Aldosterone receptor antagonism normalizes vascular function in liquorice-induced hypertension

Glucocorticoids decrease GTP cyclohydrolase and tetrahydrobiopterin-dependent vasorelaxation through glucocorticoid receptors

Excess glucocorticoids result in decreased aortic dilation and expression of guanosine triphosphate (GTP) cyclohydrolase 1 (GTPCH1) messenger RNA (mRNA), the rate-limiting enzyme in the production of the nitric oxide synthase (NOS) cofactor tetrahydrobiopterin (BH4). It was hypothesized that this response is a genomic effect mediated through the glucocorticoid receptor (GR). Endothelium-intact rat aortas were incubated with dexamethasone (DEX; 1.3 x 10(-6) M) or vehicle for 2 or 6 hours and isometric force generation was measured. Maximum acetylcholine-induced relaxation in DEX-2hr aortas was not different compared with control values; however, acetylcholine-induced relaxations in DEX-6hr aortas were significantly decreased. Coincubation with sepiapterin (10(-4) M), which produces BH4 via a salvage pathway, restored relaxation in DEX-6hr aortas to that of controls. Coincubation with the GR antagonist mifepristone (10(-6) M) completely blocked the DEX-induced decrease in relaxation. Spironolactone (10(-5) M), a mineralocorticoid receptor antagonist, had no effect. GTPCH1 mRNA expression was significantly decreased in DEX-6hr aortas compared with control values. This was blocked by mifepristone; however, spironolactone and cycloheximide did not prevent the decrease of GTPCH1 by DEX. These results support the hypothesis that GTPCH1 downregulation by glucocorticoids is mediated through the GR and contributes to reduced endothelium-dependent relaxation.

Eplerenone

Aldosterone has been implicated for many years as an important substance in the pathogenesis of heart disease. Elevated aldosterone concentrations have been documented in patients with hypertension and heart failure, leading to the use of aldosterone antagonists for the treatment of these conditions. Spironolactone has been used for nearly 2 decades for the treatment of hypertension, and more recently, has become a standard agent for the treatment of systolic heart failure. Spironolactone, however, is a nonselective antagonist of the aldosterone receptor, binding also to other steroid receptors and causing a significant percentage of patients to have sex hormone-related adverse effects such as gynecomastia. Eplerenone is the first of a new class of drugs known as selective aldosterone receptor antagonists, which selectively block the aldosterone receptor with minimal effect at other steroid receptors, thereby minimizing many of the hormonal side effects seen with spironolactone. Eplerenone has been shown to be beneficial both as monotherapy and combination therapy for lowering elevated blood pressure in patients with hypertension. The antihypertensive efficacy of eplerenone is roughly similar to that of other antihypertensive agents, although in 1 study black patients responded better with eplerenone than losartan. In addition, eplerenone has demonstrated some renoprotective effects in diabetic patients with hypertension. Recently, eplerenone was shown to significantly reduce mortality and cardiovascular morbidity in post-myocardial infarction patients with systolic heart failure currently taking standard heart failure medications. Eplerenone is generally well tolerated, although hyperkalemia with this agent is of some concern. Eplerenone is metabolized by CYP3A4 and administration with potent inhibitors of this enzyme is contraindicated because of the risk of hyperkalemia. In summary, eplerenone has proven to be beneficial in treating hypertension and post-myocardial infarction heart failure. Its exact place in therapy will in large part be determined by its cost and whether or not future studies will be able to demonstrate a clinical benefit of this agent over spironolactone or other currently available treatments.

Influence of aldosterone vs. endothelin receptor antagonism on renovascular function in liquorice-induced hypertension

BACKGROUND

The enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) provides mineralocorticoid receptor specificity for aldosterone by metabolizing glucocorticoids to their receptor inactive 11-dehydro derivatives. Inhibition of 11beta-HSD2 by liquorice-derived glycyrrhizic acid (GA) therefore results in sodium retention and hypertension. The present study investigated the effect of the aldosterone receptor antagonist spironolactone in comparison with the endothelin ET(A) receptor antagonist darusentan on renovascular endothelial function in liquorice-induced hypertension.

METHODS

GA, a recognized inhibitor of 11beta-HSD2 was supplemented to the drinking water (3 g/l) of Wistar Kyoto rats over a period of 21 days. From day 8 to 21, spironolactone (5.8+/-0.6 mg/kg/day), darusentan (45.2+/-6.5 mg/kg/day), or placebo was added to chow (n=7 per group). After the animals were killed, vascular function of isolated renal artery segments was assessed by isometric tension recording.

RESULTS

Relaxation of pre-constricted renal artery segments in response to acetylcholine (10(-10) to 10(-5) mol/l) was impaired by GA as compared with controls (12+/-4% vs 98+/-5% of norepinephrine 3x10(-7) mol/l), whereas endothelium independent relaxations were unaffected. Endothelin receptor antagonism improved renovascular endothelium-dependent relaxation (32+/-4%, P<0.05 vs placebo) whereas endothelium-dependent relaxation was completely normalized by aldosterone receptor antagonism (85+/-4%, P<0.01 vs placebo).

CONCLUSIONS

In GA-induced hypertension, both aldosterone receptor antagonism and endothelin receptor antagonism normalize blood pressure and improve renovascular function and, thus, may represent a new therapeutic approach in cardiovascular disease associated with impaired 11beta-HSD2 activity.