Protective Effect of Dietary Potassium Against Vascular Injury in Salt-Sensitive Hypertension

A review of nutritional factors in hypertension management

Hypertension is a major health problem worldwide. Its attendant morbidity and mortality complications have a great impact on patient's quality of life and survival. Optimizing blood pressure control has been shown to improve overall health outcomes. In addition to pharmacological therapies, nonpharmacological approach such as dietary modification plays an important role in controlling blood pressure. Many dietary components such as sodium, potassium, calcium, and magnesium have been studied substantially in the past decades. While some of these nutrients have clear evidence for their recommendation, some remain controversial and are still of ongoing study. Dietary modification is often discussed with patients and can provide a great benefit in blood pressure regulation. As such, reviewing the current evidence will be very useful in guiding patients and their physician and/or dietician in decision making. In this review article of nutritional factors in hypertension management, we aim to examine the role of nutritional factors individually and as components of whole dietary patterns.

Pathophysiology of salt sensitivity hypertension

Dietary salt intake is the most important factor contributing to hypertension, but the salt susceptibility of blood pressure (BP) is different in individual subjects. Although the pathogenesis of salt-sensitive hypertension is heterogeneous, it is mainly attributable to an impaired renal capacity to excrete sodium (Na(+) ). We recently identified two novel mechanisms that impair renal Na(+) -excreting function and result in an increase in BP. First, mineralocorticoid receptor (MR) activation in the kidney, which facilitates distal Na(+) reabsorption through epithelial Na(+) channel activation, causes salt-sensitive hypertension. This mechanism exists not only in models of high-aldosterone hypertension as seen in conditions of obesity or metabolic syndrome, but also in normal- or low-aldosterone type of salt-sensitive hypertension. In the latter, Rac1 activation by salt excess causes MR stimulation. Second, renospecific sympathoactivation may cause an increase in BP under conditions of salt excess. Renal beta2 adrenoceptor stimulation in the kidney leads to decreased transcription of the gene encoding WNK4, a negative regulator of Na(+) reabsorption through Na(+) -Cl (-) cotransporter in the distal convoluted tubules, resulting in salt-dependent hypertension. Abnormalities identified in these two pathways of Na(+) reabsorption in the distal nephron may present therapeutic targets for the treatment of salt-sensitive hypertension.

Reactive oxygen species and the central nervous system in salt-sensitive hypertension: possible relationship with obesity-induced hypertension

1. There are multiple and complex mechanisms of salt-induced hypertension; however, central sympathoexcitation plays an important role. In addition, the production of reactive oxygen species (ROS) is increased in salt-sensitive hypertensive humans and animals. Thus, we hypothesized that brain ROS overproduction may increase blood pressure (BP) by central sympathostimulation. 2. Recently, we demonstrated that ROS levels were elevated in the hypothalamus of salt-sensitive hypertensive animals. Moreover, intracerebroventricular anti-oxidants suppressed BP and renal sympathetic nerve activity more in salt-sensitive than non-salt-sensitive hypertensive rats. Thus, brain ROS overproduction increased BP through central sympathoexcitation in salt-sensitive hypertension. 3. Salt sensitivity of BP is enhanced in obesity and metabolic syndrome. Interestingly, it is also suggested that, in obesity-induced hypertension models, increases in BP are caused by brain ROS-induced central sympathoexcitation. 4. Recent studies suggest that increased ROS production in the brain and central sympathoexcitation may share a common pathway that increases BP in both salt- and obesity-induced hypertension.

