- Email: [email protected]
New biology of aldosterone, and experimental studies on the selective aldosterone blocker eplerenone
New biology of aldosterone, and experimental studies on the selective aldosterone blocker eplerenone John W. Funder, MD, PhD, FRACP Victoria, Australia
Background Classically, we have recognized six classes of steroid hormones: mineralocorticoids, glucocorticoids, androgens, estrogens, progestins, and vitamin D derivatives. The adrenal cortex is the principal, if not unique, source of mineralocorticoids (aldosterone) and glucocorticoids (cortisol and, in rats and mice, corticosterone), with secretion of both hormones under feedback control. For glucocorticoids, the major stimulus to secretion is adrenocorticotropic hormone (ACTH) from the pituitary gland, with cortisol exerting negative feedback at a variety of levels (hippocampus, hypothalamus, pituitary). Aldosterone secretion is elevated by either of two factors: angiotensin II, produced by the action of renin and angiotensin-converting enzyme, and plasma potassium levels. Aldosterone acts to conserve sodium (and with it, water) at epithelia such as the kidney distal tubule, colon, sweat gland, and salivary gland, and to promote potassium excretion, in this way to lower levels of both trophic stimuli. The evolutionary biology of aldosterone is important for an appreciation of its clinical roles in several respects. Mammalian evolution in general, and human evolution in particular, is characterized by restricted sodium intake (fruit/vegetable diet) and the possibility of catastrophic sodium loss (diarrhea) on top of obligate sodium loss (sweating in tropical environment). Many species, human included, have thus evolved a hunger for salt, reflecting aldosterone action on the brain. They have also evolved, not surprisingly, multiple independent mechanisms of aldosterone elevation in response to sodium deficiency. An outstanding example of this redundancy is the angiotensinogen knockout (Agen –/–) mouse, which can elevate its aldosterone secretion on a low-salt diet indistinguishably from wild type; only when a low sodium and potas-
From the Centre for Neurosciences, University of Melbourne and Prince Henry’s Institute of Medical Research, Monash Medical Centre, Victoria, Australia. Reprint requests: John W. Funder, MD, PhD, FRACP, Prince Henry’s Institute of Medical Research, Block E, Level 4, PO Box 5152, Monash Medical Centre, 246 Clayton Rd, Clayton, Victoria, Australia 3168. E-mail: [email protected] Am Heart J 2002;144:S8-11. Copyright 2002, Mosby, Inc. All rights reserved. 0002-8703/2002/$35.00 ⫹ 0 4/0/129971 doi:10.1067/mhj.2002.129971
sium diet is given is aldosterone secretion compromised in Agen –/– mice.1 What we have not evolved are mechanisms to sufficiently lower aldosterone secretion commensurately in the face of a physiologically very high salt intake, which is now the norm in many places in the developed and developing world.
The new biology of aldosterone The new biology of aldosterone might be said to have begun in 1983 with the demonstration that mineralocorticoid receptors in the rat kidney had equally high affinity for aldosterone and the physiologic glucocorticoid corticosterone; moreover, indistinguishable “mineralocorticoid receptors” could be found in the rat brain2 and subsequently, the heart.3 These data were confirmed and extended by the cloning of the human kidney mineralocorticoid receptor4; again, expression was clearly present in rat brain and heart on Northern blot analysis, as well as kidney and gut. In addition, the cloned human mineralocorticoid receptor expressed in COS cells showed equivalent high affinity for aldosterone and cortisol. For both rat and human mineralocorticoid receptors, this equivalence in affinity poses a particular physiologic problem, in that circulating free glucocorticoid levels are commonly two orders of magnitude higher than those of aldosterone, prima facie making it very difficult for aldosterone to occupy and activate such nonselective