Sodium Retention in Liver Disease: Is ENaC Activation Independent of the Renin-Angiotensin-Aldosterone System a Therapeutic Target?

Sodium reabsorption by the kidneys is driven by a combination of physiological and pathological mechanisms, largely regulated by hormonal influences and changes in blood volume or pressure. One of the primary pathways is the renin-angiotensin-aldosterone system (RAAS) [1]. When blood pressure or blood volume decreases, the kidneys release renin, eventually leading to angiotensin II production and aldosterone release from the adrenal glands, promoting sodium and water reabsorption in the distal tubules and collecting ducts of the kidneys to restore blood volume and pressure [1]. Antidiuretic hormone, also known as vasopressin, is released in response to low blood volume or high osmolarity, increasing water reabsorption in the kidneys [2]. The sympathetic nervous system also plays a role in sodium retention, activated by stress, low blood pressure, or reduced blood volume, where it directly stimulates sodium reabsorption in the kidneys and indirectly activates RAAS, further promoting sodium retention [3].

Sodium retention is a hallmark of advanced liver disease, often manifesting as ascites and peripheral edema [4‒6]. Traditionally, two main hypotheses – the underfilling and overfilling theories – have been used to explain sodium retention in liver disease [7]. The underfilling hypothesis posits that decreased effective circulating volume due to systemic vasodilation and portal hypertension is the driving force for renal sodium retention in an attempt to restore blood volume [5, 6, 8]. Conversely, the overfilling hypothesis posits that sodium retention is primarily due to intrinsic kidney changes that increase sodium reabsorption, potentially independent of systemic volume changes and RAAS activation. While underfilling has been the dominant model for decades, increasing evidence indicates that sodium retention may also occur without RAAS activation, suggesting angiotensin II and aldosterone-independent pathways. In addition to these two classic and powerful mediators of sodium reabsorption, there are new regulators that enhance it via activation of epithelial sodium channels (ENaC). For instance, plasmin has been shown to activate ENaC in patients with nephrotic syndrome [9, 10]. Therefore, there may be factors other than the systemic RAAS activation that may account for ENaC overactivity that are not stimulated by “underfilling.”

A study entitled “Epithelial Na⁺ Channel Activation after Bile Duct Ligation with Mineralocorticoid Receptor Blockade” by Wang et al. [11] supports the latter view focusing on ENaC activation by bile acids. Using a bile duct ligation model of liver disease in mice treated with spironolactone to block mineralocorticoid receptors, the study reports that bile acids directly activate ENaC, leading to increased sodium retention [11]. The key findings of this study include increased ENaC activity and fluid retention in a bile duct-ligated model of liver disease that was at least in part reversible by ENaC inhibition with benzamil. The taurocholic acid’s role in stimulating sodium reabsorption moreover is demonstrated convincingly in perfused collecting tubules. The findings support previous studies highlighting the role of bile acids in sodium retention as they can activate ENaC in the kidneys as discussed below. The study by Wang et al. [11] concludes that their findings provide experimental evidence for an aldosterone-independent mechanism for sodium and fluid retention in liver disease. Below, we render our opinion about this study and the status of the underfilling and overfilling theories.

Bile acids, such as taurocholic acid and taurohyodeoxycholic acid, are known to stimulate ENaC in the distal nephron [12‒15]. Bile acid regulation of ENaC depends on specific functional groups, suggesting a specific interaction, and does not require bile acids to be membrane permeable [12‒15]. Several lines of evidence strongly support bile acid regulation of ENaC through an allosteric mechanism, where bile acids directly bind to the channel [12‒15]. Bile acids directly bind ENaC and show specific binding affinities to both the extracellular and transmembrane domains of ENaC and modulate channel activity [12‒15]. Therefore, bile acids, like plasmin [12], are relatively new players in our understanding of the regulation of ENaC, which in the kidney collecting tubule is mainly under control of angiotensin II and aldosterone. Angiotensin II works via activation of AT1R receptors, both in the apical and basolateral membrane, while aldosterone and cortisol activate the nuclear mineralocorticoid receptor (Fig. 1).

