Immunophysiology of Intestinal Electrolyte Transport

Inducible Nitric Oxide Regulates Na-Glucose Co-transport in a Spontaneous SAMP1/YitFc Mouse Model of Chronic Ileitis

In mammalian small intestine, glucose is primarily absorbed via Na-dependent glucose co-transporter (SGLT1) on the brush border membrane (BBM) of absorptive villus cells. Malabsorption of nutrients (e.g., glucose) leads to malnutrition, a common symptom of inflammatory bowel disease (IBD), where the mucosa is characterized by chronic inflammation. Inducible nitric oxide (iNO) is known to be elevated in IBD mucosa. SAMP1/YitFc (SAMP1) mouse is a spontaneous model of chronic ileitis that develops lesions in its terminal ileum, very similar to human IBD. How SGLT1 may be affected in SAMP1 model of chronic ileitis is unknown. Ten-week-old SAMP1 mice with AKR mice as control were treated with N6-(1-iminoethyl)-L-lysine dihydrochloride (L-NIL) to inhibit iNO production. Intracellular NO levels were found to be increased in villus cells from SAMP1 mice. Moreover, SGLT1 and Na^+/K⁺-ATPase activities and BBM SGLT1 expression were significantly decreased. However, L-NIL treatment reduced the intracellular iNO production, and reversed both downregulated SGLT1 and Na⁺/K⁺-ATPase activities in SAMP1 mice. Inhibition of iNO by L-NIL treatment also significantly reversed the BBM SGLT1 protein expression in SAMP1 mice. L-NIL reversed the inflammation mediated downregulation of SGLT1 activity by restoring the BBM SGLT1 expression. Thus, regulation of SGLT1 in chronic ileitis is likely mediated by iNO.

Mechanism of leukotriene D4 inhibition of Na-alanine cotransport in intestinal epithelial cells

In a rabbit model of chronic intestinal inflammation, we previously demonstrated inhibition of neutral Na-amino acid cotransport. The mechanism of the inhibition was secondary to a decrease in the affinity for amino acid rather than the number of cotransporters. Since leukotriene (LT)D4 is known to be elevated in enterocytes during chronic intestinal inflammation, we used rat intestinal epithelial cell (IEC-18) monolayers to determine the mechanism of regulation of Na-alanine cotransport (alanine, serine, cysteine transporter 1: ASCT1) by LTD4. Na-alanine cotransport was inhibited by LTD4 in IEC-18 cells. The mechanism of inhibition of ASCT1 (solute carrier, SLC1A4) by LTD4 is secondary to a decrease in the affinity of the cotransporter for alanine without a significant change in cotransporter numbers and is not secondary to an alteration in the Na+ extruding capacity of the cells. Real-time quantitative PCR and Western blot analysis results indicate that ASCT1 message and protein levels are also unchanged in LTD4-treated IEC-18 cells. These results indicate that LTD4 inhibits Na-dependent neutral amino acid cotransport in IEC. The mechanism of inhibition is secondary to a decrease in the affinity for alanine, which is identical to that seen in villus cells from the chronically inflamed rabbit small intestine, where LTD4 levels are significantly increased.

Neutral Na-amino acid cotransport is differentially regulated by glucocorticoids in the normal and chronically inflamed rabbit small intestine

Neutral Na-amino acid cotransport by system ATB(0) [e.g., Na-alanine cotransport (NAcT)] is an important means of assimilation of amino acids in the intestine. NAcT is inhibited during chronic intestinal inflammation by an alteration in the affinity for the amino acid. How glucocorticoids, a standard of treatment for diseases characterized by chronic intestinal inflammation, may affect NAcT during chronic enteritis is not known. Thus we first demonstrated that methylprednisolone (MP) stimulated NAcT in the normal intestine. The mechanism of stimulation was secondary to an increase in cotransporter numbers without an alteration in the affinity for the amino acid. Treatment with MP reversed the reduction in NAcT in villus cells from the chronically inflamed intestine. MP also alleviated the decrease in Na-K-ATPase activity in villus cells during chronic enteritis. However, MP treatment reversed the NAcT inhibition in villus cell brush border membrane vesicles from the inflamed intestine, which suggested an effect of MP at the level of the cotransporter itself. Kinetic studies demonstrated that the reversal of NAcT inhibition by MP was secondary to restoration in the affinity for the amino acid without a change in the V(max). Unaltered steady-state mRNA and immunoreactive protein levels of NAcT also indicated that the number of cotransporters was unchanged after MP treatment in the chronically inflamed intestine. These results indicated that MP reversed NAcT inhibition in the chronically inflamed intestine by restoring the affinity of the transporter for the amino acid while it stimulated NAcT in the normal intestine by increasing the cotransporter numbers. Therefore, MP differentially regulates NAcT in the normal and chronically inflamed intestine.

