Nitric oxide donors can enhance the intestinal transport and absorption of insulin and [Asu(1,7)]-eel calcitonin in rats.

The characteristics of three NO donors, 3-(2-hydroxy-1-(1-methylethyl)-2-nitrosohydrazino)-1-propanamine (NOC5), N-ethyl-2-(1-ethyl-2-hydroxy-2-nitrosohydrazino)-ethanamine (NOC12) and S-nitroso-N-acetyl-DL-penicillamine (SNAP) as absorption enhancers for peptide drugs were examined in rats using a modified Ussing chamber method and an in situ closed loop method. Insulin and [Asu(1,7)]-eel calcitonin (ECT) were used as a model drug to investigate the effectiveness of the tested enhancers. The NO donors significantly increased the in vitro permeability of insulin across all intestinal membranes. In general, the absorption enhancement effects of these NO donors were greater in the colon than those in the jejunum and ileum. Of these NO donors, SNAP was the most effective enhancer. Their effects were concentration-dependent over the range of 0.01 to 0.1 mM. However, 0.1 mM NO donors had almost the same effects as those at 1 mM concentration. The absorption-enhancing effects of the three NO donors were inhibited by the co-administration of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide, sodium salt (carboxy-PTIO), an NO scavenger, suggesting that NO might be responsible for the efficacy of NO donors. In the in situ closed loop experiments, the three enhancers significantly improved the pharmacological availability % (PA%) of insulin in the small and large intestine. Similar results were also obtained when NO donors were added to ECT solution by an in situ closed loop method. These results suggest that NO donors possess excellent effectiveness for the use as absorption enhancers of peptide drugs and they are very effective at lower concentrations compared to the conventional enhancers.     

Nitric oxide directly impairs intestinal barrier function

Excess production of nitric oxide (NO) has been implicated in endotoxin-induced loss of gut barrier function in vivo. Thus, we tested the direct effect of NO on the barrier function of intestinal mucosal membranes suspended ex vivo in Ussing chambers and on IEC-6 enterocyte monolayers. In these experiments, ex vivo-mounted ileal membranes or IEC-6 cell enterocyte monolayers were exposed to the NO donor, S-nitroso-N-acetyl-penicillamine (SNAP) over a dose range (10 microm to 2 mM) or medium. SNAP at concentrations of 1 or 2 mM, but not 10 or 100 microM, increased the rates of bacterial translocation (BT) across both the ileal membranes and the IEC-6 monolayers by >1 log (P < 0.05), as well as the permeability of the IEC-6 monolayers to phenol red (P < 0.05). The ileal membranes exposed to 1 or 2 mM SNAP for 3 h manifested histologic evidence of mucosal injury and decreases in electrical resistance and potential difference values (P < 0.05), while the IEC-6 cells exposed to SNAP for 18 h had increased levels of cell death (P < 0.05). Since NO produced locally by stimulated enterocytes could contribute to barrier dysfunction, NO production, iNOS mRNA levels, and monolayer permeability were measured in enterocytes (IEC-6 and Caco-2) exposed to medium, endotoxin (lipopolysaccharide [25 microg/mL]) or a cytokine mixture (IL-1beta 10 ng/mL, TNF-alpha 10 ng/mL, and INF-gamma 250 U/mL) for 6 or 24 h. Endotoxin increased NO production, iNOS mRNA expression, and monolayer permeability in the IEC-6, but not the Caco-2 cells, while exposure to the cytokine mixture increased both NO production, iNOS mRNA expression, and monolayer permeability in both the IEC-6 and Caco-2 cell lines. Based on the results of these studies it appears that NO can directly increase ileal mucosal membrane and enterocyte monolayer permeability and BT and that increased NO production and iNOS mRNA expression is associated with endotoxin- and/or cytokine-induced loss of enterocyte monolayer barrier function.     

Acidic conditions exacerbate interferon-gamma-induced intestinal epithelial hyperpermeability: role of peroxynitrous acid.

