Introduction

Salt consumption has greatly changed over the millennia. Our ancestors, who lived 2 million years up to 10,000 years ago, were essentially hunters, they used to eat meat from wild animals, fruits and vegetables from uncultivated plants, and salt was that contained in natural foods. Their usual diet contained <1 g salt/day. Salt started to be used approximately 5,000 years ago when its property to preserve food was discovered. Later, the advent of cold techniques for food conservation caused a decline in salt use. Due to the recent increase in consumption of highly salted industrialized foods, salt intake is increasing again.

The terms “salt” and “sodium” are used in an interchangeable way. However, the two terms do not exactly mean the same thing. Dietary sodium is consumed as common salt: sodium chloride (NaCl). Each molecule of salt is composed of an atom of sodium (Na) and an atom of chloride (Cl). Chloride contributes more than sodium to the weight of the molecule and 1 g of salt means 0.4 g of sodium or 1 g of sodium equals 2.5 g of salt. However, in terms of health, what matters is sodium and the sodium content is what the food labels report.

The mean current salt intake in many countries is approximately 10 g/day and largely exceeds physiological needs and the recommended limit of salt <5 g/day (<2 g of sodium) [1]. The sources of dietary salt vary between developed and developing countries. In Europe and North America, more than 75 % of dietary salt comes from processed food, where it is added at manufacturing stage [2]: 10 % is the natural sodium content and approximately 15 % is added at home during cooking. This latter, rather than packaged food, is the major source of salt consumption in the developing countries [3, 4].

Salt, Blood Pressure, and Cardiovascular Risk

The first observation linking salt to blood pressure (BP) is reported in the Yellow Emperor’s Classic of Internal Medicine, where it was stated: “the pulse hardens when too much salt is added” [5]. Then, many evidences originated from epidemiological studies [6, 7] and randomized, clinical trials on dietary salt intake [8, 9] have shown both a relationship between BP and salt and a dose-response effect to salt reduction.

The Intersalt study [6] was performed in 52 centres throughout the world. Salt consumption, judged by 24-hour urinary sodium excretion, and blood pressure (BP) were measured following strict and uniform protocols. This study confirmed the link between salt intake and BP both within and between populations. Moreover, it was demonstrated that the increase in systolic (SBP) and diastolic (DBP) BP is age-related and is steeper with higher sodium excretion. The EPIC cohort [7], which examined 23,100 community-living adults aged 45-79 years, identified differences in urinary sodium as markers of sodium intake associated with clinically relevant BP differences.

Because high BP favours cardiovascular disease (CVD) [10], a worldwide reduction of salt intake could substantially reduce the incidence of stroke, heart failure, and renal disease [11, 12••]. Despite a statistically significant heterogeneity among the studies, a meta-analysis of 13 prospective cohort studies on dietary salt intake and incidence of CVD [13] reported that a 2 g/day increase in sodium intake (5 g of salt) was associated with an overall increased risk of stroke (relative risk (RR) 1.23, 95 % confidence interval (CI) 1.06-1.43, p = 0.007) and all CV events (RR 1.14, 95 % CI 0.99-1.32, p = 0.07). Major sources of heterogeneity among studies were the geographic regions, differing substantially for dietary sodium intake, the estimation of sodium intake, which was based on a single 24-hour urinary collection, and the length of follow-up for the estimation of salt effect on CVD.

The causal effect of salt on CV risk is still debated, because observational studies of individuals on a free diet have reported an inverse relationship between CV events and salt intake [14••, 15–17]. A J-shaped curve relationship between salt and CVD was recently described [18••].

Salt Sensitivity

BP response to changes in salt intake varies considerably among individuals, independent of blood pressure levels [19]; this phenomenon is described as salt sensitivity of BP [20, 21]. Evidence support that this mechanism is partially under genetic control, because normotensive and hypertensive salt-sensitive individuals tend to have family history for hypertension more frequently than do those who are salt-resistant [22, 23]. It also is known that salt sensitivity is more frequent in subjects of African origin and in older people [21].

