NHE2 is the main apical NHE in mouse colonic crypts but an alternative Na+-dependent acid extrusion mechanism is upregulated in NHE2-null mice.
The mechanism of apical Na(+)-dependent H(+) extrusion in colonic crypts is controversial. With the use of confocal microscopy of the living mouse distal colon loaded with BCECF or SNARF-5F (fluorescent pH sensors), measurements of intracellular pH (pH(i)) in epithelial cells at either the crypt base or colonic surface were reported. After cellular acidification, the addition of luminal Na(+) stimulated similar rates of pH(i) recovery in cells at the base of distal colonic crypts of wild-type or Na(+)/H(+) exchanger isoform 2 (NHE2)-null mice. In wild-type crypts, 20 microM HOE694 (NHE2 inhibitor) blocked 68-75% of the pH(i) recovery rate, whereas NHE2-null crypts were insensitive to HOE694, the NHE3-specific inhibitor S-1611 (20 microM), or the bicarbonate transport inhibitor 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS; 1 mM). A general NHE inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA; 20 microM), inhibited pH(i) recovery in NHE2-null mice (46%) but less strongly than in wild-type mice (74%), suggesting both EIPA-sensitive and -insensitive compensatory mechanisms. Transepithelial Na(+) leakage followed by activation of basolateral NHE1 could confound the outcomes; however, the rates of Na(+)-dependent pH(i) recovery were independent of transepithelial leakiness to lucifer yellow and were unchanged in NHE1-null mice. NHE2 was immunolocalized on apical membranes of wild-type crypts but not NHE2-null tissue. NHE3 immunoreactivity was near the colonic surface but not at the crypt base in NHE2-null mice. Colonic surface cells from wild-type mice demonstrated S1611- and HOE694-sensitive pH(i) recovery in response to luminal sodium, confirming a functional role for both NHE3 and NHE2 at this site. We conclude that constitutive absence of NHE2 results in a compensatory increase in a Na(+)-dependent, EIPA-sensitive acid extruder distinct from NHE1, NHE3, or SITS-sensitive transporters.
Guan
Yanfang
Y
Dept. of Molecular and Cellular Physiology, Univ. of Cincinnati College of Medicine, 231 Albert Sabin Way, Medical Sciences Bldg., Cincinnati, OH 45267-0576, USA.
Dong
Jin
J
Tackett
Lixuan
L
Meyer
Jamie W
JW
Shull
Gary E
GE
Montrose
Marshall H
MH
D000143:Acids / Q000378:metabolism*; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D027682:Cation Transport Proteins / Q000235:genetics / Q000378:metabolism; D003106:Colon / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D005786:Gene Expression Regulation; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D012964:Sodium / Q000378:metabolism*; D000074082:Sodium-Hydrogen Exchanger 1; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics* / Q000378:metabolism*; D015854:Up-Regulation
D000143:Acids; D027682:Cation Transport Proteins; D008565:Membrane Proteins; C490997:Slc9a1 protein, mouse; C513111:Slc9a2 protein, mouse; C000613386:Slc9a3 protein, mouse; D000074082:Sodium-Hydrogen Exchanger 1; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D012964:Sodium; C039614:ethylisopropylamiloride