Aldosterone inhibition and heart failure: too good to be true?

Central mineralocorticoid receptors and cardiovascular disease

The mineralocorticoid receptor (MR) is expressed in many cell types throughout the body, including specific neurons, and mediates diverse functions, many of which are just now being appreciated. MR that pertain to the central modulation of cardiovascular function and health are addressed herein.

Hypercholesterolemia suppresses inwardly rectifying K+ channels in aortic endothelium in vitro and in vivo

Inwardly rectifying K+ (Kir) channels are responsible for maintaining endothelial membrane potential and play a key role in endothelium-dependent vasorelaxation. In this study, we show that endothelial Kir channels are suppressed by hypercholesterolemic levels of lipoproteins in vitro and by serum hypercholesterolemia in vivo. Specifically, exposing human aortic endothelial cells to acetylated low-density lipoprotein or very low density lipoprotein resulted in a time- and concentration-dependent decrease in Kir current that correlated with the degree of cholesterol loading. The suppression was fully reversible by cholesterol depletion. Furthermore, a decrease in Kir current resulted in depolarization of endothelial membrane potential. Most important, the flow sensitivity of Kir currents was also impaired by cholesterol loading. Specifically, flow-induced increase in Kir current was suppressed by 70%, and flow-induced hyperpolarization was almost completely abrogated. Furthermore, we show that hypercholesterolemia in vivo also strongly suppresses endothelial Kir currents and causes a shift in endothelial membrane potential, as determined by comparing the currents in aortic endothelial cells freshly isolated from healthy or hypercholesterolemic pigs. Therefore, we suggest that suppression of Kir current is one of the important factors in hypercholesterolemia-induced endothelial dysfunction.

Influence of serum cholesterol on atherogenesis and intimal hyperplasia after angioplasty: inhibition by amlodipine

The objectives of the present study were to determine whether serum hypercholesterolemia (HC) promotes the development of spontaneous and angioplasty-induced lesions and whether amlodipine inhibits these lesions and cellular processes underlying their genesis. Rabbits were fed normal, 0.5%, or 2% cholesterol diets for 9 wk, which resulted in the development of increasing HC. After week one, balloon dilation of the abdominal aorta was performed while the thoracic aorta was not disturbed and monitored for the development of spontaneous lesions. Lesion size increased with the degree of HC and was accompanied by increased collagen synthesis and smooth muscle cell (SMC) proliferation at each site. Amlodipine (5 mg/kg p.o.) inhibited lesion size by 50% (P < 0.01) at both sites in cholesterol-fed animals but not at angioplasty sites in animals on a normal diet. Local collagen synthesis was inhibited at both sites by amlodipine in the diet animals. The increase in HC was accompanied by a 1.7-fold increase in basal Ca2+ uptake in SMCs in the thoracic aorta, which was not altered by amlodipine, nifedipine, Ni2+, or La3+, revealing an uninhibitable calcium leak during atherogenesis. In culture, cholesterol enrichment increased SMC proliferation, collagen synthesis, and the secretion of a soluble SMC mitogen, which were inhibited by amlodipine (10(-9) M). Finally, in SMC membranes, amlodipine uniquely restored the cholesterol-expanded membrane bilayer width without any effect on membrane fluidity. This study establishes a causal role between serum HC and the development of spontaneous and angioplasty-induced lesions and the ability of amlodipine to disrupt this action by a novel remodelling action on the SMC membrane.

Brain mineralocorticoid receptors: orchestrators of hypertension and end-organ disease

PURPOSE OF REVIEW

'New' tasks have been discovered for aldosterone and its receptor, the mineralocorticoid receptor, within both epithelial tissues of vectorial ion and water transport, such as the kidney, and non-epithelial organs, including the brain, heart and vessels. Promising results of clinical trials using low doses of mineralocorticoid receptor antagonists to forestall end-organ disease is resulting in an increase in their use, yet the biology of the mineralocorticoid receptor is far from clear.

RECENT FINDINGS

Mineralocorticoid receptors within the kidney, heart and blood vessels mediate direct effects of aldosterone, including tissue inflammation, hypertrophy and fibrosis, that are independent of blood pressure. Activation, by aldosterone, of mineralocorticoid receptors in the brain increases central sympathetic nervous system drive to the periphery, thereby producing hypertension through multiple mechanisms, and increases levels of proinflammatory cytokines in both the circulation and peripheral tissues. Blocking of the mineralocorticoid receptor of the forebrain lowers the levels of peripheral tissue cytokines, including those induced by ischemic injury in the heart. Aldosterone is produced within the heart, blood vessels and brain, potentially liberating regulation of local concentrations of the steroid from peripheral mechanisms of control. A conundrum yet to be explained is the ligand-dependent functional specificity of the mineralocorticoid receptor in some non-epithelial tissues, which may be crucial to our understanding the end-organ pathophysiology of hypertension.