Salt sensitivity is a negative prognostic indicator, because it is associated with increased incidence of cardiovascular complications [13], microalbuminuria [24], and endothelial dysfunction [25]. In previous studies, it has been assessed with different standardized protocols, including acute salt load, chronic dietary sodium interventions, and chronic sodium volume depletion.

The pathogenetic mechanisms of salt sensitivity are not clear; however, one hypothesis proposed is a larger renal reabsorption of sodium. The kidney has a central role in the relationship between sodium intake, total body sodium, fluid balance, and BP [26]. Guyton et al. [26] contributed to understand the role of kidneys by developing a complex computer model of the control mechanisms and feedback loops that regulate BP. A fundamental concept was pressure natriuresis, a mechanism through which the kidneys increase urinary output of fluids and salt to overcome excess of BP when it rises above normal. Cross-transplantation experiments in rats with inherited hypertension have strengthened the key role of the kidney in BP regulation by demonstrating that the kidney brings a “hypertensive” message. In fact, these studies have shown that the transplanted kidney from a young prehypertensive rat into a normotensive rat caused a chronic BP increase [27]. Indirect and more circumstantial evidence also are available in humans. A study on renal transplantation showed that recipients of a kidney from a donor with a negative family history for hypertension had lower BP than recipients from hypertensive families [28]. This concept has been consolidated in recent years by the identification of the molecular mechanisms that lead to increased sodium reabsorption in some forms of monogenic hypertension [29, 30].

Sodium contributes to more than 80 % of the extracellular fluid osmolality and kidney participates in maintaining a constant extracellular environment that is necessary for a normal cell function. Sodium handling starts with filtration by the glomerulus and then it is reabsorbed along the tubule through an integrated system of carriers, channels, and pumps. The bulk of sodium reabsorption occurs in the proximal tubule and Henle’s loop while the final qualitative changes are made in the collecting tubule. Sodium entry at this site occurs through selective sodium channels in the apical membrane, and the number of open channels is under hormonal control, being affected by aldosterone and atrial natriuretic peptide. Like most mammals, humans developed in a salt-poor environment. As a consequence, all the mechanisms that are related to sodium handling are focused on retaining sodium.

Salt Sensitivity and Genes

The studies published on the genetics of salt sensitivity have mainly followed a candidate gene approach. In humans, only one genome wide linkage scan, the Genetic Epidemiology Network of Salt Sensitivity (GenSalt) study [31], the largest family dietary sodium intervention, has addressed this issue. It was performed in Han Chinese rural population that underwent a high dietary salt intake. The dietary intervention included a 7-day low sodium intake (3 g of NaCl) followed by a 7-day high sodium intake (17 g of NaCl). Considerable heterogeneity exists among the different studies, mainly due to the definition and measurement of the phenotype and to ethnic differences. This heterogeneity results in conflicting findings and makes reliable comparisons difficult. Standardization in the measurement of the phenotype and replication in ethnically homogeneous populations is crucial to identify the true susceptibility genetic factors.

The genes associated with salt sensitivity phenotype have been recently reviewed by Felder et al. [32•], Kelly and He [33•], and Sanada et al. [34•]. In the present review, we report the most important findings.

Renin-Angiotensin-Aldosterone System

The genes of the renin-angiotensin-aldosterone system (RAAS) have been extensively studied because of their role in renal sodium handling. The aspartyl protease renin cleaves angiotensinogen (AGT) to angiotensin I, which is converted to angiotensin II (AGTII) by the angiotensin-converting (ACE) enzyme. AGTII binds to specific receptors on the cell membrane. AGTII produces vasoconstriction raising systemic BP. Moreover, AGTII both directly stimulates sodium transport in the proximal tubule and increases aldosterone release that leads to increased activity of the epithelial Na+ (ENaC) channel. Through this mechanism, AGTII increases sodium and water reabsorption, expanding extracellular fluid volume, and increasing cardiac output and BP.