OBJECTIVE: Nitric oxide (NO*) derived from exogenous donors has been shown to increase the permeability of cultured intestinal epithelial monolayers, an effect that is augmented by mildly acidic conditions. Because interferon-gamma (IFN-gamma) also increases intestinal epithelial permeability, at least partly by an NO*-dependent mechanism, we sought to determine whether IFN-gamma-induced hyperpermeability is increased under acidic conditions. METHODS: Human intestinal epithelial (Caco-2BBe) cells were grown as monolayers on permeable supports in bicameral chambers. Permeability was assessed by measuring transepithelial electrical resistance (TER) or the transepithelial passage of fluorescein disulfonic acid. Inducible nitric oxide synthase (iNOS) messenger RNA expression was determined by northern blot analysis. Concentrations of nitrite and nitrate (NO2-/NO3-), stable oxidation products of NO*, were determined using the Greiss reaction. Cellular adenosine triphosphate (ATP) levels were determined using the luciferin/luciferase method. MEASUREMENTS AND MAIN RESULTS: Incubation of Caco-2BBe monolayers with INF-gamma (1000 units/mL) at an extracellular pH (pHo) of 7.4 increased permeability to fluorescein disulfonic acid and decreased TER. However, incubation of monolayers with IFN-gamma under mildly acidic conditions (i.e., pHo 7.0-6.6) accelerated the decrease in TER and augmented the increase in permeability induced by the cytokine. IFN-gamma-induced iNOS messenger RNA expression and NO2-/NO3- accumulation in medium were unaffected by acidic conditions. At pHo 7.4, incubation of Caco-2BBe monolayers with IFN-gamma (1000 units/mL) for 72 hrs had no effect on intracellular ATP content compared with monolayers simultaneously incubated under the same conditions but in the absence of the cytokine. However, when the cells were incubated for 72 hrs with the same concentration of IFN-gamma under mildly acidic conditions (i.e., pHo 7.0 or 6.6), ATP levels were significantly decreased. At pHo 7.0, IFN-gamma-induced increases in permeability were ameliorated by addition of the following agents: 2-phenyl-4,4,5,5- tetramethylimidazoline-1-oxyl-3-oxide (a NO* scavenger), N(G)-monomethyl-L-arginine (a iNOS inhibitor), dimethyl sulfoxide (a hydroxyl radical scavenger), and ascorbate (a peroxynitrous acid scavenger). CONCLUSION: Mild acidosis augments IFN-gamma-induced intestinal epithelial hyperpermeability and ATP depletion, possibly by fostering the formation of peroxynitrous acid and/or hydroxyl radical.     

Lipopolysaccharide-induced enterocyte-derived nitric oxide induces intestinal monolayer permeability in an autocrine fashion

Studies indicate that endotoxin (LPS) causes intestinal injury, increases inducible nitric oxide synthase (iNOS) activity, leads to increased NO production, and promotes bacterial translocation (BT). To investigate the mechanism by which LPS causes gut injury and to test the hypothesis that NO produced by enterocytes promotes gut injury in an autocrine fashion, rat intestinal epithelial cell (IEC-6) monolayers were tested. IEC-6 monolayers grown in a bicameral system were incubated with media or with LPS (25 microg/mL) and tested for permeability to phenol red, BT, and nitrate/nitrite (NO2/NO3) production. To determine the direct effect of NO on permeability, monolayers were incubated with the NO donor S-nitroso-acetylpenicillinamide (SNAP; 1 mM) and tested for permeability. Next, the protective effects of two NOS inhibitors (L-NMMA and L-NIL) were tested. Finally, to determine if LPS-induced permeability occurs via a poly (ADP-ribose) synthetase- (PARS) dependent pathway, monolayers incubated with LPS alone or with the PARS inhibitor, INH2BP (100 microM) were tested. LPS significantly increased IEC-6 permeability to phenol red, as well as increased NO2/NO3 by 20-fold (P < 0.001) and increased BT 10-fold (P < 0.001). SNAP mimicked the effect of LPS and significantly increased both permeability to phenol red and BT. Inhibition of iNOS significantly decreased the LPS-induced increase in monolayer permeability and BT (P < 0.05). Monolayers incubated with INH2BP had significantly decreased permeability to phenol red and BT, suggesting that LPS-induced NO production increases monolayer permeability at least in part via a PARS-dependent mechanism. In summary, LPS-induced disruption of monolayer barrier function appears to be related, at least in part, to enterocyte produced NO. This supports the hypothesis that NO produced by LPS-stimulated enterocytes promotes injury in an autocrine fashion and highlights the fact that enterocytes can be a target as well as a producer of NO.     

