Selvi Krishnan

Stimulation of sodium chloride absorption from secreting rat colon by short-chain fatty acids.

Inhibition of electroneutral NaCl absorption bycyclic adenosine monophosphate (cAMP) results in fluidmalabsorption in cholera. Short-chain fatty acids (SCFA)stimulate electroneutral NaCl absorption from the colon. The present study investigatedeffects of elevated cAMP on SCFA-stimulated NaClabsorption in rat distal colon. The effect of SCFA onfluxes of 22Na and 36Cl wasstudied under voltage-clamp conditions, in the presence and absence of secretagoguesinducing mucosal cAMP elevation [ie, theophylline,cholera toxin (CT) and forskolin]. The effect ofbutyrate concentration on Na absorption in CT- andtheophyllinetreated mucosa was compared with control normal mucosa.cAMP was measured in isolated colonocytes in thepresence of secretagogues with and without SCFA using aradioassay method. All secretagogues were noted to inhibit net Na absorption and to induce net Clsecretion. In the presence of SCFA, net Na absorptionwas normal, and net Cl secretion was partly reversed.The flux data indicated that NaCl absorption from secreting colon was stimulated by SCFA and thatCl secretion was partially inhibited. The effects ofSCFA on NaCl absorption were similar regardless of thesecretagogue used. The kinetics of butyratestimulated Na absorption were altered by theophylline andCT, which decreased Km(6.87 and 7.17,respectively, compared to 10.75 mM for control) andincreased Vmax(4.55 and 8.33 compared to 3.45 mM/eq/cm2/hr for control). cAMPproduction by colonocytes in response to secretorystimuli was significantly reduced (34.4%) by butyratebut not by acetate or propionate. In conclusion,SCFA-stimulated Na absorption is up-regulated by cAMP and maybe an absorptive pathway that can be utilized in thetherapy of cholera. Effects of butyrate on cAMPgeneration are also likely to be useful in secretorydiarrhea.

Glucose‐dependent Insulinotropic Polypeptide (GIP) Stimulates Transepithelial Glucose Transport

The purpose of this study was to characterize the effects of glucose‐dependent insulinotropic peptide (GIP) on small intestinal glucose transport in vitro. Stripped proximal jejunum from fasted mice was mounted in Ussing chambers. The serosal side was bathed in Regular Ringer solution containing 5 mmol/l glucose, and the mucosal side, with solution containing 10 mmol/l 3‐O‐methyl glucose (3OMG). Intercellular cyclic adenosine monophosphate (cAMP), mucosa‐to‐serosa fluxes of 3OMG (Jms3OMG), and short‐circuit current (ISC) were measured in the presence and absence of GIP. GIP increased cAMP by 2.5‐fold in isolated enterocytes, consistent with a direct effect of GIP on these epithelial cells. GIP also increased ISC and Jms3OMG by 68 and 53%, respectively, indicating that the increase in Jms3OMG was primarily electrogenic, with a small electroneutral component. The stimulatory effect of GIP on Jms3OMG was concentration dependent. In addition, 1,000 nmol/l and 10 nmol/l GIP increased Jms3OMG by 70 and 30% over control, respectively, consistent with receptor activation. Phlorizin (20 μmol/l), an inhibitor of Na+‐glucose cotransporter (SGLT‐1), abolished the increase in ISC and decreased Jms3OMG by ∼65%. These results indicate that stimulation of SGLT‐1 activity by GIP partially accounts for the increase in Jms30MG. These studies are the first to demonstrate direct stimulation of intestinal glucose transport by GIP independent of its insulinotropic properties. GIP stimulates cellular accumulation of cAMP and thereby upregulates glucose transport. The GIP‐induced increase in glucose transport appears to be mediated, at least in part, by SGLT‐1.

A novel cellular survival factor – the B2 subunit of vacuolar H + -ATPase inhibits apoptosis

The ubiquitous vacuolar H+-ATPase, a multisubunit proton pump, is essential for intraorganellar acidification. Disruption of its function leads to disturbances of organelle function and cell death. Here, we report that overexpression of the B2 subunit of the H+-ATPase inhibits apoptosis. This antiapoptotic effect is not mediated by an increase in H+-ATPase activity but through activation of the Ras-mitogen-activated protein kinase (MAPK)-signaling pathway that results in the serine phosphorylation of Bad at residues 112 and 155. Increased Bad phosphorylation reduces its translocation to mitochondria, limits the release of mitochondrial cytochrome c and apoptosis-inducing factor and increases the resistance of the B2 overexpressing cells to apoptosis. Screening experiments of kinase inhibitors, including inhibitors of cAMP-activated protein kinase, protein kinase C, protein kinase B, (MAPK/extracellular signal-regulated (ERK) kinase) MEK and Ste-MEK113, a cell permeable ERK activation inhibitor peptide, revealed that the B2 subunit of H+-ATPase acts upstream of MEK activation in the MEK/ERK pathway to ameliorate apoptosis.