The INS/DEL polymorphism (I/D, rs4646994 or rs4340 or rs1799752 or rs13447447) is the most studied variant in the ACE gene, because it has been shown associated with serum ACE levels [35]. Most studies could not find any association with salt sensitivity [34•, 36]. Although an association of the I allele with BP increase after a high salt intake has been reported, under either a recessive [37] or a dominant [38] model, whereas the D allele has been described associated to BP changes in another study [39].

Two studies [40, 41] reported a significant association between salt sensitivity and M235T polymorphism (rs699 T>C) in the AGT gene. Patients with the TT genotype had the greatest increase in DBP, whereas no relationship was found with SBP [40]. Accordingly to this, the TT and the CT genotypes showed a significant reduction in SBP and DBP after low-sodium diet [41]. In AGT promoter a functional variant, in complete linkage disequilibrium with M235T, has been described (-6G>A, rs5051). The A allele is associated with increased gene transcription compatible with increased angiotensinogen level [42]. The AA genotype was found associated with greater BP decrease after sodium reduction [43].

The 11-beta-hydroxysteroids dehydrogenase (HSD11B2) gene has been studied since in the kidney the enzyme converts cortisol to the inactive cortisone. In this way, it prevents the improper activation by cortisol of the mineralcorticoid receptors that have similar affinities for cortisol and aldosterone. Mutations in HSD11B2 cause the syndrome of apparent mineralcorticoid excess and hypertension [44]. Three polymorphisms have been studied in relation to salt sensitivity: G-534A (rs45483293) with one negative and one positive association, a CA-repeat in intron 1 and G-209A (rs45598932) that showed positive associations [34].

The GenSalt study identified several signals of association to low sodium intake in RAAS genes: rs4524238 and rs3772616 in angiotensin type 1 receptor (AGT1R), rs1557501 and rs2269372 in renin-binding protein (RENBP), and rs5479 in HSD11B2 [45]. These results refer to a Chinese population in which linkage disequilibrium structure can be different compared with a Western population and this could affect replication.

Renal Ion Transporters

Renal ion channels, transporters, and exchangers involved in regulation of sodium balance, blood volume, and BP have been extensively studied. The GenSalt study reported the first association between common variants in the ENaC genes and salt sensitivity. In the kidney, ENaC is localized in the distal nephron where the final tubular sodium reabsorption occurs. One exonic and five intronic SNPs of ENaC γ-subunit were found associated with SBP response to low sodium [46]. Two of the five intronic SNPs have a possible enhancer activity on gene expression, but they need functional validation.

In the ascending limb of Henle’s loop and in the distal convoluted tubule, transcellular sodium reabsorption is directly coupled to chloride reabsorption. Therefore, an alteration at this site could affect both sodium and chloride homeostasis. Two members of the chloride channels (CLC) gene family are predominantly expressed in the kidney: the CLCNKA in the thin ascending limb (TAL) of Henle’s loop [47] and CLCNKB in the TAL, distal nephron, and macula densa [48]. The 1p36 locus harbours the two genes and emerged as a candidate when it was identified a strong activating mutation (Thr481Ser) in CLCNKB causing a sevenfold increase in Cl- transport when expressed in Xenopus oocytes [49]. The 481Ser allele was associated to higher BP in young normotensives [50]. Despite this association was not replicated [51–53], we described four new SNPs of CLCNKA associated with salt-sensitivity in never-treated hypertensives challenged with an acute sodium load [53].

ENaC is regulated by the serum/glucocorticoid regulated kinase 1 (SGK1) and by the Neural precursor cell Expressed Developmentally Down-regulated 4 Like (NEDD4L). SGK1 stimulates ENaC activity in the kidney by either increasing the residential time of the channels on the apical membrane or increasing ENaC gene transcription. NEDD4L affects the rate of sodium reabsorption in the distal nephron through an interaction with the PY motif and regulating ENaC cell surface expression by ubiquination. Rao et al. [54] recently demonstrated that carriers of the T allele at rs2758151 and of G allele at rs9402571 in SGK1 have significantly higher SBP on high salt intake.