The role of bile acids is especially relevant in liver disease, where elevated bile acids excreted by the kidney may lead to increased sodium reabsorption. The challenge is to understand their relative contribution; when in liver disease, there is usually strong activation of the RAAS as well. Therefore, the actions of angiotensin II and aldosterone may be dominant. By performing their studies in animals treated with spironolactone, Wang et al. [11] using the bile ligation model of liver disease tried to demonstrate that sodium retention occurs independently of aldosterone. A key finding of this study is that the natriuresis caused by blockade of the sodium channel using benzamil is greater than in sham-operated mice. This clearly shows activation of ENaC in this model. Of note, there were subtle sex differences that merit attention since ENaC activity is higher in females than males [16]. Notwithstanding potential important sex differences, the study by Wang et al. [11] confirms previous studies showing that bile acids can activate ENaC in the collecting duct [17, 18].

Activation of the renal ENaC may be particularly relevant in liver disease, where elevated bile acid levels may promote sodium retention even when the action of aldosterone is blocked by spironolactone. As pointed out by Wang et al. [11], however, one cannot be certain of the dose of spironolactone was sufficient to block the sodium channel. Therefore, we would be cautious to say that sodium retention via ENaC is aldosterone-independent. We think that ENaC activation from the various influences plays a role and that this may depend on different patients and phases of the disease. Measurements of renin (ideally angiotensin II) and aldosterone should be considered in guiding therapeutic interventions. Such measurements need to be interpreted with caution when patients are on diuretics and mineralocorticoid and sodium blockers (amiloride) that interfere with interpretation. Ideally, such measurements should be done when individuals are off these medications. Increasing bilirubin levels as a surrogate for bile acid renal excretion should be considered as an additional potential factor causing sodium retention via ENaC activation. It would be of interest to know if they also activate vascular ENaC and whether they exert an effect on blood pressure which is typically reduced in advanced liver disease. In fact, it is tempting to speculate that the real purpose for sodium retention and activation of ENaC in liver disease may be to sustain blood pressure.

Regarding therapeutic implications, it is logical not to oppose angiotensin II with ACE inhibitors or AT1 blockers when patients with liver disease are very hypotensive. Bile acid-induced ENaC activation could be a therapeutic target to reduce sodium and fluid retention in liver disease. Targeting circulating bile acids using bile acid sequestering agents like cholestyramine could be considered. Moreover, the uptake of bile acids by the proximal tubular epithelial cells of the kidney via the apical sodium-dependent bile acid transporter can be blocked by novel compounds as a way to lower circulating levels [19]. However, increased delivery of bile acids to the distal nephron by these compounds could activate ENaC. Therefore, lowering the bile acid levels using cholestyramine ought to be better than the use of these compounds.

Available blockers of ENaC like amiloride should be tested alone or in combination with maneuvers designed to lower bile acid excretion by the kidney. Amiloride may be more effective than spironolactone whenever the levels of aldosterone are not very high in the first place and also have an additive effect to mineralocorticoid blockade in many cases. Perhaps the safest approach could be to start with amiloride. This will increase levels of aldosterone even further such that adding a mineralocorticoid antagonist may then be even more effective to attenuate sodium retention. Such a combination may not be tolerated, however, because of hyperkalemia. Therefore, therapies aimed at lowering bile acid levels in combination with either only amiloride or a mineralocorticoid blocker may be a better option. Given the known sex differences in ENaC activation, studies in this area should pay attention to gender responses. Finally, the finding of an activated ENaC in liver disease can be viewed as a bridge between the underfilling and overfilling hypothesis. Both hypotheses have elements that complement each other and now experimental support.

D. Batlle is coinventor of patents entitled “Active Low Molecular Weight Variants of Angiotensin Converting Enzyme 2 (ACE2)” and “Soluble ACE2 Variants and Uses Therefor.” D. Batlle is founder of Angiotensin Therapeutics Inc. and has received consultant fees from AstraZeneca, Renibus, and Advicenne unrelated to this work. The other authors report no conflicts.

We acknowledge the support of Joseph and Bessie Feinberg Foundation and National Institutes of Health Grants 1R21 AI166940-01 and 1R43HL160432-01A1.

Daniel Batlle led the conceptual idea and wrote the final paper, Mohammed Z. Rehman wrote a draft, Thomas Brannon searched the literature, and Robert Rosa reviewed the draft of the paper and made suggestions for improvements.