receptors. What allows aldosterone occupancy in epithelial cells is the expression, at high levels (3-4 ⫻ 106 copies per cell) of the enzyme 11-hydroxysteroid dehydrogenase (11HSD) type 2, which converts cortisol and corticosterone into their receptor inactive 11-keto congeners, cortisone and 11-dehydrocorticosterone.5,6 Aldosterone is characterized by (and named after) its unique aldehyde (CHO) group at carbon 18, in place of the otherwise universal methyl group. This aldehyde group cyclizes with the hydroxyl group at carbon 11 to form an 11,18 hemiacetal and to make aldosterone not a substrate for 11HDS2 (Figure 1). Over the past decade, it has also become clear that inappropriate aldosterone levels for salt status can have major pathophysiologic effects through mineralocorticoid receptor occupancy in the brain and heart. Gomez-Sanchez et al7 showed (in salt-sensitive rats) that infusion of the mineralocorticoid receptor blocker
American Heart Journal Volume 144, Number 5
RU 28318 intracerebroventricularly, at doses without effect when infused systemically, completely blocked the elevation in blood pressure (BP) normally seen when such rats are given 0.9% NaCl solution to drink. In ground-breaking studies on cardiac fibrosis, Brilla and Weber8 showed that infusion of robust doses of aldosterone to uninephrectomized rats with 0.9% NaCl solution to drink for 8 weeks produced hypertension, perivascular and interstitial cardiac fibrosis, and cardiac hypertrophy. Importantly, infusion of aldosterone into rats on a low-salt diet did not elevate BP or produce the cardiac fibrosis and hypertrophy seen with the aldosterone/salt combination. Studies by Young et al9 showed that these cardiac effects of aldosterone/salt are not secondary to the induced BP rise. When rats were infused peripherally with aldosterone under standard conditions and intracerebroventricularly with RU 28318 at peripherally ineffective doses, the BP remained clamped at normotensive control levels. In contrast, the trophic effects of aldosterone on the heart in terms of hypertrophy and fibrosis were indistinguishable from those seen in animals infused with aldosterone alone.9 Further evidence for a direct trophic effect comes from the data of Sato and Funder10 in studies on isolated rat neonatal cardiomyocytes. Aldosterone, but not corticosterone, substantially increased leucine incorporation into protein, an effect potentiated by glucose through protein kinase C-dependent mechanisms.10 In subsequent studies using a deoxycorticosterone (DOCA)/salt rather than the aldosterone/salt model, Fujisawa et al11 made two additional sets of observations. Whereas in the aldosterone-infused rats no evidence of cardiac hypertrophy or fibrosis could be seen until 3 to 4 weeks after starting the infusion, rats given weekly injections of DOCA showed substantial elevations of BP by day 4 and cardiac type III collagen accumulation by day 2 after the initial dose. Second, at days 4 and 8 (but not at days 2, 16, or 32), sections of the heart are characterized on TUNEL staining by massive necrosis and apoptosis and on hematoxylin and eosin staining by substantial perivascular infiltration of inflammatory cells.11 In subsequent studies (Labib et al, unpublished) using the same model, elevated levels of some (eg, TNF␣, ED-1) but not all (eg, surprisingly, TGF) of the common inflammatory markers were seen in vessel walls and perivascular tissue. It is in this context that the results of the recent RALES trial need to be placed.
The RALES trial In this trial, two groups of patients were compared, both with severe (New York Heart Association class III-IV) heart failure. One group continued to receive their standard medication (angiotensin-converting en-
Funder S9
Figure 1
Flow chart of a principal cell in the distal tubule of the kidney, showing metabolism of cortisol to the receptor-inactive cortisone, allowing aldosterone (with its 11-OH group protected by cyclization) to access mineralocorticoid receptors. HSD, Hydroxysteroid dehydrogenase.
