Effects of aldosterone on coronary function.

Our understanding of the effects of aldosterone and its mechanisms has increased substantially in recent years, probably because of the importance of the mineralocorticoid receptor (MR) antagonists in several major cardiovascular diseases. Recent clinical studies have confirmed the benefits of MR antagonists in patients with heart failure, left ventricular dysfunction after myocardial infarction, hypertension or diabetic nephropathy. However, it would be a gross oversimplification to conclude that the role of aldosterone is unequivocally negative. Aldosterone is synthesized in the adrenal glands and binds to specific MRs in target epithelial cells. The steroid-receptor complex penetrates the cell nucleus where it modulates gene expression and activates specific aldosterone-induced proteins that control sodium reabsorption. Recent studies have shown that aldosterone also impacts a wide range of non-epithelial tissues such as the heart and blood vessels. Remarkably, aldosterone can also be synthesized in extra-adrenal tissues and it may act in a rapid non-genomic manner.We note the existence of glucocorticoids that exhibit plasma concentrations much higher than those of aldosterone and that are structurally very similar to aldosterone. It is thus possible that glucocorticoids may bind to the aldosterone receptor in some cell types. Diverse experimental models and several strains of transgenic mice have allowed us to better understand the effects of aldosterone on the heart. Specifically, it seems that a slight increase in cardiac aldosterone concentrations induces a decreased coronary reserve in mice by decreasing the BKCa potassium channels associated with coronary smooth muscle cells. Taken together, these experiments indicate that vascular cells are the primary targets of aldosterone in the cardiovascular system. The hormone directly affects NO and EDHF-mediated coronary relaxation. Both mechanisms may contribute to the deleterious cardiovascular effects of MR stimulation.

Aldosterone-induced coronary dysfunction in transgenic mice involves the calcium-activated potassium (BKCa) channels of vascular smooth muscle cells.

BACKGROUND: Cardiomyocyte-specific overexpression of aldosterone synthase in male (MAS) mice induces a nitric oxide-independent coronary dysfunction. Because calcium-activated potassium (BKCa) channels are essential for vascular smooth muscle cell (VSMC) relaxation, we hypothesized that aldosterone alters their expression and/or function in VSMCs. METHODS AND RESULTS: Left coronary artery segments were isolated from MAS or male wild-type mice and mounted in a wire myograph. Responses to acetylcholine were assessed (in the presence of a nitric oxide synthase inhibitor) without or with the cyclooxygenase inhibitor diclofenac, the KCa inhibitors charybdotoxin plus apamin, or the BKCa inhibitor iberiotoxin. Expression of BKCa was quantified in hearts by real-time quantitative polymerase chain reaction and Western blot and in isolated coronary arteries by polymerase chain reaction. The effect of aldosterone on BKCa expression also was studied in cultured rat aortic VSMCs. Acetylcholine-mediated coronary relaxation was markedly decreased in MAS mice and was prevented by spironolactone. Diclofenac did not affect the MAS-induced impairment in the responses to acetylcholine, whereas charybdotoxin plus apamin virtually abolished the relaxation in both male wild-type and MAS mice. After iberiotoxin, relaxation to acetylcholine was decreased to a larger extent in male wild-type than in MAS, leading to similar levels of relaxation. BKCa-alpha and -beta1 subunit expressions were significantly decreased in MAS heart and coronary arteries. In cultured VSMCs, aldosterone induced a concentration-dependent decrease in BKCa expression, which was prevented by spironolactone. CONCLUSIONS: Aldosterone overexpression altered VSMC BKCa expression and coronary BKCa-dependent relaxation. The resulting alteration of relaxing responses may contribute to the deleterious effects of aldosterone in cardiovascular diseases. BKCa channels may therefore be useful therapeutic targets in cardiovascular diseases.

Cardiac specific increase in aldosterone production induces coronary dysfunction in aldosterone synthase-transgenic mice.

