased urinary aldosterone and piasma renin levels compared with placebo. Consistent with the lower affinity of eplerenone for androgen and progesterone receptors seen in animat studies, men administered spironotactone experienced significant increases in testosterone ~evels (P --< 0.02) compared with eplerenone. 64

Will Low Circulating Aldosterone Levels Mitigate the Renoprotective Effects of Aldosterone-Receptor Antagonists ? Age-related changes in the renin-aldosterone system in healthy humans are well documented. In analogy with plasma renin, aging is associated with a decrease in basal, uptight,posture, and volume-depleted plasma aldosterone levels. 65,66 Also, it is well known that patients with diabetes often tend toward hypoaldosteronism. 67 In addition, recent data suggest that certain older individuals with mild renal insufficiency might develop slight hypovolemia and tend to have decreased plasma aldoster0ne levels~ Whether such decrements in circulating aldosterone wouid mitigate against the therapeutic benefit of aldosterone-receptor b]ockade must be considered, A review of recent investigations suggests that this is not the case. Although the site of mi~eralocor-

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ticoid production was believed to be only the adrenal cortex, recent studies indicate that aldosterone synthesis occurs at extra-adrenal sites, including the endothelium and VSMCs of blood cells, showing direct evidence that vascular cells per se are aldosteronogenic. 36,37 Duprez et a168 proposed that locally produced aldosterone in the vascular endothelial cell may act on the VSMCs through binding to the receptor, thereby acting in a paracrine manner. Second, this ostensible dissociation is consistent with the formulation that determinants of aldosterone effects include both ambient aldosterone levels and aldosterone responsiveness per se. As described previously, Horiuchi et a111 showed an increased concentration of mineralocorticoid receptors in the kidneys of M-SHRSPs (a substrain of SHRSPs), in which the development of malignant nephrosclerosis occurs without salt loading. These observations suggest that an increased mineralocorticoid response may occur in the absence of an increase in circulating aldosterone levels. Takeda et 0.]69 showed that vascular aldosterone and aldosterone synthase messenger R N A levels in the mesenteric arteries of 2-week-old SHRSPs were significantly increased compared with those of age-matched normotensive Wistar-Kyoto rats, although vascular aldosterone levels did not differ. 69 These observations are consistent with an increase in aldosterone responsiveness. Thus, aldosterone response can be readily dissociated from circulating aldosterone levels. Only the use of aldosterone-receptor antagonists as a pharmacological probe will unmask the pathogenetic role of aldosterone. SUMMARY AND POTENTIAL CLINICAL IMPLICATIONS

Recent observations indicate that it is no longer appropriate to consider the endocrine or paracrine properties of aldosterone restricted to classic target cells. H e m o d y n a m i c and humoral actions of aldosterone have important clinical implications for the pathogenesis of progressive renal disease and consequently may influence future antihypertensive strategies. Although ACE inhibitors are very effective in retarding disease progression, additional benefit theoretically may be achieved with concurrent aldosterone-receptor blockade. As observed in clinical studies of

congestive heart failure, as well as in animal models of renal disease, antagonism of aldosterone protects against end-organ damage through both hemodynamic and direct cellular actions. With the advent of selective aldosterone-receptor antagonists, it is now feasible to conduct proofof-principle antihypertensive studies to assess whether end-organ damage, including progressive renal disease, can be more effectively prevented without the dose-limiting side effects of nonspecific aldosterone-receptor blockade. REFERENCES

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