title: The Na⁺/H⁺ exchanger isoform 3 is required for active paracellular and transcellular Ca²⁺ transport across murine cecum.
abstract: Intestinal calcium (Ca²⁺) absorption occurs via paracellular and transcellular pathways. Although the transcellular route has been extensively studied, mechanisms mediating paracellular absorption are largely unexplored. Unlike passive diffusion, secondarily active paracellular Ca²⁺ uptake occurs against an electrochemical gradient with water flux providing the driving force. Water movement is dictated by concentration differences that are largely determined by Na⁺ fluxes. Consequently, we hypothesized that Na⁺ absorption mediates Ca²⁺ flux. NHE3 is central to intestinal Na⁺ absorption. NHE3 knockout mice (NHE3-/-) display impaired intestinal Na⁺, water, and Ca²⁺ absorption. However, the mechanism mediating this latter abnormality is not clear. To investigate this, we used Ussing chambers to measure net Ca²⁺ absorption across different segments of wild-type mouse intestine. The cecum was the only segment with net Ca²⁺ absorption. Quantitative RT-PCR measurements revealed cecal expression of all genes implicated in intestinal Ca²⁺ absorption, including NHE3. We therefore employed this segment for further studies. Inhibition of NHE3 with 100 μM 5-(N-ethyl-N-isopropyl) amiloride decreased luminal-to-serosal and increased serosal-to-luminal Ca²⁺ flux. NHE3-/- mice had a >60% decrease in luminal-to-serosal Ca²⁺ flux. Ussing chambers experiments under altered voltage clamps (-25, 0, +25 mV) showed decreased transcellular and secondarily active paracellular Ca²⁺ absorption in NHE3-/- mice relative to wild-type animals. Consistent with this, cecal Trpv6 expression was diminished in NHE3-/- mice. Together these results implicate NHE3 in intestinal Ca(2+) absorption and support the theory that this is, at least partially, due to the role of NHE3 in Na⁺ and water absorption.
authors.affiliation: Dept. of Pediatrics, 4-585 Edmonton Clinic Health Academy, 11405 87th Ave., Univ. of Alberta, Edmonton, Alberta, T6G 2R7, Canada. todd2@ualberta.ca.
authors.forename: Juraj
authors.initials: J
authors.lastname: Rievaj
authors.forename: Wanling
authors.initials: W
authors.lastname: Pan
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Cordat
authors.forename: R Todd
authors.initials: RT
authors.lastname: Alexander
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002432:Cecum / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D005786:Gene Expression Regulation; D007408:Intestinal Absorption / Q000187:drug effects; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism*; D017136:Ion Transport; D008297:Male; D008564:Membrane Potentials; D049990:Membrane Transport Modulators / Q000494:pharmacology; D051379:Mice; D018345:Mice, Knockout; D018408:Patch-Clamp Techniques; D012333:RNA, Messenger / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012430:Ruthenium Red / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D050916:TRPV Cation Channels / Q000037:antagonists & inhibitors / Q000378:metabolism; D014867:Water / Q000378:metabolism
chemical_list: D015220:Calcium Channels; D049990:Membrane Transport Modulators; D012333:RNA, Messenger; C000613386:Slc9a3 protein, mouse; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D050916:TRPV Cation Channels; C446923:Trpv6 protein, mouse; D014867:Water; D012430:Ruthenium Red; D000584:Amiloride; D012964:Sodium; D002118:Calcium; C039614:ethylisopropylamiloride
keywords: NHE3; calcium absorption; cecum