Mineralocorticoid receptor-independent activation of ENaC in a mouse model of cholestatic liver disease

Abstract

Advanced liver disease is associated with renal sodium retention, contributing to ascites, edema, and electrolyte disturbances. According to the current paradigm, portal hypertension with peripheral vasodilation leads to a deficit in the 'effective' extracellular fluid and blood volume, which stimulates the renin-angiotensin-aldosterone system (RAAS) to promote renal sodium and fluid retention. However, numerous studies over the past several decades document that roughly half of examined liver disease patients exhibiting fluid retention did not have stimulated RAAS. This suggests that a deficit in effective volume is insufficient to explain fluid retention in these patients. In advanced liver diseases, plasma and urinary bile acids levels can be greatly elevated, and recent studies have shown that bile acids directly stimulate epithelial Na+ channel (ENaC) transport. Here, we investigated ENaC regulation by bile acids in isolated perfused tubules and in whole animals in the context of a cholestatic liver disease model. We hypothesized that bile acids directly contribute to sodium retention through the in vivo activation of ENaC. Taurocholic acid enhanced sodium flux in isolated perfused rabbit tubules under low flow conditions, but had no effect on potassium flux. When we performed common bile duct ligations (BDL) to induce chronic cholestasis, plasma bile acids dramatically increased (p<0.0001). BDL mice also had increased plasma aldosterone compared to sham controls (p<0.0001). Accordingly, BDL mice had lower plasma K (p=0.005) and higher plasma bicarbonate (p=0.002), as well as higher indices of kidney injury, including the presence of casts and increased transcript of kidney injury marker 1 (Kim1; p<0.0001) and neutrophil gelatinase-associated lipicalin (NGAL; p<0.0001). Treating mice with the mineralocorticoid receptor antagonist spironolactone normalized bicarbonate levels and reduced transcripts of Kim1 (p=0.0003) and NGAL (p=0.0001), but did not normalize plasma K (BDL vs sham: p=0.004 for males and p<0.0001 for females). Using metabolic cages, we measured the effect of BDL with spironolactone treatment on the response to benzamil, an ENaC blocker. BDL increased the natriuretic response to benzamil compared to sham in both male (P<0.0001) and female (p=0.03) animals over a 24-hour preiod. We detected no effect on the kaliuretic response to benzamil over the same period. Altogether, our data support our hypothesis that urinary bile acids, elevated in liver disease, can activate ENaC-mediated transport independent of effects on mineralocorticoid receptor signaling. NIH R01 DK125439 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.