The importance of potassium in managing hypertension

Dietary potassium intake has been demonstrated to significantly lower blood pressure (BP) in a dose-responsive manner in both hypertensive and nonhypertensive patients in observational studies, clinical trials, and several meta-analyses. In hypertensive patients, the linear dose-response relationship is a 1.0 mm Hg reduction in systolic BP and a 0.52 mm Hg reduction in diastolic BP per 0.6 g per day increase in dietary potassium intake that is independent of baseline potassium deficiency. The average reduction in BP with 4.7 g (120 mmol) of dietary potassium per day is 8.0/4.1 mm Hg, depending race and on the relative intakes of other minerals such as sodium, magnesium, and calcium. If the dietary sodium chloride intake is high, there is a greater BP reduction with an increased intake of dietary potassium. Blacks have a greater decrease in BP than Caucasians with an equal potassium intake. Potassium-induced reduction in BP significantly lowers the incidence of stroke (cerebrovascular accident, CVA), coronary heart disease, myocardial infarction, and other cardiovascular events. However, potassium also reduces the risk of CVA independent of BP reductions. Increasing consumption of potassium to 4.7 g per day predicts lower event rates for future cardiovascular disease, with estimated decreases of 8% to 15% in CVA and 6% to 11% in myocardial infarction.

Changing standard chow diet promotes vascular NOS dysfunction in Dahl S rats

We hypothesized that vascular nitric oxide synthase (NOS) function and expression is differentially regulated in adult Dahl salt-sensitive rats maintained on Teklad or American Institutes of Nutrition (AIN)-76A standard chow diets from 3 to 16 wk old. At 16 wk old, acetylcholine (ACh)-mediated vasorelaxation and phenylephrine (PE)-mediated vasoconstriction in the presence and absence of NOS inhibitor, N(ω)-nitro-L-arginine methyl ester (L-NAME), was assessed in small-resistance mesenteric arteries and aortas. Rats maintained on either diet throughout the study had similar responses to ACh and PE in the presence or absence of L-NAME in both vascular preparations. We reasoned that changing from one diet to another as adults may induce vascular NOS dysfunction. In the absence of L-NAME, small arteries from Teklad-fed rats switched to AIN-76 diet and vice versa had similar responses to ACh and PE. Small-arterial NOS function was maintained in rats switched to AIN-76A from Teklad diet, whereas NOS function in response to ACh and PE was lost in the small arteries from rats changed to Teklad from AIN-76A diet. This loss of NOS function was echoed by reduced expression of NOS3, as well as phosphorylated NOS3. The change in NOS phenotype in the small arteries was observed without changes in blood pressure. Aortic responses to ACh or PE in the presence or absence of L-NAME were similar in all diet groups. These data indicate that changing standard chow diets leads to small arterial NOS dysfunction and reduced NOS signaling, predisposing Dahl salt-sensitive rats to vascular disease.

Marinobufagenin and cyclic strain may activate endothelial NADPH oxidase, contributing to the adverse impact of salty diets on vascular and cerebral health

Limited but provocative ecologic epidemiology suggests that dietary salt may play a central role in the genesis of not only of stroke, but also dementia, including Alzheimer's disease. Impairment of nitric oxide bioactivity in the cerebral microvasculature is a likely mediator of this effect. Salted diets evoke increased adrenal secretion of the natriuretic steroid marinobufagenin (MBG), which promotes natriuresis via inhibition of renal tubular Na+/K+-ATPase; this effect is notably robust in salt-sensitive rodent strains in which other compensatory natriuretic mechanisms are subnormally efficient. MBG-mediated inhibition of sodium pumps in vascular smooth muscle likely plays a role in the hypertension induced by salty diets in these rodents. However, salt sensitivity in humans is associated with increased vascular mortality and ventricular hypertrophy independent of blood pressure; this suggests that MBG may be pathogenic via mechanisms unrelated to blood pressure control. Indeed, recent evidence indicates that MBG, via interaction with alpha1 isoforms of the sodium pump, can activate various intracellular signaling pathways at physiological concentrations too low to notably inhibit pump activity. An overview of current evidence suggests the hypothesis that MBG - as well as the cyclic strain induced by hypertension per se - may induce endothelial oxidative stress by activating NADPH oxidase. If so, this could rationalize the increase in vascular and systemic oxidative stress observed in salt-sensitive rodents fed salty diets, or in rodents infused with MBG; moreover, if this effect is a particularly prominent determinant of oxidative stress in cerebrovascular endothelium, it might help to explain the virtual absence of stroke and dementia in low-salt societies. As a corollary of this hypothesis, it can be predicted that spirulina-derived phycobilins, which appear to mimic the physiological role of bilirubin as an inhibitor of NAPDH oxidase complexes, may have potential for ameliorating the adverse health impacts of MBG and of salty diets. Potassium-rich diets are also likely to be protective in this regard, as they should suppress MBG production via their natriuretic impact, while their stimulatory effect on sodium pump activity may exert a hyperpolarizing effect on plasma membranes that suppresses NADPH oxidase activity.