Carriers of the GG genotype at rs4149601 and of the CC genotype at rs2288774 in NEDD4L were more salt-sensitive and had lower plasma renin compared with noncarriers [55], suggesting a tendency to increased sodium reabsorption in carriers. Manunta et al. [56] described a combination of risk alleles in α-adducin (ADD1) (rs4961), WNK1 (rs880054), and NEDD4L (rs4149601) that affects renal Na handling and BP response to thiazide treatment. WNK1 (WNK lysine deficient protein kinase 1) is a regulator of sodium transporters in the kidney.

Carey et al. [57] identified two common variants (rs7571842 and rs10177833) in the sodium-bicarbonate cotransporter gene (SLC4A5) that are strongly associated with salt sensitivity of blood pressure in two white independent populations. In renal tubular cells, SLC4A5 is involved in mantainance of costant intracellular pH.

We recently identified SNPs in genes that regulate intracellular calcium and vascular tone associated with SBP. In hypertensives very accurately phenotyped for salt-sensitivity, we reported an association between SBP increase and SNPs in sodium/calcium exchanger member 1 (SLC9A1, rs434082 G>A) and sodium/potassium/calcium exchanger (SLC24A3, rs3790261 G>A) [58]. The involvement of SLC8A1 in the pathogenesis of salt sensitivity has been previously demonstrated [59]. In the same study [58], a cluster of SNPs in intron 1 of the type 1 cGMP-dependent protein kinase (PRKG1) gene was found associated with DBP increase in response to an acute salt load. PRKG1 is a nitrovasodilator effector that has been shown to mediate vascular smooth muscle cells relaxation.

α-Adducin

The first paper describing the involvement of α-adducin (ADD1) in salt sensitivity was published by our group [60] and showed that the 460W allele of a functional polymorphism in ADD1 coding region (G460W, rs4961) was associated with a greater BP decrease after acute sodium load or long-term thiazide treatment. Adducin is a heterodimeric protein encoded by three closely related genes, α, β, and γ, mapping on different chromosomes [61]. Adducin promotes and regulates the binding of spectrin with actin and directly binds actin and bundles actin filaments. It is present in many tissues and within regions of cell-cell contacts. Adducin can modulate the lattice structure of the cytoskeleton and the exposure of transmembrane proteins [62].

The G460W replacement, as described in silico analysis, causes a change from an aliphatic group to an aromatic group, which could be potentially damaging because it affects protein stability [63].

This variant increases the number and the activity of Na+-K+-ATPase pump [64], the driving force of sodium transport reabsorption in renal epithelial cells. This probably explains why carriers of the 460W allele, compared with wild-type carriers, are salt sensitive [60], have a less steep pressure natriuresis relationship [65], and present enhanced proximal reabsorption of sodium [66]. We also demonstrated a joint effect of the 460W allele and the DEL/DEL ACE genotype on BP response to an acute sodium load [67]. An independent study reported similar results in carriers of the 460W allele on SBP changes from high- to a low-sodium intervention [68].

The GenSalt study examined seven SNPs in ADD1, including rs4961, as potential predictors of salt sensitivity [69], but reported no association for these variants. In this Chinese population a low-frequency variant, rs17833172, was significantly associated both with SBP and DBP response to high sodium and with DBP response to low sodium. Lack of association between rs4961 and salt sensitivity also was reported by two other studies [70, 71].