zyme inhibitor, diuretic, and so forth) plus placebo; the second group received standard medication plus low-dose (average 26 mg/d) spironolactone, a nonselective aldosterone antagonist. Although recruitment was halted halfway through the trial because of the divergence between the two groups, ⬎800 patients in each group were followed for three years, with only three salient differences between groups. First, the placebo group consistently ran plasma (K⫹) values 0.2 to 0.3 mEq lower than the aldosterone antagonist– treated group. Second, the spironolactone group showed a 10% incidence of gynecomastia (reflecting the nonselectivity of spironolactone and its action as an androgen antagonist) compared with 1% for placebo. Finally, the spironolactone-treated group showed a 30% improvement in mortality rate and a 35% improvement in morbidity compared with placebo in patients receiving standard “best practice” treatment.12 Although the mechanism of therapeutic action of aldosterone blockade under such circumstances remains to be established, a substudy of the RALES trial strongly suggests an effect on fibrosis, known to be involved in both sudden cardiac death and progression of heart failure. In this substudy, circulating levels of the N-terminal portion of the procollagen type III precursor were measured before and after spironolactone treatment and similarly in the placebo group. Survival was clearly better in patients with low circulating precursor levels; importantly, spironolactone administration improved outcomes in patients in whom it dropped initially elevated precursor levels rather than in the treated population as a whole.13 The profibrotic effect of aldosterone/salt and the protective effect of
S10 Funder
Figure 2
Effects of aldosterone (gray bars) and eplerenone plus aldosterone (white bars) on inflammatory markers in perivascular area in rat heart after 7, 14, and 28 days’ exposure. COX-2, cyclooxygenase-2; MCP-1, momocyte chemoattractant protein-1; TGF1, transforming growth factor-1. (Reprinted with permission from: Rocha R, Rudolph AE, Frierdich GE, et al. Aldosterone induces a vascular inflammatory phenotype in the rat heart. AJP-Heart and Circulatory Physiology 2002;283:8102–10.)
aldosterone antagonism in terms of vascular and interstitial fibrosis thus form the context for the series of experimental studies on end-organ protection by the selective aldosterone blocker eplerenone.
Eplerenone: Selective aldosterone blocker The three tissues in which aldosterone-induced vasculitis appears to play a major role are the brain, kidney, and heart. It has long been known that prolonged exposure to aldosterone (eg, in the syndrome of glucocorticoid-remediable aldosteronism) is characterized by a preponderance of death from stroke rather than heart attack, compared with other causes of hypertension. One model of this state is the stroke-prone spontaneously hypertensive rat (SHRSP), which, even when receiving 0.9% NaCl to drink, shows elevated plasma renin and aldosterone levels. Such SHRSP characteristically die at 13 to 18 weeks of age from stroke. When treated with eplerenone, however, 6 of 7 SHRSP sur-
American Heart Journal November 2002
vived for 18 weeks, at which point they were killed and necropsied. At necropsy, Rocha et al14 found marked fibrinoid necrosis in the cerebral vessels of untreated SHRSP, with considerably thickened walls and morphologic evidence of tissue damage; changes essentially absent from equivalent sections of eplerenone-treated SHRSP. Similar studies in SHRSP show an equivalent level of end-organ protection in the kidney.15 Consistent with their state of advanced hypertension and elevated plasma renin and aldosterone, SHRSP show evidence of renal damage, both functionally (eg, albuminuria) and structurally (eg, glomerulosclerosis, tubular dilatation, and casts). Just as in the cerebral vessels, eplerenone treatment of SHRSP protects the kidney, in terms of morphometric indexes of renal damage and proteinuria, despite a continuing elevated level of angiotensin. What needs to be emphasized in terms of both cerebrovascular and renal protective effects of eplerenone is that at the doses used, no reduction in BP was seen (ie, the protective effect has a humoral rather than a hemodynamic basis), which is consistent with the direct effects of aldosterone on the heart previously cited.9,10 In an additional series of in vivo studies, Rocha et al16 have