Association of dietary sodium and potassium intakes with albuminuria in normal-weight, overweight, and obese participants in the Reasons for Geographic and Racial Differences in Stroke (REGARDS) Study

BACKGROUND

Among obese adults, sodium intake has been associated with cardiovascular disease. Few data are available on sodium intake and albuminuria, a marker of kidney damage and risk factor for cardiovascular disease.

OBJECTIVE

We examined the relation between dietary sodium and potassium intakes and the ratio of sodium to potassium (Na/K) with albuminuria by BMI in the Reasons for Geographic and Racial Differences in Stroke (REGARDS) Study (n = 30,239 adults aged ≥45 y).

DESIGN

A modified Block 98 food-frequency questionnaire was used for dietary assessment in 21,636 participants, and nutritional variables were categorized by sex-specific quintiles. Normal weight, overweight, and obese were defined as BMI (in kg/m(2)) categories of 18.5-24.9, 25-29.9, and ≥30, respectively. Albuminuria was defined as a ratio (mg/g) of urinary albumin to creatinine of ≥30.

RESULTS

The prevalences of albuminuria were 11.5%, 11.6%, and 16.0% in normal-weight, overweight, and obese participants, respectively. The multivariable-adjusted ORs for albuminuria in a comparison of the highest with the lowest quintile of Na/K intake (≥1.12 to <0.70 for men and ≥1.07 to <0.62 for women) were 0.89 (95% CI: 0.65, 1.22), 1.08 (95% CI: 0.85, 1.36), and 1.28 (95% CI: 1.02, 1.61) in normal-weight, overweight, and obese participants, respectively. The highest quintile of dietary sodium was associated with an increased OR for albuminuria in obese participants (OR: 1.44; 95% CI: 1.00, 2.07) but not in normal-weight or overweight participants. Dietary potassium was not associated with albuminuria.

CONCLUSION

In obese adults, higher dietary Na/K and sodium intakes were associated with albuminuria.

Dietary Potassium Intake and Risk of Stroke

Background and Purpose—

Potassium intake has been inconsistently associated with risk of stroke. Our aim was to conduct a meta-analysis of prospective studies to assess the relation between potassium intake and stroke risk.

Methods—

Pertinent studies were identified by a search of PubMed from January 1966 through March 2011 and by reviewing the reference lists of retrieved articles. We included prospective studies that reported relative risks with 95% CIs of stroke for ≥3 categories of potassium intake or for potassium intake analyzed as a continuous variable. Study-specific results were pooled using a random-effects model.

Results—

Ten independent prospective studies, with a total of 8695 stroke cases and 268 276 participants, were included in the meta-analysis. We observed a statistically significant inverse association between potassium intake and risk of stroke. For every 1000-mg/day increase in potassium intake, the risk of stroke decreased by 11% (pooled relative risk, 0.89; 95% CI, 0.83 to 0.97). In the 5 studies that reported results for stroke subtypes, the pooled relative risks were 0.89 (95% CI, 0.81 to 0.97) for ischemic stroke, 0.95 (95% CI, 0.83 to 1.09) for intracerebral hemorrhage, and 1.08 (95% CI, 0.92 to 1.27) for subarachnoid hemorrhage.

Conclusions—

Dietary potassium intake is inversely associated with risk of stroke, in particular ischemic stroke.