Stimulation of Cl(-) secretion by L-alanine oligopeptides in the mammalian large intestine

A variety of oligopeptides are probably released within the intestinal tissue under inflammatory conditions or during peptide absorption. To examine whether some of these peptides can affect intestinal transport functions, we determined the effects of L-alanine oligopeptide on short-circuit current (I(sc)) and transmucosal conductance (G(t)) in submucosa-mucosa preparations from the mouse cecum and guinea pig distal colon in vitro in Ussing chambers. L-Alanyl-L-alanine (Ala-Ala, 10 mM) added to the serosal side increased I(sc) and G(t), giving a peak followed by a sustained phase (the peak increase in I(sc) was 45 +/-6 microA/cm(2) and the increase in G(t) was 0.55+/-0.11 mS/cm(2)). The tripeptide, L-alanyl-L-alanyl-L-alanine (Ala-Ala-Ala, 10 mM), added to the serosal side also induced increases in I(sc) and G(t) by a similar degree. On the other hand, luminal Ala-Ala, and serosal L-alanine and L-alanine (10 mM) caused significantly smaller increases in I(sc) and G(t) ( approximately 15 microA/cm(2) and approximately 0. 15 mS/cm(2), respectively). The Ala-Ala induced increase in I(sc) was partially inhibited by serosal bumetanide (0.1 mM) and mucosal 5-nitro-2-(3-phenylpropylamino)benzoic acid (0.1 mM), and largely suppressed by removing Cl(-) from the bathing solution. The increase in I(sc) was largely suppressed by serosal low Ca(2+) and tetrodotoxin, but was not affected by indomethacin. In the guinea pig distal colon, serosal Ala-Ala (10 mM) evoked a transient increase in I(sc) by 23+/-7 microA/cm(2) and an increase in G(t) by 1.2+/-0.3 mS/cm(2). These results suggest that Ala-Ala, and probably also Ala-Ala-Ala, added to the serosal side stimulated electrogenic Cl(-) secretion mainly through the activation of submucosal secretomotor neurons in the mammalian large intestine.

Cholera toxin induces synthesis of phospholipase A2-activating protein

The mechanism of cholera toxin (CT)-stimulated arachidonate metabolism was evaluated. CT caused rapid in vitro synthesis of prostaglandin E2 (PGE2) in murine smooth muscle-like cells (BC3H1), reaching maximal levels within 3 to 4 min. In comparison, cyclic AMP (cAMP) levels were unchanged, and addition of dibutyryl cAMP did not affect PGE2 synthesis. CT-induced PGE2 synthesis was prevented by actinomycin D or cycloheximide, indicating a need for de novo protein synthesis. Northern blot analysis of total RNA from BC3H1 cells revealed that exposure to CT resulted in an increase in abundance of mRNA encoding phospholipase A2 (PLA2)-activating protein (PLAP). PLAP is a regulatory protein that increases the enzymatic activity of cellular PLA(2), which in turn causes increased hydrolysis of arachidonate from membrane phospholipids. Furthermore, CT evoked the accumulation of PLAP mRNA in J774 (murine monocyte/macrophage) and Caco-2 (human intestinal epithelial) cells in vitro, but the responses were more delayed than that of BC3H1 cells. A protein band of approximately 35 kDa, which corresponded to the size of PLAP, was observed in sodium dodecyl sulfate extracts of Caco-2 cells by Western blot (immunoblot) analysis using affinity-purified antibodies to PLAP synthetic peptides. Synthesis of PLAP protein was increased after 2 h of exposure to CT. Exposure of mouse intestinal loops to either CT or live Salmonella typhimurium for 3 h increased mucosal PLAP mRNA levels. The role of PLAP in CT-induced PGE2 synthesis provides an attractive explanation for the reported suppression of CT-induced intestinal secretion by inhibitors of protein synthesis.

Comparison of the mechanisms of action of cholera toxin and the heat-stable enterotoxins of Escherichia coli

The mechanisms which enable cholera toxin (CT) and the Escherichia coli heat-stable enterotoxins (STa and STb) to stimulate intestinal secretion of water and electrolytes are only partially understood. CT evokes the synthesis of 3',5'-cyclic AMP (cAMP), and STa is known to elevate intestinal levels of 3',5'-cyclic GMP (cGMP). Neither of these recognized second messengers appears to mediate E. coli STb responses. We compared the secretory effects of CT, STa, and STb using the pig intestinal loop model and also measured the effects of toxin challenge on the synthesis of cAMP, cGMP, and prostaglandins (e.g., prostaglandin E2 [PGE2]), as well as on the release of 5-hydroxytryptamine (5-HT) from intestinal enterochromaffin cells. All three enterotoxins elicited fluid accumulation within a 2-h observation period. A combination of maximal doses of STa with STb yielded additive effects on fluid accumulation, which suggested different mechanisms of action for these toxins. Similarly, challenge of pig intestinal loops with a combination of CT and STb resulted in additive effects on fluid accumulation and luminal release of 5-HT. Unlike its effect on intestinal tissues from other animals, CT did not appear to elicit a dose-dependent cAMP response measurable in mucosal extracts from pig small intestine. In contrast, luminal fluid from CT-challenged pig intestinal loops contained dose-related amounts of cAMP and PGE2 that had been secreted from the mucosa. cAMP responses to STa or STb could not be demonstrated in either mucosal tissue or luminal fluid. In contrast, cGMP levels were increased in the intestinal fluid of loops challenged with STa but not in those challenged with STb. While the mechanisms of action of CT and STa are thought to involve impulse transmission via the enteric nervous system, we demonstrated significant stimulation of PGE2 synthesis and 5-HT release for CT and STb but very little for STa. We conclude from these data that the mechanisms of action of STa, STb, and CT are distinct, although the mode of action of STb may have some similarity to that of CT. Since STb stimulated the release of both PGE2 and 5-HT from the intestinal mucosa, the data suggested the potential for an effect of STb on the enteric nervous system.