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Postprandial Effects on ENaC-Mediated Sodium Absorption.

ABSTRACT: Recent studies have suggested that postprandial increases in insulin directly contribute to reduced urinary sodium excretion. An abundance of research supports the ability of insulin to augment epithelial sodium channel (ENaC) transport. This study hypothesized that ENaC contributes to the increase in renal sodium reabsorption following a meal. To test this, we used fasted or 4?hour postprandial Sprague Dawley rats to analyze ENaC expression and activity. We also assessed total expression of additional sodium transporters (Na+-Cl- cotransporter (NCC), Na+-K+-2Cl- cotransporter (NKCC2), and Na+-K+-ATPase (NKA)) and circulating hormones involved in the renin-angiotensin-aldosterone system (RAAS). We found that after carbohydrate stimulus, ENaC open probability increased in split-open isolated collecting duct tubules, while ENaC protein levels remained unchanged. This was supported by a lack of change in phosphorylated Nedd4-2, an E3 ubiquitin ligase protein which regulates the number of ENaCs at the plasma membrane. Additionally, we found no differences in total expression of NCC, NKCC2, or NKA in the postprandial rats. Lastly, there were no significant changes in RAAS signaling between the stimulated and fasted rats, suggesting that acute hyperinsulinemia increases ENaC activity independent of the RAAS signaling cascade. These results demonstrate that insulin regulation of ENaC is a potential mechanism to preserve sodium and volume loss following a meal, and that this regulation is distinct from classical ENaC regulation by RAAS.

SUBMITTER: Blass G

PROVIDER: S-EPMC6414683 | biostudies-literature | 2019 Mar

REPOSITORIES: biostudies-literature

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Project description:Reissner's membrane epithelium forms much of the barrier that produces and sustains the large ionic differences between cochlear endolymph and perilymph. We have reported that Reissner's membrane contributes to normal cochlear function by absorbing Na(+) from endolymph via amiloride-sensitive channels in gerbil inner ear. We used mouse Reissner's membrane to 1) identify candidate genes involved in the Na(+) transport pathway, 2) determine whether their level of expression was regulated by the synthetic glucocorticoid dexamethasone, and 3) obtain functional evidence for the physiological importance of these genes. Transcripts were present for alpha-, beta-, and gamma-subunits of epithelial Na(+) channel (ENaC); corticosteroid receptors GR (glucocorticoid receptor) and MR (mineralocorticoid receptor); GR agonist regulator 11beta-hydroxysteroid dehydrogenase (HSD) type 1 (11beta-HSD1); Na(+) transport control components SGK1, Nedd4-2, and WNKs; and K(+) channels and Na(+)-K(+)-ATPase. Expression of the MR agonist regulator 11beta-HSD2 was not detected. Dexamethasone upregulated transcripts for alpha- and beta-subunits of ENaC ( approximately 6- and approximately 3-fold), KCNK1 ( approximately 3-fold), 11beta-HSD1 ( approximately 2-fold), SGK1 ( approximately 2-fold), and WNK4 ( approximately 3-fold). Transepithelial currents from the apical to the basolateral side of Reissner's membrane were sensitive to amiloride (IC(50) approximately 0.7 muM) and benzamil (IC(50) approximately 0.1 muM), but not EIPA (IC(50) approximately 34 muM); amiloride-blocked transepithelial current was not immediately changed by forskolin/IBMX. Currents were reduced by ouabain, lowered bath Na(+) concentration (from 150 to 120 mM), and K(+) channel blockers (XE-991, Ba(2+), and acidification from pH 7.4 to 6.5). Dexamethasone-stimulated current and gene expression were reduced by mifepristone, but not spironolactone. These molecular, pharmacological, and functional observations are consistent with Na(+) absorption by mouse Reissner's membrane, which is mediated by apical ENaC and/or other amiloride-sensitive channels, basolateral Na(+)-K(+)-ATPase, and K(+)-permeable channels and is under the control of glucocorticoids. These results provide an understanding and a molecular definition of an important transport function of Reissner's membrane epithelium in the homeostasis of cochlear endolymph.

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Postprandial Effects on ENaC-Mediated Sodium Absorption.

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