Cell-specific expression of epithelial sodium channel alpha, beta, and gamma subunits in aldosterone-responsive epithelia from the rat: localization by in situ hybridization and immunocytochemistry.

TLDR

A highly selective, amiloride‑sensitive epithelial sodium channel (rENaC) composed of α, β, and γ subunits was cloned from rat colon and proposed to mediate electrogenic sodium reabsorption in aldosterone‑responsive epithelia. The study aimed to determine whether rENaC accounts for in vivo sodium absorption by mapping its mRNA and protein localization in rat kidney, colon, salivary, and sweat glands. The authors used in situ hybridization to detect mRNA subunits and immunocytochemistry to visualize protein distribution across renal and extra‑renal epithelia. They found that α, β, and γ subunit mRNAs and proteins are co‑expressed in the distal nephron (DCT, CNT, CCD, OMCD) but absent in IMCD, where another amiloride‑sensitive transporter likely operates, and that rENaC is also present on distal colon epithelium and secretory ducts of salivary and sweat glands, supporting its role in aldosterone‑responsive sodium transport.

Abstract

A highly selective, amiloride-sensitive, epithelial sodium channel from rat colon (rENaC), composed of three homologous subunits termed alpha, beta, and gamma rENaC, has been cloned by functional expression and was proposed to mediate electrogenic sodium reabsorption in aldosterone-responsive epithelia. To determine whether rENaC could account for sodium absorption in vivo, we studied the cellular localization of the sodium channel messenger RNA subunits by in situ hybridization and their cellular and subcellular distribution by immunocytochemistry in the kidney, colon, salivary, and sweat glands of the rat. In the kidney, we show that the three subunit mRNAs are specifically co-expressed in the renal distal convoluted tubules (DCT), connecting tubules (CNT), cortical collecting ducts (CCD), and outer medullary collecting ducts (OMCD), but not in the inner medullary collecting ducts (IMCD). We demonstrate co-localization of alpha, beta, and gamma subunit proteins in the apical membrane of a majority of cells of CCD and OMCD. Our data indicate that alpha, beta, and gamma subunit mRNAs and proteins are co-expressed in the distal nephron (excepting IMCD), a localization that correlates with the previously described physiological expression of amiloride-sensitive electrogenic sodium transport. Our data, however, suggest that another sodium transport protein mediates electrogenic amiloride-sensitive sodium reabsorption in IMCD. We also localized rENaC to the surface epithelial cells of the distal colon and to the secretory ducts of the salivary gland and sweat gland, providing further evidence consistent with the hypothesis that the highly selective, amiloride-sensitive sodium channel is physiologically expressed in aldosterone-responsive cells.