Submitted on October 5, 2006
Accepted on August 9, 2007
Differential Regulation of Cholera Toxin-Inhibited Na-H exchange Isoforms by Butyrate in Rat Ileum
Sandeep B. Subramanya1, Vazhaikkurichi M Rajendran2, Balakrishnan S Ramakrishna3, and Henry J. Binder4*
1 G.I. Sciences, Christian Medical College, Vellore, India
2 Dept of Internal Med - Dig. Dis., Yale Univ School of Med, New Haven, Connecticut, United States
3 G.I.Sciences, Christian Medical College Hospital, Vellore, Tamil Nadu, India
4 Internal Medicine, Yale University, 06520, Connecticut, United States
* To whom correspondence should be addressed. E-mail: henry.binder{at}yale.edu.
Electroneutral Na absorption occurs in the intestine via sodium-hydrogen exchanger (NHE) isoforms NHE2 and NHE3. Bicarbonate and butyrate both stimulate electroneutral Na absorption through NHE. Bicarbonate-, but not butyrate-dependent Na absorption, is inhibited by cholera toxin (CT). Long term exposure to butyrate also influences expression of apical membrane proteins in epithelial cells. These studies investigated the effects of short- and long-term in vivo exposure to butyrate on apical membrane NHE expression and activity in rat ileal epithelium that had been exposed to CT. Ileal loops were exposed to CT in vivo for 5 hours and apical membrane vesicles were isolated. 22Na uptake was measured using the inhibitor HOE694 to identify NHE2 and NHE3 activity and western blot analyses were performed. CT reduced total NHE activity by 70% in apical membrane vesicles with inhibition of both NHE2 and NHE3. Reduced NHE3 activity and protein expression remained low following removal of CT but increased to control values following incubation of the ileal loop with butyrate for two hours. In parallel there was a 40% decrease in CT-induced increase in cAMP content. In contrast, NHE2 activity partially increased following removal of CT and further increased to control levels by butyrate. NHE2 protein expression did not parallel its activity. These results indicate that CT has varying effects on the two apical NHE isoforms, inhibiting NHE2 activity without altering its protein expression, and reducing both NHE3 activity and protein expression. Butyrate restores both CT-inhibited NHE2 and NHE3 activities to normal levels but via different mechanisms.