Hypertension, periodontal disease, and potassium intake in nonsmoking, nondrinker african women on no medication

The purpose of this cross-sectional study was to investigate the association of periodontitis and/or tooth loss with hypertension by excluding the common confounders. Eighty-one Tanzanian women who were aged 46-58 years, nonsmokers, nonalcoholic drinkers, and on no medication underwent clinical examination. Multiple-regression analysis showed that the severity of periodontitis was significantly correlated with increased systolic blood pressure and diastolic blood pressure. Simple-regression analysis indicated that the severity of periodontitis was inversely correlated with 24-hour urinary excretion of potassium (r = -0.579, P = 0.0004) and also inversely with the frequency of intakes of green vegetables (r = -0.232, P = 0.031) and fruits (r = -0.217, P = 0.0043). Low-potassium intake in the diet mostly accompanied by low dietary fiber intake increases BP as well as periodontal inflammation. Potassium intake may be an important factor linking periodontitis and hypertension in middle-aged nonsmoking and nonalcoholic women on no medication, although chronic inflammation such as periodontitis may cause hypertension through a more direct mechanism.

Preventive dietary potassium supplementation in young salt-sensitive Dahl rats attenuates development of salt hypertension by decreasing sympathetic vasoconstriction

AIM

Increased potassium intake attenuates the development of salt-dependent hypertension, but the detailed mechanisms of blood pressure (BP) reduction are still unclear. The aims of our study were (i) to elucidate these mechanisms, (ii) to compare preventive potassium effects in immature and adult animals and (iii) to evaluate the therapeutic effects of dietary potassium supplementation in rats with established salt hypertension.

METHODS

  Young (4-week-old) and adult (24-week-old) female salt-sensitive Dahl rats were fed a high-salt diet (5% NaCl) or a high-salt diet supplemented with 3% KCl for 5 weeks. The participation of vasoconstrictor (renin-angiotensin and sympathetic nervous systems) and vasodilator systems [prostanoids, Ca(2+) -activated K(+) channels, nitric oxide (NO)] was evaluated using a sequential blockade of these systems.

RESULTS

Preventive potassium supplementation attenuated the development of severe salt hypertension in young rats, whereas it had no effects on BP in adult rats with moderate hypertension. Enhanced sympathetic vasoconstriction was responsible for salt hypertension in young rats and its attenuation for potassium-induced BP reduction. Conversely, neither salt hypertension nor its potassium-induced attenuation were associated with significant changes of the vasodilator systems studied. The relative deficiency of vasodilator action of NO and Ca(2+) -activated K(+) channels in salt hypertensive Dahl rats was not improved by potassium supplementation.

CONCLUSIONS

The attenuation of enhanced sympathetic vasoconstriction is the principal mechanism of antihypertensive action exerted by preventive potassium supplementation in immature Dahl rats. Dietary potassium supplementation has no preventive effects on BP in adult salt-loaded animals or no therapeutic effects on established salt hypertension in young rats.

Renal preservation effect of ubiquinol, the reduced form of coenzyme Q10

BACKGROUND

The aim of this study was to evaluate the renal preservation effect of ubiquinol, the reduced form of coenzyme Q10 (CoQ10).

METHODS

Three-week-old heminephrectomized male Sprague-Dawley rats were divided into three groups (10 animals each): diet with normal (0.3%) salt, high (8%) salt, and high salt plus 600 mg/kg body weight/day of ubiquinol, for 4 weeks. Systolic blood pressure (SBP), urinary albumin (u-alb), superoxide anion generation (lucigenin chemiluminescence) and ubiquinol levels in renal tissues were examined.

RESULTS

Salt loading increased SBP (111.0 ± 3.6 vs. 169.4 ± 14.3 mmHg, p < 0.01) and u-alb (43.8 ± 28.0 vs. 2528.7 ± 1379.0 µg/day, p < 0.02). These changes were associated with stimulation of superoxide generation in the kidney (866.3 ± 102.8 vs. 2721.4 ± 973.3 RLU/g kidney, p < 0.01). However, ubiquinol decreased SBP (143.9 ± 29.0 mmHg, p < 0.05), u-alb (256.1 ± 122.1 µg/day, p < 0.02), and renal superoxide production (877.8 ± 195.6 RLU/g kidney, p < 0.01), associated with an increase in renal ubiquinol levels.

