Salt-loading and nitric oxide inhibition after the menopause

Before the menopause, the cardiovascular risk profile of women is very different to that of men. The exact mechanisms of the protective effect of the ovaries and uterus are still far from clear, although recent studies have provided us with valuable insight. Certainly, the prevalence of hypertension increases significantly after menopause, and is a strong risk factor for future cardiovascular morbidity and mortality [1]. Nitric oxide, salt and hypertension Previous studies have shown that the presence of oestrogen may prevent hypertension, mediating its effect by regulating the production of nitric oxide (NO) [2], accounting for some of the gender differences in blood pressure between age-matched males and premenopausal women. Nitric oxide is a potent vasodilator that is synthesized constitutively by endothelial nitric oxide synthase (eNOS), an isoform of NOS in endothelial cells, and plays a major role in regulating the tone of the arterial circulation by activation of soluble guanylate cyclase and the consequent increase in the concentration of cyclic guanosine monophosphate (cGMP) on vascular smooth muscle cells [3]. Indeed, NO has a central role in the pathophysiology of human hypertension, and this has been confirmed experimentally. For example, it was shown that, in rats prone to hypertension, the ability to generate NO is considerably less than in those that are not prone [4]. Also, patients with essential hypertension have less plasma nitrate and nitrite, suggesting reduced NO synthesis [5]. Endogenous inhibitors of NOS are thought to play an important role in the development of hypertension, and asymmetrical dimethlylarginine (ADMA) is one of them. This inhibitor rises in essential hypertension, and correlates with the increase in blood pressure [5]. This compound was initially studied in renal failure, but it is now appreciated that it could play a part in causing endothelial dysfunction in insulin resistance and ischaemic heart disease [6]. Endothelial dysfunction can usually be studied by quantifying the change in brachial artery diameter after inducing forearm ischaemia (i.e. a measure of large artery hyperaemic response, mediated mainly by endothelial-derived NO). Nonetheless, a continuum is also likely to exist between endothelial activation (e.g. by cytokines), endothelial dysfunction (resulting in thrombogenesis and atherogenesis) and endothelial damage (resulting in overt vascular damage and atherosclerosis) in hypertension. Certainly, the presence of some sort of endothelial disturbance in hypertension probably contributes to the observation that, despite the blood vessels being exposed to high-pressures, the main complications of hypertension (i.e. myocardial infarction and stroke) are paradoxically thrombotic in nature rather than haemorrhagic: ‘the thrombotic paradox of hypertension’ or ‘Birmingham paradox’ [7]. It is now well established that a high intake of dietary salt can cause and exacerbate hypertension. Recent meta-analyses have supported the notion that a modest reduction in dietary salt significantly reduces blood pressure, and also contributes to a reduction in cardiovascular morbidity and mortality [8,9]. Apart from directly increasing circulating volume, salt has been shown to contribute to hypertension by affecting endothelial function, especially in salt-sensitive hypertension, where circulating markers of endothelial damage (e.g. von Willebrand factor, soluble E-selectin) are raised [10]. Furthermore, laboratory studies suggest that NO synthesis is impaired in salt-sensitive hypertension, but a salt challenge stimulates NO production in salt insensitive rats; conversely, the infusion of l-arginine and l-citrulline (NO precursors) prevented hypertension in both groups [11]. It is therefore plausible that, in salt-sensitive individuals, excessive intake of dietary salt could injure endothelium, and be a factor in causing hypertension by affecting the production of NO. Nonetheless, is there any link between this pathway and the menopause? Oestrogen, endothelial function and nitric oxide Oestradiol has numerous physiological and pathophysiological effects on the cardiovascular system. There is also convincing evidence that oestrogen exerts beneficial effects on endothelium-dependent vasodilation by stimulating eNOS and the subsequent generation of NO. Many physiological studies of oestrogen replacement in premenopausal women undergoing surgical ovariectomy have shown that endothelial dysfunction develops after the procedure (i.e. loss of endogenous oestrogen) and administration of exogenous 17β- oestradiol restores vascular reactivity to premenopausal levels [12,13]. Oestrogen restores endothelial function by enhancing NO synthesis through at least two known mechanisms [14]: (i) a genomic mechanism, involving augmented messenger RNA transcription and synthesis of NOS; and (ii) the