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1 Internal Medicine, Yale University, New Haven, CT, USA
* To whom correspondence should be addressed. E-mail: vazhaikkurichi.rajendran{at}yale.edu.
Bicarbonate and butyrate stimulate electroneutral Na absorption via apical membrane Na-H exchange (NHE) in rat distal colon. Cyclic AMP down-regulates NHE-3 isoform and inhibits HCO3--dependent, but not butyrate-dependent Na absorption. This study sought to determine whether: 1) the apical membrane NHE-2 and NHE-3 isoforms differentially mediated HCO3- and butyrate-dependent Na absorption; and 2) cyclic AMP had different effects on NHE-2 and NHE-3 isoforms. The effect of amiloride analogues that are specific inhibitors of NHE-2 and NHE-3 isoforms (50 µ M HOE694 and 2 µM S3226, respectively) on unidirectional 22Na transepithelial fluxes performed across isolated mucosa from rat distal colon under voltage clamp conditions was examined. HCO3--stimulation of Na absorption was inhibited by EIPA, a non-specific inhibitor of all NHE isoforms; by S3226; by dibutyryl cAMP; but not by HOE694. In contrast, butyrate stimulation of Na absorption was not altered by dibutyryl cAMP; was not inhibited by HOE694, in the absence of dibutyryl cAMP, but in the presence of dibutyryl cAMP was HOE694-sensitive. In contrast, S3226 inhibited butyrate-stimulated Na absorption in the absence of dibutyryl cAMP, but not in its presence. We conclude that: 1) HCO3--stimulated Na absorption is mediated solely by NHE-3 isoform, while butyrate-stimulated Na absorption is mediated by either NHE-3 or NHE-2 isoform; and 2) dibutyryl cAMP selectively inhibits NHE-3 isoform, but stimulates NHE-2 isoform. Dibutyryl cAMP does not inhibit butyrate-stimulated Na absorption as a result of its differential effects on NHE-2 and NHE-3 isoforms.
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