also demonstrated the cardioprotective effects of eplerenone in aldosterone-infused uninephrectomized rats drinking 0.9% NaCl solution. At the dose of eplerenone used, BP was significantly reduced after 14 and 28 days, although to levels that remained clearly hypertensive. In groups of rats killed at 7, 14, and 28 days, markers of vasculitis/perivascular inflammation in the heart (ED-1, MCP, osteopontin, COX-2) were clearly and progressively elevated in aldosteronetreated animals; in the presence of eplerenone, however, the changes were reversed to (or close to) control (Figure 2).16 Other markers of inflammation (eg, TGF, ICAM, VCAM) were marginally if at all elevated by aldosterone treatment, evidence that some but perhaps not all inflammatory pathways are activated by aldosterone/salt. Finally, in terms of the vascular-protective effect of eplerenone, Ward et al17 have recently published studies on the effects of eplerenone on the response to experimental angioplasty in the pig. Four groups of animals were used (control, spironolactone, eplerenone, and aldosterone infused) in studies comparing coronary and iliac artery angioplasty. In sections of iliac artery, no differences were seen between groups in terms of vascular architecture 4 weeks after angioplasty. In the coronary arteries, however, eplerenone-treated animals showed significantly larger vessel diameter and, perhaps more importantly, substantially preserved luminal area compared with control. Aldosterone-infused pigs tended to have lumen areas less than control, and spironolactone-infused pigs
American Heart Journal Volume 144, Number 5
were intermediate between control and eplerenone, although in neither instance were the differences significant on a population basis. Although aldosterone infusion did not reduce vessel cross-sectional area or lumen size compared with control, it did increase medial collagen density by ⬃75%. Although the mechanism of coronary protection after angioplasty by eplerenone remains to be explored, these studies point to a tonic effect of normal circulating levels of aldosterone in such circumstances, given the effects of receptor blockade by eplerenone.
Conclusions Over the past decade, the effect of aldosterone/salt on a variety of cardiovascular functions has been shown to reflect direct effects through mineralocorticoid receptors in nonepithelial tissue as well as classic effects on sodium retention and potassium excretion at epithelia. The clinical importance of aldosterone receptor blockade has been convincingly shown for heart failure by the RALES trial. Experimental studies in a variety of hyperaldosteronemic models of hypertension have shown that selective aldosterone blockade by eplerenone attenuates the proinflammatory effect of aldosterone/salt imbalance and confers substantial end-organ (brain, kidney, heart) protection, even in the presence of elevated angiotensin levels. In a normoaldosteronomic model (pig angioplasty), eplerenone is similarly protective of coronary arteries. Although the extent of end-organ protection in clinical practice awaits outcome trials, the results of preclinical studies suggest a critical role for eplerenone in the optimal treatment of hypertension as well as heart failure.
References 1. Okubo S, Niimura F, Nishimura H, et al. Angiotensin-independent mechanism for aldosterone synthesis during chronic extracellular fluid volume depletion. J Clin Invest 1997;99:855-60. 2. Krozowski ZS, Funder JW. Renal mineralocorticoid receptors and hippocampal corticosterone-binding species have identical intrinsic steroid specificity. Proc Natl Acad Sci USA 1983;80:6056-60.
Funder S11
3. Pearce P, Funder JW. High affinity aldosterone binding sites (type I receptors) in rat heart. Clin Exp Pharmacol Physiol 1987;14:85966. 4. Arriza JL, Weinberger C, Cerelli G, et al. Cloning of human mineralocorticoid receptor complementary DNA: structural and functional kinship with the glucocorticoid receptor. Science 1987;237: 268-75. 5. Funder JW, Pearce PT, Smith R, et al. Mineralocorticoid action: target tissue specificity is enzyme, not receptor, mediated. Science 1988;242:583-5. 6. Edwards CRW, Stewart PM, Burt D, et al. Localisation of 11-hydroxysteroid dehydrogenase—tissue specific protector of the mineralocorticoid receptor. Lancet 1988;2:986-9. 7. Gomez-Sanchez EP, Fort C, Thwaites D. Central mineralocorticoid receptor antagonism blocks hypertension in Dahl S/JR rats. Am J Physiol 1992;262:E96-9. 