SUMMARY

New technology is rapidly adding layers of complexity to, rather than simplifying, our understanding of the facile terms 'hemodynamic homeostasis' and 'end-organ' disease, but within this new knowledge lies the promise of better, more precise treatment of hypertension and its sequelae.

Phosphatidylinositol 3-kinase and calcium-activated transcription pathways are required for VLDL-induced smooth muscle cell proliferation

Little is known regarding the molecular mechanisms of atherogenicity of triglyceride-rich lipoproteins such as very low-density lipoproteins (VLDLs). We examined the effect of VLDL on proliferation of rat aortic smooth muscle cells, intracellular Ca2+ handling, and activity of cAMP-responsive element binding protein (CREB) and nuclear factor of activated T cells (NFAT) transcription factors. VLDL, isolated from human serum, dose- and time-dependently promoted proliferation. After 4 hours of exposure to VLDL (0.15 g/L proteins), the caffeine-induced Ca2+ release was inhibited and the IP3-sensitive Ca2+ release induced by ATP (10 micromol/L) was markedly prolonged. In quiescent cells, CREB was phosphorylated (pCREB) and NFAT was present in the cytosol, whereas in cells exposed to VLDL for 4 to 24 hours, pCREB disappeared and NFAT was translocated to the nucleus. VLDL-induced NFAT translocation and proliferation were blocked by cyclosporin A and LY294002 involving calcineurin and phosphatidylinositol 3-kinase (PI3K) pathways. Indeed, VLDLs rapidly phosphorylate protein kinase B and glycogen synthase kinase-3beta in a PI3K-dependent way. These results provide the first evidence that VLDLs induce smooth muscle cell proliferation by activating the PI3K pathway and nuclear NFAT translocation. Blockade of the Ca2+-induced Ca2+ release mechanism and dephosphorylation of pCREB contribute but were not sufficient to induce a proliferating phenotype.

Working heart function in diabetes is not improved by spironolactone treatment

Aldosterone antagonism has emerged as an important strategy for end-stage congestive heart failure. To evaluate the potential contribution of aldosterone towards the cardiac complications of diabetes, this study examined the effects of chronic aldosterone receptor blockade (with spironolactone) on isolated working heart function in streptozotocin (STZ) - induced diabetic rats. Wistar rats were divided into four groups: control, control spironolactone-treated, diabetic, and diabetic spironolactone-treated. Following chronic spironolactone treatment (8 weeks), cardiac function was assessed in terms of the rate of contraction (+dP/dT), rate of relaxation (-dP/dT), and left ventricular developed pressure (LVDP). Untreated diabetic rats exhibited marked cardiac dysfunction when compared with age matched controls (p < 0.001). Long-term spironolactone treatment did not improve these parameters. These data demonstrate the lack of beneficial effects of aldosterone receptor blockade on isolated working heart function in diabetes.

Surrogate end points in heart failure

OBJECTIVE

To describe the potential utility of a true surrogate marker of heart failure outcomes, historically investigate the validity of surrogates most commonly evaluated in heart failure, and identify specific end points offering the most potential for future use.

DATA SOURCES

A MEDLINE search (1966-June 2001) was completed to identify relevant literature. Additional references were also retrieved from selected articles. Search terms included b-type natriuretic peptide, cardiac remodeling, end-diastolic volume, heart failure, and surrogate end points.

DATA SYNTHESIS

By definition, true surrogate end points must predict outcomes associated with disease progression and response to therapy. A validated surrogate measure of mortality would render significant utility in both heart failure drug development and clinical practice. Improvements in traditional functional markers of heart failure, including ejection fraction and exercise capacity, have produced inconsistent results in regard to mortality in a number of clinical trials. Numerous measures of cardiac remodeling and neurohormonal activation, however, have proven to be reliable predictors of disease progression and therapeutic response. These findings have contributed significantly to the current understanding of heart failure pathophysiology and approach to treatment. Specifically, measures such as left-ventricular end-diastolic volume (LVEDV) and, potentially, b-type natriuretic peptide (BNP) concentrations may correlate with disease progression and parallel the mortality reductions observed with angiotensin-converting enzyme inhibitor and beta-blocker therapy.

CONCLUSIONS

Currently, LVEDV and plasma BNP offer the greatest potential as surrogate end points in heart failure. Further investigation of these measures is required before routine implementation in drug development and clinical practice can be justified.