Aldosterone applied mucosally or serosally is equally effective in stimulating ENaC activity in rat distal colon

Abstract

Background: Aldosterone (ALDO), a mineralocorticoid hormone, is essential for regulating blood volume and electrolyte homeostasis. ALDO acts via intracellular mineralocorticoid receptors to stimulate epithelial Na+ channel (ENaC)-mediated Na+ absorption and large conductance K+ (BK) channel-mediated K+ secretion in both kidney and distal colon. We recently showed that ALDO stimulates iron absorption in rat distal colon and SK-CO15 cells (human colon cancer cell line), raising the prospect that ALDO might be used as adjuvant therapy in iron-deficient patients refractory to oral iron. Since, parenteral ALDO (or Fludrocortisone, a synthetic mineralocorticoid agonist) would adversely affect cardiovascular function, an alternative route of administration is required. Parenteral ALDO gains access to target epithelial cells via their serosal (basolateral) membranes, and in vitro studies have always employed serosally applied ALDO to determine its effects on colonic ion transport, but it is not known if mucosally (apically) applied ALDO has similar effects. Hypothesis: We hypothesized that like serosal ALDO, mucosal ALDO would diffuse across apical cell membranes and induce ENaC activity in normal rat distal colon. Aim: To determine whether mucosally applied ALDO stimulates amiloride-sensitive electrogenic Na+ transport in rat distal colon. Methods: Muscularis stripped normal rat (Sprague-Dawley; 126 – 150 g) distal colon was mounted under voltage clamp condition in Ussing chambers. ALDO (10 mM) was added to either the mucosal or serosal bath. ENaC activity was measured as the change in short-circuit current (Isc) following the mucosal addition of amiloride (10 mM). ALDO in mucosal and serosal bath solutions was measured using mass-spectroscopy. Results: With serosal addition of ALDO, transcriptionally induced ENaC activity began to increase after 4 hr and reached maximum/plateau by 5 hr. By contrast, with mucosal addition of ALDO, ENaC activity began to increase after 8 hr and slowly reached maximum/plateau by 9.5 hr. Maximal amiloride-sensitive ENaC activities were similar irrespective of whether ALDO was applied mucosally or serosally. Following addition of ALDO to the mucosal or serosal bath, a similar amount (40%) of ALDO had moved from mucosa to serosa or serosa to mucosa after 24 hr. Conclusions: We conclude that: (1) ALDO induces ENaC activity to the same extent, whether applied mucosally or serosally, and (2) the delayed effect of mucosally applied ALDO may reflect slower diffusion across apical membranes than across basolateral membranes. We speculate that developing a colon-specific delivery system may avoid the adverse cardiovascular effects of oral mineralocorticoid agonists (e.g. Fludrocortisone) if used to enhance colonic iron absorption in iron-deficient patients refractory to oral iron. Transition GrantSupport; Offce of Research and Graduate Education, WVU Health Sciences Center. This is the full abstract presented at the American Physiology Summit 2024 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.