8. Brilla CG, Weber KT. Mineralocorticoid excess, dietary sodium, and myocardial fibrosis. J Lab Clin Med 1992;120:893-901. 9. Young M, Head G, Funder J. Determinants of cardiac fibrosis in experimental hypermineralocorticoid states. Am J Physiol 1995; 269:E657-62. 10. Sato A, Funder JW. High glucose stimulates aldosterone-induced hypertrophy via type I mineralocorticoid receptors in neonatal rat cardiomyocytes. Endocrinology 1996;137:4145-53. 11. Fujisawa G, Dilley R, Fullerton MJ, et al. Experimental cardiac fibrosis: differential time course of responses to mineralocorticoidsalt administration. Endocrinology 2001;142:3625-31. 12. Pitt B, Zannad F, Remme WJ, et al. The effect of spironolactone on morbidity and mortality in patients with severe heart failure. N Engl J Med 1999;341:709-17. 13. Zannad F, Alla F, Dousset B, et al. Limitation of excessive extracellular matrix turnover may contribute to survival benefit of spironolactone therapy in patients with congestive heart failure: insights from the randomized aldactone evaluation study (RALES). Circulation 2000;102:2700-6. 14. Rocha R, Stier CT Jr. Pathophysiological effects of aldosterone in cardiovascular tissues. Trends Endocrinol Metab 2001;12:308-14. 15. Rocha R, Stier CT Jr, Kifor I, et al. Aldosterone: a mediator of myocardial necrosis and renal arteriopathy. Endocrinology 2000; 141:3871-8. 16. Rocha R, Rudolph AE, Frierdich GG, et al. Aldosterone induces a vascular inflammatory phenotype in the rat heart. Am J Physiol Heart Circ Physiol. 2002;283:1802-10. 17. Ward MR, Kanellakis P, Ramsey D, et al. Eplerenone suppresses constrictive remodeling and collagen accumulation after angioplasty in porcine coronary arteries. Circulation 2001;104:467-72.
Background Classically, we have recognized six classes of steroid hormones: mineralocorticoids, glucocorticoids, androgens, estrogens, progestins, and vitamin D derivatives. The adrenal cortex is the principal, if not unique, source of mineralocorticoids (aldosterone) and glucocorticoids (cortisol and, in rats and mice, corticosterone), with secretion of both hormones under feedback control. For glucocorticoids, the major stimulus to secretion is adrenocorticotropic hormone (ACTH) from the pituitary gland, with cortisol exerting negative feedback at a variety of levels (hippocampus, hypothalamus, pituitary). Aldosterone secretion is elevated by either of two factors: angiotensin II, produced by the action of renin and angiotensin-converting enzyme, and plasma potassium levels. Aldosterone acts to conserve sodium (and with it, water) at epithelia such as the kidney distal tubule, colon, sweat gland, and salivary gland, and to promote potassium excretion, in this way to lower levels of both trophic stimuli. The evolutionary biology of aldosterone is important for an appreciation of its clinical roles in several respects. Mammalian evolution in general, and human evolution in particular, is characterized by restricted sodium intake (fruit/vegetable diet) and the possibility of catastrophic sodium loss (diarrhea) on top of obligate sodium loss (sweating in tropical environment). Many species, human included, have thus evolved a hunger for salt, reflecting aldosterone action on the brain. They have also evolved, not surprisingly, multiple independent mechanisms of aldosterone elevation in response to sodium deficiency. An outstanding example of this redundancy is the angiotensinogen knockout (Agen –/–) mouse, which can elevate its aldosterone secretion on a low-salt diet indistinguishably from wild type; only when a low sodium and potas-
From the Centre for Neurosciences, University of Melbourne and Prince Henry’s Institute of Medical Research, Monash Medical Centre, Victoria, Australia. Reprint requests: John W. Funder, MD, PhD, FRACP, Prince Henry’s Institute of Medical Research, Block E, Level 4, PO Box 5152, Monash Medical Centre, 246 Clayton Rd, Clayton, Victoria, Australia 3168. E-mail: [email protected] Am Heart J 2002;144:S8-11. Copyright 2002, Mosby, Inc. All rights reserved. 0002-8703/2002/$35.00 ⫹ 0 4/0/129971 doi:10.1067/mhj.2002.129971
sium diet is given is aldosterone secretion compromised in Agen –/– mice.1 What we have not evolved are mechanisms to sufficiently lower aldosterone secretion commensurately in the face of a physiologically very high salt intake, which is now the norm in many places in the developed and developing world.