BACKGROUND: Elevated circulating aldosterone level is associated with impaired cardiovascular function. Although the mechanisms are not fully understood, aldosterone antagonists decrease total and cardiovascular mortality in heart failure and myocardial infarction. Aldosterone induces cardiac fibrosis in experimental models, and it is synthesized locally in rat heart. These observations suggest pathological effects of aldosterone in heart that remain unclear. METHODS AND RESULTS: Transgenic mice (TG) that overexpress the terminal enzyme of aldosterone biosynthesis, aldosterone synthase (AS), in heart have been raised by gene targeting with the alpha-myosin heavy chain promoter. AS mRNA increased 100-fold and aldosterone concentration 1.7-fold in hearts of male TG mice relative to wild-type. No structural or myocardial alterations were evidenced, because ventricle/body weight, AT1 and AT2 receptor binding, and collagen content were unchanged in TG. No alteration in cardiac function was evidenced by echocardiography, isolated perfused heart, or whole-cell patch clamp experiments. In contrast, coronary function was impaired, because basal coronary flow was decreased in isolated perfused heart (-55% of baseline values), and vasodilatation to acetylcholine, bradykinin, and sodium nitroprusside was decreased by 75%, 60%, and 75%, respectively, in TG mice compared with wild-type, showing that the defect was not related to NO production. CONCLUSIONS: Increased cardiac aldosterone production in male mice induces a major coronary endothelium-independent dysfunction with no detectable alterations in cardiac structure and function. However, coronary dysfunction may be harmful for coronary adaptation to increased flow demand.

Asymmetric septal hypertrophy in heterozygous cMyBP-C null mice.

OBJECTIVE: Cardiac myosin-binding protein C (cMyBP-C) gene mutations are involved in familial hypertrophic cardiomyopathy (FHC). Many of these mutations produce truncated proteins, which are unstable in the cardiac tissue of patients, suggesting that haploinsufficiency could account for the development of the phenotype. However, existing mouse models of cMyBP-C gene mutations have represented hypomorphic alleles without evidence of asymmetric septal hypertrophy, a key FHC phenotypic feature. In the present study, we generated a new model of cMyBP-C null mice and characterized the phenotype in both homozygotes and heterozygotes at different ages. METHODS: The mouse model was based upon the targeted deletion of exons 1 and 2, which contain the transcription initiation site, and the phenotype was determined by molecular, functional and morphological analyses. RESULTS: Herein, we demonstrate that inactivation of one or two mouse cMyBP-C alleles leads to different cardiac disorders at different post-natal time windows. The homozygous cMyBP-C null mice do not express the cMyBP-C gene, develop eccentric left ventricular hypertrophy with decreased fractional shortening at 3-4 months of age and a markedly impaired relaxation after 9 months. This is associated with myocardial disarray and an increase of interstitial fibrosis. The heterozygous cMyBP-C null mice present a slight but significant decrease of cMyBP-C amount and develop asymmetric septal hypertrophy associated with fibrosis at 10-11 months of age. CONCLUSION: These data provide evidence that heterozygous cMyBP-C null mice represent the first model with a key feature of human FHC that is asymmetric septal hypertrophy.

Aldosterone enhances ischemia-induced neovascularization through angiotensin II-dependent pathway.

BACKGROUND: We analyzed the role of aldosterone in ischemia-induced neovascularization and the involvement of angiotensin II (Ang II) signaling in this effect. METHODS AND RESULTS: Ischemia was induced by right femoral artery ligature in mice treated or not with aldosterone (4.5 microg/day), aldosterone plus spironolactone (aldosterone receptor blocker; 20 mg/kg per day), or aldosterone plus valsartan (angiotensin type 1 [AT1] receptor blocker; 20 mg/kg per day). After 21 days, neovascularization was evaluated by microangiography, capillary density measurement, and laser-Doppler perfusion imaging. Protein level of vascular endothelial growth factor (VEGF) was determined by Western blot analysis in hindlimbs. mRNA levels of renin-angiotensin system components were also assessed by semiquantitative reverse transcription-polymerase chain reaction. Angiographic score, capillary number, and foot perfusion were improved in ischemic/nonischemic leg ratio by 1.4-, 1.5-, and 1.4-fold, respectively, in aldosterone-treated mice compared with controls (P<0.05). Aldosterone proangiogenic effect was associated with 2.3-fold increase in VEGF protein content (P<0.05). Treatments with spironolactone or with neutralizing VEGF antibody hampered the proangiogenic effect of aldosterone (P<0.05 versus aldosterone-treated mice). Interestingly, AT1 receptor blockade completely abrogated the aldosterone proangiogenic effect, emphasizing the involvement of Ang II-related pathway in aldosterone-induced vessel growth. In this view, angiotensinogen mRNA content was 2.2-fold increased in aldosterone-treated mice in reference to controls (P<0.05), whereas that of renin, angiotensin-converting enzyme, and AT1 receptor subtype was unaffected. Aldosterone treatment also decreased AT2 mRNA content by 2-fold (P<0.05 versus controls), suggesting that aldosterone may switch the Ang II pathway toward activation of vessel growth. CONCLUSIONS: This study shows for the first time that aldosterone increases neovascularization in the setting of ischemia through activation of Ang II signaling.