3B and C), it is likely to be mediated by NBCe1, which is also expressed in the crypt basolateral membrane (Yu et al. 2009). We therefore performed experiments in the absence of CO2/HCO3?, and used different concentrations of the NHE inhibitor HOE642 to inhibit sequentially NHE1, and NHE1 and NHE2, assessing pHi recovery individually in the surface cells near cryptal mouth and the basal regions of the crypts, as demonstrated visually by Cinar et al. check details
(2007). Figure 4 shows that in the basal regions of the crypt, pHi recovery after an acid load is mediated ?40% by NHE1 and ?60% by NHE2, and no residual proton flux remains with addition of 50 ��m HOE642. In the surface regions, NHE1 and NHE2 mediate <30% of pHi recovery. Given that the selective NHE3 inhibitor S1611 has been shown to inhibit rat NHE1 at higher concentrations, we used NHE3 KO mice to assess how <a href="http://www.selleck.cn/products/AZD6244.html
">Selumetinib much of the residual HOE642-insensitive pHi recovery in the WT was mediated by NHE3. When identical experiments were performed in NHE3 KO mice, the proton flux in the near-surface cells of the cryptal mouth region in the presence of 50 ��m HOE642 was 4.2 mm min?1, in comparison to ?25 mm min?1 in WT mice. The remaining ?15% of Na+-dependent proton efflux, which is only seen in the surface colonocytes, is due to non-identified transport mechanisms. We next measured the HCO3? secretory rates in the different segments of isolated murine colonic mucosa. As we had previously observed that basal alkalinization rates in colonic mucosa are strongly Cl? dependent and mediated by the chloride anion exchanger DRA (Slc26a3), we first measured basal HCO3? secretory rates in WT and NBCn1-deficient proximal and mid-distal colonic mucosa, but performed luminal Cl? substitution before stimulation with agonists (Fig. 5A and
. The removal of luminal Cl? results in inhibition of Cl?�CHCO3? exchange-mediated HCO3? export, a strong reduction in basal colonic mucosal alkalinization rates and an DNA Damage inhibitor
enhancement of the agonist-stimulated HCO3? secretory response (Xiao et al. 2012b). The basal, carbachol (CCh)- and FSK-stimulated colonic HCO3? secretory rates were not significantly different in the proximal or the mid-distal isolated colonic mucosa of NBCn1-deficient and WT mice (Fig. 5A and
. We next assessed the fluid absorptive rate in the mid-distal colon in anaesthetized, ventilated and acid�Cbase-controlled mice by a luminal perfusion technique, as well as the HCO3? secretory rate in these conditions (Fig. 5C and D). No significant differences in the fluid absorptive rates (Fig. 5D) were observed between NBCn1-deficient and WT mice during luminal perfusion with unbuffered iso-osmolar saline, pH 7.0 (as well as after stimulation of fluid absorption by switching to an iso-osmolar CO2/HCO3?-containing solution of identical pH 7.0; data not shown).