The new biology of aldosterone The new biology of aldosterone might be said to have begun in 1983 with the demonstration that mineralocorticoid receptors in the rat kidney had equally high affinity for aldosterone and the physiologic glucocorticoid corticosterone; moreover, indistinguishable “mineralocorticoid receptors” could be found in the rat brain2 and subsequently, the heart.3 These data were confirmed and extended by the cloning of the human kidney mineralocorticoid receptor4; again, expression was clearly present in rat brain and heart on Northern blot analysis, as well as kidney and gut. In addition, the cloned human mineralocorticoid receptor expressed in COS cells showed equivalent high affinity for aldosterone and cortisol. For both rat and human mineralocorticoid receptors, this equivalence in affinity poses a particular physiologic problem, in that circulating free glucocorticoid levels are commonly two orders of magnitude higher than those of aldosterone, prima facie making it very difficult for aldosterone to occupy and activate such nonselective receptors. What allows aldosterone occupancy in epithelial cells is the expression, at high levels (3-4 ⫻ 106 copies per cell) of the enzyme 11-hydroxysteroid dehydrogenase (11HSD) type 2, which converts cortisol and corticosterone into their receptor inactive 11-keto congeners, cortisone and 11-dehydrocorticosterone.5,6 Aldosterone is characterized by (and named after) its unique aldehyde (CHO) group at carbon 18, in place of the otherwise universal methyl group. This aldehyde group cyclizes with the hydroxyl group at carbon 11 to form an 11,18 hemiacetal and to make aldosterone not a substrate for 11HDS2 (Figure 1). Over the past decade, it has also become clear that inappropriate aldosterone levels for salt status can have major pathophysiologic effects through mineralocorticoid receptor occupancy in the brain and heart. Gomez-Sanchez et al7 showed (in salt-sensitive rats) that infusion of the mineralocorticoid receptor blocker
American Heart Journal Volume 144, Number 5
RU 28318 intracerebroventricularly, at doses without effect when infused systemically, completely blocked the elevation in blood pressure (BP) normally seen when such rats are given 0.9% NaCl solution to drink. In ground-breaking studies on cardiac fibrosis, Brilla and Weber8 showed that infusion of robust doses of aldosterone to uninephrectomized rats with 0.9% NaCl solution to drink for 8 weeks produced hypertension, perivascular and interstitial cardiac fibrosis, and cardiac hypertrophy. Importantly, infusion of aldosterone into rats on a low-salt diet did not elevate BP or produce the cardiac fibrosis and hypertrophy seen with the aldosterone/salt combination. Studies by Young et al9 showed that these cardiac effects of aldosterone/salt are not secondary to the induced BP rise. When rats were infused peripherally with aldosterone under standard conditions and intracerebroventricularly with RU 28318 at peripherally ineffective doses, the BP remained clamped at normotensive control levels. In contrast, the trophic effects of aldosterone on the heart in terms of hypertrophy and fibrosis were indistinguishable from those seen in animals infused with aldosterone alone.9 Further evidence for a direct trophic effect comes from the data of Sato and Funder10 in studies on isolated rat neonatal cardiomyocytes. Aldosterone, but not corticosterone, substantially increased leucine incorporation into protein, an effect potentiated by glucose through protein kinase C-dependent mechanisms.10 In subsequent studies using a deoxycorticosterone (DOCA)/salt rather than the aldosterone/salt model, Fujisawa et al11 made two additional sets of observations. Whereas in the aldosterone-infused rats no evidence of cardiac hypertrophy or fibrosis could be seen until 3 to 4 weeks after starting the infusion, rats given weekly injections of DOCA showed substantial elevations of BP by day 4 and cardiac type III collagen accumulation by day 2 after the initial dose. Second, at days 4 and 8 (but not at days 2, 16, or 32), sections of the heart are characterized on TUNEL staining by massive necrosis and apoptosis and on hematoxylin and eosin staining by substantial perivascular infiltration of inflammatory cells.11 In subsequent studies (Labib et al, unpublished) using the same model, elevated levels of some (eg, TNF␣, ED-1) but not all (eg, surprisingly, TGF) of the common inflammatory markers were seen in vessel walls and perivascular tissue. It is in this context that the results of the recent RALES trial need to be placed.