Endothelial System

The vascular endothelium plays a key role in BP regulation. The endothelial cells produce nitric oxide (NO), a very potent vasodilator synthesized from the aminoacid L-arginine by the endothelial nitric oxide synthase (eNOS) [72]. The endothelium expresses sodium channels, similar to those expressed in kidney tubular cells [73], regulated by the aldosterone [74] and whose activity is negatively correlated with NO release. Excess daily salt intake enhances vasoconstriction by decreasing production of NO via eNOS and increasing endothelial cell stiffness [75]. In humans, two studies reported an association between T-786C (rs2070744) polymorphism in the promoter of eNOS and BP response to salt intake [76, 77]. NAD(P)H dehydrogenase [NAD(P)H] is expected to reduce NO availability as NO also interact with superoxide ions generating peroxynitrite. Therefore, NAD(P)H oxidase gene could be a candidate gene in salt sensitivity and it appeared to be a susceptibility factor for salt sensitivity in women [78].

Despite a model of transgenic mice for endothelin-1 gene (ET-1), encoding a contracting factor produced by vascular endothelium, supported the involvement of this gene in salt-sensitivity [79], the GeneSalt study did not find any association between salt-sensitivity and variants in eNOS and ET-1.

The G1065A (rs5351) variant in the endothelin receptor subtype B (ETRB), encoding for a receptor mediating vasodilatation via nitric oxide, was described as significantly associated with salt sensitivity [80].

Sympathetic Nervous and Dopaminergic Systems

Increased sympathetic activity has been demonstrated in salt-sensitive hypertension and results in a) renal vasoconstriction with decreased renal blood flow and glomerular filtration rate, b) increased renal vascular resistance, c) increased renal tubular sodium and water reabsorption, and d) increased renal release of renin and norepinephrine [81–83].

In the sympathetic system, β-2 adrenergic receptors (ADRB2) may affect volume homeostasis through increased renin secretion [82]. Although not replicated by the GenSalt study, different reports identified an association between nonsynonymous coding variants in ADRB2 (G16R, rs1042713 and Q27E, rs1042714) and salt sensitivity [84–86].

The renal dopaminergic system, either by itself or via a synergistic interaction with other natriuretic factors, has a key role in renal natriuresis in presence of sodium excess [87]. An important player of the dopaminergic system in salt sensitivity is the G protein-coupled receptor kinase 4 (GRK4) [88•], which regulates type 1 dopamine receptors (D1R).

Functional polymorphisms (R65L rs2960306, A142V rs1024323, and A486V rs1801058) that increase basal GRK4 activity, decreasing D1R and increasing AGT1R activities, affect renal sodium excretion [89–92]. Animal studies also demonstrated that transgenic mice overexpressing GRK4-A142V were hypertensive, whereas transgenic mice overexpressing the variant GRK4-A486V developed hypertension only when placed on a high-salt diet [87]. However, negative results also were reported in human studies [93].

β3 Subunit of G Protein

G-proteins are transmembrane signal transducers that transmit signals from the cell surface to the intracellular environment. The GenSalt study reported that the minor allele at a new variant in guanine nucleotide-binding protein β-polypeptide 3 (GNB3), rs1129649, was associated with significantly decreased mean arterial pressure in response to low-sodium intervention [94].

Conclusions

Dietary sodium is an important contributor to hypertension, although blood pressure response to sodium, that is salt sensitivity, is very heterogeneous among individuals.

Despite few reports of an inverse relationship between cardiovascular events and salt intake, consensus exists on a causal relationship between chronic high-salt intake, high BP, and CVD. Controversy also exists about the beneficial effects that a reduction in population salt intake to the recommended level of <5 g/day could have on health in all countries around the world.

Many studies have been conducted to elucidate the genetic determinants underlying BP response to salt intake. Considerable heterogeneity exists among these studies, mainly due to the definition and measurement of the phenotype and to ethnic differences, which makes comparison more difficult. This heterogeneity generated many conflicting results.

Much work is still necessary to delineate the genetic architecture of salt sensitivity. This could allow identifying specific genetic profiles of BP susceptibility to salt and to characterize individuals who could benefit from low-sodium dietary intervention to prevent hypertension.