more rapid, non-genomic effect that occurs without alterations in gene expression. A third mechanism may be by reducing oxidative stress and NO breakdown. Is there a link between hypertension, nitric oxide, salt and the menopause? In this issue of the journal, Scuteri et al. [15] address this question in a study of 15 Caucasian normotensive postmenopausal healthy women, who were fed a low sodium diet (70 mmol/day) for 4 days, followed by a salt-loading for the next 7 days (260 mmol/day). After 7 days of salt-loading (which tripled and doubled urinary salt and NO metabolite excretion, respectively), blood pressure rose significantly and baseline brachial artery diameter rose by 21% (with a trend towards reduced reactivity to hyperaemia). Although ADMA did not rise, there was a significant inverse correlation between the change in ADMA and urinary NO metabolite excretion. Furthermore, using multivariate analysis, the blood pressure rise observed during the transition from low to high salt intake was associated with an increase in ADMA. Because there was a rise in NO metabolites with increased salt intake, but a paradoxical blood pressure rise, the authors concluded that this effect is mediated by a reduction in NO bioavailability by ADMA in postmenopausal women. Several issues in the study by Scuteri et al. [15] warrant further discussion. It is well known that mean systolic blood pressure rises more steeply in women than in men aged 40–55 years [16]. A recent report from the cross-sectional data of the World Health Organization-coordinated Cardiovascular Diseases and Alimentary Comparison (WHO-CARDIAC) Study reported a strong positive sodium–blood pressure association, independent of age, in postmenopausal women, implicating increased salt sensitivity after menopause as one of the mechanisms which increased cardiovascular disease risk. Indeed, in salt-sensitive hypertensives, salt-loading increases plasma ADMA levels and blood pressure, and reduces plasma NO metabolites; by contrast, a low-salt diet reverses these abnormalities [17]. A larger study is required to explain whether it is primarily increased salt sensitivity (secondary to the lack of oestrogen and up-regulation of ADMA) that enhances the risk of raised blood pressure post menopause. These factors are important in view of the implications for the prevention and treatment of hypertension, especially in terms of prescribing oestrogen (post menopause), and the development of ADMA inhibitors. Of course, this would also further highlight the importance of dietary salt restriction. Interestingly, there was no statistically significant change in night-time blood pressure between low- and high- salt intakes (although there was a trend to higher blood pressure during salt-loading). There is some evidence that a high-salt diet in salt-sensitive individuals may reduce the normal circadian fluctuation in blood pressure [18]. Thus, does the menopause induce or enhance salt sensitivity? Would oestrogen replacement potentially reverse this deleterious effect? Finally, is there a similar change in the 24-h ADMA and NO metabolite profile after the menopause? Scuteri et al. [15] also show that brachial artery reactivity is reduced after salt-loading, although basal brachial artery diameter and urinary NO metabolites were higher than in salt-deficient diets. Thus, it could be suggested that salt could potentially increase rather than decrease NO production, and the reduction in brachial artery diameter could be due to the already dilated artery with limited further expansion. This interpretation is important because it questions the primary hypothesis that oestrogen is linked to NO, and this needs to be addressed in future studies. Indeed, the basal over-expression of NO may be responsible for increased endothelial damage because the peroxynitrite cation (a NO metabolite) is cytotoxic [19]. Finally, Scuteri et al. [15] studied only postmenopausal Caucasian women with an average age of 55 years. It remains to be seen whether the blood pressure responses and plasma ADMA and urinary NO metabolites in the present study are applicable in older women and age-matched men, as well as in other non- Caucasian ethnic groups. Furthermore, women undergoing a natural menopause (a more gradual process, with no defined, precise point of changing from pre- to post menopause) are probably quite different from those undergoing a surgical menopause following bilateral oophrectomy, where the point of menopause is much more precise. Thus, NO inhibition may well be a mechanism for the blood pressure increase during salt-loading in normotensive postmenopausal women. Larger studies will need to ascertain the importance and implications of this mechanism in other patient groups (e.g. following surgical menopause), as well as the relationship with hypertensive target organ damage, the effects of treatment and its predictive value for prognosis in such patients.