The RALES trial In this trial, two groups of patients were compared, both with severe (New York Heart Association class III-IV) heart failure. One group continued to receive their standard medication (angiotensin-converting en-
Funder S9
Figure 1
Flow chart of a principal cell in the distal tubule of the kidney, showing metabolism of cortisol to the receptor-inactive cortisone, allowing aldosterone (with its 11-OH group protected by cyclization) to access mineralocorticoid receptors. HSD, Hydroxysteroid dehydrogenase.
zyme inhibitor, diuretic, and so forth) plus placebo; the second group received standard medication plus low-dose (average 26 mg/d) spironolactone, a nonselective aldosterone antagonist. Although recruitment was halted halfway through the trial because of the divergence between the two groups, ⬎800 patients in each group were followed for three years, with only three salient differences between groups. First, the placebo group consistently ran plasma (K⫹) values 0.2 to 0.3 mEq lower than the aldosterone antagonist– treated group. Second, the spironolactone group showed a 10% incidence of gynecomastia (reflecting the nonselectivity of spironolactone and its action as an androgen antagonist) compared with 1% for placebo. Finally, the spironolactone-treated group showed a 30% improvement in mortality rate and a 35% improvement in morbidity compared with placebo in patients receiving standard “best practice” treatment.12 Although the mechanism of therapeutic action of aldosterone blockade under such circumstances remains to be established, a substudy of the RALES trial strongly suggests an effect on fibrosis, known to be involved in both sudden cardiac death and progression of heart failure. In this substudy, circulating levels of the N-terminal portion of the procollagen type III precursor were measured before and after spironolactone treatment and similarly in the placebo group. Survival was clearly better in patients with low circulating precursor levels; importantly, spironolactone administration improved outcomes in patients in whom it dropped initially elevated precursor levels rather than in the treated population as a whole.13 The profibrotic effect of aldosterone/salt and the protective effect of
S10 Funder
Figure 2
Effects of aldosterone (gray bars) and eplerenone plus aldosterone (white bars) on inflammatory markers in perivascular area in rat heart after 7, 14, and 28 days’ exposure. COX-2, cyclooxygenase-2; MCP-1, momocyte chemoattractant protein-1; TGF1, transforming growth factor-1. (Reprinted with permission from: Rocha R, Rudolph AE, Frierdich GE, et al. Aldosterone induces a vascular inflammatory phenotype in the rat heart. AJP-Heart and Circulatory Physiology 2002;283:8102–10.)
aldosterone antagonism in terms of vascular and interstitial fibrosis thus form the context for the series of experimental studies on end-organ protection by the selective aldosterone blocker eplerenone.
