Central Role for ENaC in Development of Hypertension

Increased Na⁺ Reabsorption in Proximal Nephron Accompanied by an Appropriate Decrease in ENaC Activity: Studies in Normotensive Black and White Individuals

We tested the hypothesis that black individuals, a group at high risk for developing hypertension, have a greater level of ENaC activity when compared with white individuals, a group at lower risk. Participants were from a cohort of young people (mean age 17 yr), all of whom were normotensive (but inclusive of prehypertensives); thus, we avoided the confounding influences associated with hypertension and in particular its treatment. Black individuals of the cohort had consistently demonstrated lower aldosterone levels than the white individuals (Figure 2) (11), suggesting to us that greater Na⁺ retention was suppressing the renin-angiotensin-aldosterone axis in black individuals. Black and white individuals were treated for 1 wk with a 5-mg/d dose of amiloride. The BP response was used to gauge ENaC activity (12). Measurements of BP were made after an overnight hospitalization in the General Clinical Research Center so as to minimize differences between the conditions at baseline and the posttreatment period. The results indeed were surprising to us. In response to amiloride, systolic and diastolic BP decreased in white but not in black individuals, with the differences between groups significant for both systolic (P = 0.034) and diastolic (P = 0.010) BP. Black individuals seemed to have less ENaC activity to inhibit. In a cross-sectional sampling of participants from the same cohort, the urinary excretion of K⁺ was significantly lower in black than in white individuals (3.2 ± 0.1 versus 3.8 ± 0.1 mmol/mmol creatinine; P < 0.0001), and the serum K⁺ concentration was significantly higher in black than in white individuals (4.35 ± 0.05 versus 4.21 ± 0.03 [SE] mmol/L; P = 0.012), both findings indicating less K⁺ secretion in black individuals and consistent with less ENaC activity.

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Figure 2.

Plasma aldosterone concentrations in white and black school-aged children (pmol/L, mean ± SD). The difference between groups was significant at P < 0.001. Illustration by Josh Gramling—Gramling Medical Illustration.

We concluded that the lower aldosterone levels in black individuals was the primary event and that ENaC activity was secondarily reduced—the opposite of what we had conjectured previously. We deduced from the findings that increased Na⁺ reabsorption was occurring in another nephron region that led to suppressed aldosterone secretion and in turn a reciprocal decrement in Na⁺ reabsorption by ENaC. Na⁺ transport by ENaC adjusted appropriately to the increase in Na⁺ reabsorption because the BP in black and white individuals were similar.

“Normal” Aldosterone Levels Lead to an Inappropriate Level of ENaC Activity

Unlike what we observed in young people without hypertension, ENaC may not adjust to an increase in Na⁺ reabsorption in adults with the same facility. In a study of normotensive individuals from the Framingham Offspring Study cohort (n = 1688; mean age 55 yr), Vasan et al. (13) found a significant relationship between normal aldosterone levels and the BP 4 yr later. Specifically, there was a 16% increase in risk for an increase in BP and a 17% increase in risk for actually becoming hypertensive with each quartile increment in serum aldosterone level. The authors suggested that the regulation of aldosterone secretion developed along the lines that improved survival during an early ancestral period, when there was a scarcity of salt and water. There was no similarly compelling need for a full downward adjustment in aldosterone secretion to accommodate the higher intakes of Na⁺ that typify modern-day diets (Figure 3). Thus, many people, some more than others, may be exposed to an excess of aldosterone with the potential for an inappropriately increased reabsorption of Na⁺ by ENaC.

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Figure 3.

Aldosterone excretion rates in the Yanomamo Indians (21), who consumed an extremely low Na⁺ diet, probably typical of the early ancestral period, when regulation of aldosterone secretion evolved to optimize the conservation of Na⁺ for purposes of survival (Salt-Scarce World), and in individuals who consume a diet replete with Na⁺ (Today). There presumably was no similar survival advantage for aldosterone secretion to decrease to where there would be an avoidance of an excess of Na⁺ retention. Under low-salt conditions, excretion of aldosterone matches the amount required for Na⁺ and water homeostasis. Today, with diets containing an abundance of salt, aldosterone excretion may exceed what is required, leading to an inappropriately elevated level of ENaC activity. The “required” aldosterone excretion rate when diets are replete with Na⁺ was arbitrarily set at 3 μg/d. Illustration by Josh Gramling—Gramling Medical Illustration.