CONCLUSION

Ubiquinol, the reduced form of CoQ10, effectively ameliorates renal function, probably due to its antioxidant effect. Thus, ubiquinol may be a candidate for the treatment of patients with kidney disease.

Effects of potassium chloride and potassium bicarbonate on endothelial function, cardiovascular risk factors, and bone turnover in mild hypertensives

To determine the effects of potassium supplementation on endothelial function, cardiovascular risk factors, and bone turnover and to compare potassium chloride with potassium bicarbonate, we carried out a 12-week randomized, double-blind, placebo-controlled crossover trial in 42 individuals with untreated mildly raised blood pressure. Urinary potassium was 77+/-16, 122+/-25, and 125+/-27 mmol/24 hours after 4 weeks on placebo, potassium chloride, and potassium bicarbonate, respectively. There were no significant differences in office blood pressure among the 3 treatment periods, and only 24-hour and daytime systolic blood pressures were slightly lower with potassium chloride. Compared with placebo, both potassium chloride and potassium bicarbonate significantly improved endothelial function as measured by brachial artery flow-mediated dilatation, increased arterial compliance as assessed by carotid-femoral pulse wave velocity, decreased left ventricular mass, and improved left ventricular diastolic function. There was no significant difference between the 2 potassium salts in these measurements. The study also showed that potassium chloride reduced 24-hour urinary albumin and albumin:creatinine ratio, and potassium bicarbonate decreased 24-hour urinary calcium, calcium:creatinine ratio, and plasma C-terminal cross-linking telopeptide of type 1 collagen significantly. These results demonstrated that an increase in potassium intake had beneficial effects on the cardiovascular system, and potassium bicarbonate may improve bone health. Importantly, these effects were found in individuals who already had a relatively low-salt and high-potassium intake.

Regulation of the epithelial sodium channel [ENaC] in kidneys of salt-sensitive Dahl rats: insights on alternative splicing

The epithelial sodium channel [ENaC] is critical for the maintenance of sodium balance, extracellular fluid volume and long term blood pressure control. Monogenic disorders causing ENaC hyperactivity have led to a severe form of hereditary hypertension in humans, known as Liddle's syndrome. Similarly, in animal models, ENaC hyperactivity has been well documented in kidneys of salt-sensitive [S] Dahl rats [a genetic model of salt-sensitive hypertension] versus their normotensive control [Dahl salt-resistant [R] rats]. The purpose of the present review is to highlight the differential regulation of ENaC in kidneys of Dahl S versus R rats. A systematic overview of the putative role of alternative splicing of the main alpha subunit of ENaC [alpha ENaC] in modulating ENaC expression in kidneys of Dahl rats will be discussed. Finally, a better understanding of the meaningful contribution of ENaC in the pathogenesis of salt-sensitive hypertension will be achieved upon completion of this review.

Recent advances in understanding integrative control of potassium homeostasis

The potassium homeostatic system is very tightly regulated. Recent studies have shed light on the sensing and molecular mechanisms responsible for this tight control. In addition to classic feedback regulation mediated by a rise in extracellular fluid (ECF) [K(+)], there is evidence for a feedforward mechanism: Dietary K(+) intake is sensed in the gut, and an unidentified gut factor is activated to stimulate renal K(+) excretion. This pathway may explain renal and extrarenal responses to altered K(+) intake that occur independently of changes in ECF [K(+)]. Mechanisms for conserving ECF K(+) during fasting or K(+) deprivation have been described: Kidney NADPH oxidase activation initiates a cascade that provokes the retraction of K(+) channels from the cell membrane, and muscle becomes resistant to insulin stimulation of cellular K(+) uptake. How these mechanisms are triggered by K(+) deprivation remains unclear. Cellular AMP kinase-dependent protein kinase activity provokes the acute transfer of K(+) from the ECF to the ICF, which may be important in exercise or ischemia. These recent advances may shed light on the beneficial effects of a high-K(+) diet for the cardiovascular system.