Eplerenone: Selective aldosterone blocker The three tissues in which aldosterone-induced vasculitis appears to play a major role are the brain, kidney, and heart. It has long been known that prolonged exposure to aldosterone (eg, in the syndrome of glucocorticoid-remediable aldosteronism) is characterized by a preponderance of death from stroke rather than heart attack, compared with other causes of hypertension. One model of this state is the stroke-prone spontaneously hypertensive rat (SHRSP), which, even when receiving 0.9% NaCl to drink, shows elevated plasma renin and aldosterone levels. Such SHRSP characteristically die at 13 to 18 weeks of age from stroke. When treated with eplerenone, however, 6 of 7 SHRSP sur-
American Heart Journal November 2002
vived for 18 weeks, at which point they were killed and necropsied. At necropsy, Rocha et al14 found marked fibrinoid necrosis in the cerebral vessels of untreated SHRSP, with considerably thickened walls and morphologic evidence of tissue damage; changes essentially absent from equivalent sections of eplerenone-treated SHRSP. Similar studies in SHRSP show an equivalent level of end-organ protection in the kidney.15 Consistent with their state of advanced hypertension and elevated plasma renin and aldosterone, SHRSP show evidence of renal damage, both functionally (eg, albuminuria) and structurally (eg, glomerulosclerosis, tubular dilatation, and casts). Just as in the cerebral vessels, eplerenone treatment of SHRSP protects the kidney, in terms of morphometric indexes of renal damage and proteinuria, despite a continuing elevated level of angiotensin. What needs to be emphasized in terms of both cerebrovascular and renal protective effects of eplerenone is that at the doses used, no reduction in BP was seen (ie, the protective effect has a humoral rather than a hemodynamic basis), which is consistent with the direct effects of aldosterone on the heart previously cited.9,10 In an additional series of in vivo studies, Rocha et al16 have also demonstrated the cardioprotective effects of eplerenone in aldosterone-infused uninephrectomized rats drinking 0.9% NaCl solution. At the dose of eplerenone used, BP was significantly reduced after 14 and 28 days, although to levels that remained clearly hypertensive. In groups of rats killed at 7, 14, and 28 days, markers of vasculitis/perivascular inflammation in the heart (ED-1, MCP, osteopontin, COX-2) were clearly and progressively elevated in aldosteronetreated animals; in the presence of eplerenone, however, the changes were reversed to (or close to) control (Figure 2).16 Other markers of inflammation (eg, TGF, ICAM, VCAM) were marginally if at all elevated by aldosterone treatment, evidence that some but perhaps not all inflammatory pathways are activated by aldosterone/salt. Finally, in terms of the vascular-protective effect of eplerenone, Ward et al17 have recently published studies on the effects of eplerenone on the response to experimental angioplasty in the pig. Four groups of animals were used (control, spironolactone, eplerenone, and aldosterone infused) in studies comparing coronary and iliac artery angioplasty. In sections of iliac artery, no differences were seen between groups in terms of vascular architecture 4 weeks after angioplasty. In the coronary arteries, however, eplerenone-treated animals showed significantly larger vessel diameter and, perhaps more importantly, substantially preserved luminal area compared with control. Aldosterone-infused pigs tended to have lumen areas less than control, and spironolactone-infused pigs
American Heart Journal Volume 144, Number 5
were intermediate between control and eplerenone, although in neither instance were the differences significant on a population basis. Although aldosterone infusion did not reduce vessel cross-sectional area or lumen size compared with control, it did increase medial collagen density by ⬃75%. Although the mechanism of coronary protection after angioplasty by eplerenone remains to be explored, these studies point to a tonic effect of normal circulating levels of aldosterone in such circumstances, given the effects of receptor blockade by eplerenone.
Conclusions Over the past decade, the effect of aldosterone/salt on a variety of cardiovascular functions has been shown to reflect direct effects through mineralocorticoid receptors in nonepithelial tissue as well as classic effects on sodium retention and potassium excretion at epithelia. The clinical importance of aldosterone receptor blockade has been convincingly shown for heart failure by the RALES trial. Experimental studies in a variety of hyperaldosteronemic models of hypertension have shown that selective aldosterone blockade by eplerenone attenuates the proinflammatory effect of aldosterone/salt imbalance and confers substantial end-organ (brain, kidney, heart) protection, even in the presence of elevated angiotensin levels. In a normoaldosteronomic model (pig angioplasty), eplerenone is similarly protective of coronary arteries. Although the extent of end-organ protection in clinical practice awaits outcome trials, the results of preclinical studies suggest a critical role for eplerenone in the optimal treatment of hypertension as well as heart failure.
References 1. Okubo S, Niimura F, Nishimura H, et al. Angiotensin-independent mechanism for aldosterone synthesis during chronic extracellular fluid volume depletion. J Clin Invest 1997;99:855-60. 2. Krozowski ZS, Funder JW. Renal mineralocorticoid receptors and hippocampal corticosterone-binding species have identical intrinsic steroid specificity. Proc Natl Acad Sci USA 1983;80:6056-60.
Funder S11
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