ENaC’s Pivotal Position Lends Itself to a “Two-Hit Model” for Development of Hypertension

If there is increased Na⁺ reabsorption in a proximal region of the nephron, then a downstream site must respond with an equivalent in magnitude reduction in Na⁺ reabsorption if normal Na⁺ and volume homeostasis are to be maintained. A principal downstream site is ENaC, with aldosterone delivering the level of stimulation to ENaC in keeping with the prevailing state of Na⁺ balance. If ENaC fails to decrease reciprocally its activity and because there is no additional downstream site to adjust to the increase in Na⁺ reabsorption by ENaC, then net Na⁺ retention will take place. Thus, a scenario can be proposed, one requiring “two hits”: An increase in Na⁺ reabsorption at a nephron site proximal to ENaC or possibly at ENaC itself (hit 1) together with a failure on the part of aldosterone secretion to decrease fully (hit 2), leaving ENaC operating at an inappropriately increased level. The orchestration of the kidney’s functions of course is more complex and fluid than such a simple scheme implies (Figure 4). For example, the pressure natriuresis that develops in response to the increase in BP would work toward restoring Na⁺ homeostasis followed by normalization of BP. Whether a hypertensive state ensues would also depend on the pressure-natriuretic responsiveness of the kidney and thus the possible requisite for a third hit. Nonetheless, ENaC, because of its distal location in the nephron and because of its regulation being driven by aldosterone, would seem to be at the center of the development of commonly encountered forms of hypertension. Proof of the concept will rest with demonstration of a combination of molecular or genetic variations acting together to increase risk for hypertension. That such a pairing of genes can affect BP was suggested in an association study in which individuals with a HindIII restriction site on the Y-chromosome were at increased risk for hypertension if they were also carriers of a variant in the aldosterone synthase gene (14).

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Figure 4.

An ENaC-mediated mechanism for the development of hypertension. In the normotensive state, there is no increase in Na⁺ reabsorption (example 1), or there is an increase in Na⁺ reabsorption that is accompanied by an appropriate downward adjustment in aldosterone secretion (example 2). An increase in BP occurs when there is an increase in Na⁺ reabsorption, with incomplete suppression of aldosterone secretion leading to incomplete suppression of ENaC activity (example 3). Because of the pivotal position of ENaC within the nephron, there being no downstream Na⁺ reabsorptive site to adjust reciprocally to the increase in Na⁺ reabsorption by ENaC, an increase in BP can ensue. Illustration by Josh Gramling—Gramling Medical Illustration.

Study Design and Procedures

The study used a two-by-two factorial design with four treatment groups: Amiloride (10 mg/d), a direct inhibitor of ENaC; spironolactone (25 mg/d); the combination of both drugs; and placebo. Patients (n = 98) were between 18 and 75 yr of age (mean age 46 yr), with a systolic BP >140 and/or a diastolic BP >90 mmHg while receiving hydrochlorothiazide (minimum dose of 25 mg) or furosemide (minimum dose of 40 mg) or equivalent doses of similar diuretics and amlodipine 5 or 10 mg or equivalent doses of a similar calcium channel blocker. Other drugs could be used (e.g., β blockers), but triamterene, angiotensin-converting enzyme inhibitors, and/or angiotensin receptor blockers were discontinued for 1 mo before the study was started because of the added risk for hyperkalemia. Exclusion criteria were an elevated plasma renin activity and evidence of a secondary form of hypertension. BP measurements were made at baseline and at weeks 1, 3, 5, 7, and 9 of active treatment. The study was designed with the primary end points being the changes from baseline in systolic and diastolic BP.

Outcomes

The reductions in systolic and diastolic BP (mmHg; mean ± SEM) in responses to the additional treatment were, respectively, 9.8 ± 1.6 and 3.4 ± 1.0 for amiloride and 4.6 ± 1.6 and 1.8 ± 1.0 for spironolactone. Amiloride significantly reduced both systolic and diastolic BP (P < 0.001). Spironolactone produced a significant reduction in systolic BP (P = 0.006) and a marginally significant reduction in diastolic BP (P = 0.07). Figure 5 shows the adjusted mean changes from baseline in BP. The largest drop in BP occurred at 3 wk, but it continued to decrease until week 7, at which point values stabilized for each of the treatment groups. The amiloride-spironolactone combination group showed the greatest reduction in BP followed by amiloride alone and spironolactone alone. We observed, however, no significant interaction of amiloride with spironolactone.

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Figure 5.

Changes from baseline in systolic and diastolic BP. The decrease in systolic BP was significant for all of the treatment groups when compared with placebo: P < 0.001 for amiloride, P < 0.010 for spironolactone, and P < 0.001 for the combination of amiloride and spironolactone. The decrease in diastolic BP was significant for the amiloride-treated group (P = 0.003) and for the combination of amiloride and spironolactone (P = 0.002) when compared with placebo. Illustration by Josh Gramling—Gramling Medical Illustration.

The BP-lowering response was greater for amiloride than for spironolactone, but a response similar to that seen with amiloride might have occurred had a larger dose of spironolactone been used. The superior response to amiloride suggests that it was indeed ENaC inhibition that led to the reduction in BP rather than another aldosterone response site for sodium reabsorption (19,20). The results of targeting ENaC for inhibition are consistent with a significant role for ENaC in the maintenance if not the development of common forms of hypertension.