Metabolic syndrome and oxidative stress

Metabolic syndrome is an obesity-associated collection of disorders, each of which contributes to cardiovascular risk. Metabolic syndrome is also associated with overproduction of reactive oxygen species (ROS). ROS contribute to the interrelationship between metabolic syndrome and salt-sensitive hypertension, which are both caused by obesity and excess salt consumption and are major threats to health in developed countries. ROS can induce insulin resistance, which is indispensable for the progression of metabolic syndrome, and salt-sensitive hypertension stimulates ROS production, thereby promoting the development of metabolic syndrome. Moreover, ROS activate mineralocorticoid receptors (MRs) and the sympathetic nervous system, which can contribute to the development of metabolic syndrome and salt-sensitive hypertension. Salt-induced progression of cardiovascular disease (CVD) is also accelerated in animal models with metabolic syndrome, probably owing to further stimulation of ROS overproduction and subsequent ROS-induced MR activation and sympathetic excitation. Therefore, ROS contribute to the progression of the metabolic syndrome itself and to the CVD accompanying it, particularly in conjunction with excessive salt consumption.

Potassium softens vascular endothelium and increases nitric oxide release

In the presence of aldosterone, plasma sodium in the high physiological range stiffens endothelial cells and reduces the release of nitric oxide. We now demonstrate effects of extracellular potassium on stiffness of individual cultured bovine aortic endothelial cells by using the tip of an atomic force microscope as a mechanical nanosensor. An acute increase of potassium in the physiological range swells and softens the endothelial cell and increases the release of nitric oxide. A high physiological sodium concentration, in the presence of aldosterone, prevents these changes. We propose that the potassium effects are caused by submembranous cortical fluidization because cortical actin depolymerization induced by cytochalasin D mimics the effect of high potassium. In contrast, a low dose of trypsin, known to activate sodium influx through epithelial sodium channels, stiffens the submembranous cell cortex. Obviously, the cortical actin cytoskeleton switches from gelation to solation depending on the ambient sodium and potassium concentrations, whereas the center of the cell is not involved. Such a mechanism would control endothelial deformability and nitric oxide release, and thus influence systemic blood pressure.

Primary aldosteronism (PA) and endothelial progenitor cell (EPC) bioavailability

OBJECTIVE

Patients with primary aldosteronism (PA) experience more cardiovascular events than patients with essential hypertension matched for risk factor profile. Endothelial progenitor cells (EPC) represent a bone marrow-derived cell population implicated in vascular healing whose number correlates to the cardiovascular risk factor profile. Aldosterone has been reported to decrease EPC proliferation in rats.

DESIGN AND PATIENTS

We assessed (i) the growth characteristics of EPC from six PA patients and six matched normotensive controls; (ii) the growth characteristics of EPC treated with increasing doses of aldosterone.

MEASUREMENTS

Senescence and cell-cycle analysis of EPC from PA patients and normotensive controls and of aldosterone-treated EPC from healthy volunteers.

RESULTS

No difference was found in the senescence rate between EPC from PA patients (72.4% senescent cells) and controls (70.7%, P > 0.05). No difference was also found in the cell-cycle distribution determined by FACS (controls: 75.2% cells in G0/G1 phase; PA: 73.5%, P > 0.05). Incubation of EPC with aldosterone did not modify their senescence rate (controls: 72.4% senescent cells; aldosterone 10 nmol/l: 70.9%; aldosterone 100 nmol/l 71.6%, P > 0.05 for all comparisons) and cell-cycle distribution (controls: 73.3% cells in G0/G1 phase; aldosterone 10 nmol/l: 74.9%; aldosterone 100 nmol/l: 75.4%, P > 0.05 for all comparisons). No expression of the mineralocorticoid receptor (MR) transcript was found in EPC by RT-PCR analysis.

CONCLUSIONS

High aldosterone levels, both in PA patients and in vitro, exert no direct or indirect effect on EPC growth characteristics.