Talniflumate increases survival in a cystic fibrosis mouse model of distal intestinal obstructive syndrome

Intestinal disease in cystic fibrosis (CF) mice closely mirrors aspects of obstructive syndromes in CF patients. The pathogenesis involves accumulation of mucoid debris in the crypts that fuse with intestinal content to form obstructing mucofeculant impactions. Treatment involves modalities that increase the fluidity of the luminal content, such as osmotic laxatives and liquid diets. We investigated the effects of talniflumate (Lomucin, Genaera Corporation, Plymouth Meeting, PA), a compound that may be beneficial to treatment of CF intestinal disease based on three mechanisms of action: mucus synthesis inhibition by blockade of the murine calcium-activated chloride channel 3 (mCLCA3), nonsteroidal anti-inflammatory effects, and inhibition of Cl(-)/HCO (-)(3) exchanger(s) involved in intestinal NaCl absorption. Cohorts of CF mice were fed control diet or diets containing either talniflumate (0.4 mg/g chow) or ibuprofen (0.4 mg/g chow) for 21 days to assess survival. Talniflumate significantly increased CF mouse survival from 26 to 77%, whereas ibuprofen had no effect (22% survival). Oral talniflumate did not alter crypt goblet cell numbers or change intestinal expression of mCLCA3 but tended to decrease crypt mucoid impaction. Ussing chamber studies indicated that talniflumate slightly increased the basal short-circuit current of CF intestine, but the change was not sensitive to secretagogue stimulation or bumetanide inhibition. In contrast, intracellular pH measurements of intact intestinal villous epithelium indicated that talniflumate significantly inhibited apical membrane Cl(-)/HCO (-)(3) exchange by >50%. We conclude that oral talniflumate increases the survival of CF mice, possibly by the beneficial effects of decreasing small intestinal NaCl absorption through the inhibition of apical membrane Cl(-)/HCO (-)(3) exchanger(s).     

Ammonia inhibits cAMP-regulated intestinal Cl- transport. Asymmetric effects of apical and basolateral exposure and implications for epithelial barrier function.

The colon, unlike most organs, is normally exposed to high concentrations of ammonia, a weak base which exerts profound and diverse biological effects on mammalian cells. The impact of ammonia on intestinal cell function is largely unknown despite its concentration of 4-70 mM in the colonic lumen. The human intestinal epithelial cell line T84 was used to model electrogenic Cl- secretion, the transport event which hydrates mucosal surfaces and accounts for secretory diarrhea. Transepithelial transport and isotopic flux analysis indicated that physiologically-relevant concentrations of ammonia (as NH4Cl) markedly inhibit cyclic nucleotide-regulated Cl- secretion but not the response to the Ca2+ agonist carbachol. Inhibition by ammonia was 25-fold more potent with basolateral compared to apical exposure. Ion substitution indicated that the effect of NH4Cl was not due to altered cation composition or membrane potential. The site of action of ammonia is distal to cAMP generation and is not due simply to cytoplasmic alkalization. The results support a novel role for ammonia as an inhibitory modulator of intestinal epithelial Cl- secretion. Secretory responsiveness may be dampened in pathological conditions associated with increased mucosal permeability due to enhanced access of lumenal ammonia to the basolateral epithelial compartment.     

Repeated measurement of intestinal permeability as an assessment of colitis severity in HLA-B27 transgenic rats.

We report on the development of a method for repeated monitoring of mucosal permeability that allows assessment of the severity of colitis and evaluation of treatment efficacy in HLA-B27 transgenic rats. We determined the extent to which intestinal permeability related to stool condition, colon weight, and histological pathology in precolitic and diseased rats up to 29 weeks old. Intestinal permeability was measured by the urinary excretion of iodixanol at 24 h after oral administration. Mean permeability values increased significantly with age in HLA-B27 rats but remained decreased in the background strain Fischer-344 (F-344) control animals. Macroscopic evaluation of HLA-B27 rat colons between 20 and 24 weeks old showed colonic thickening with colonic wet weights increased from 3.4+/-0.13 mg/kg b.wt. in F-344 rats to 6.79+/-0.73 mg/kg b.wt. (p<.05) in HLA-B27 rats. Histological examination of HLA-B27 rat colons confirmed the colonic inflammation as a chronic active mononuclear cell infiltrate. The increase in colon weight was associated with an increase in permeability: 1.16+/-0.17 mg iodixanol versus 5.37+/-1.3 mg of iodixanol in F-344 and HLA-B27 rats, respectively. Three weeks treatment of HLA-B27 rats with cyclosporin A, but not sulfasalazine, showed a dose-dependent decrease in mucosal permeability and colon weight. Neither treatment improved stool condition. We conclude that the measurement of intestinal permeability by iodixanol excretion is a useful biochemical marker that is associated with increases in colonic weight and histological evaluation of inflammation. These data indicate that this technique may be valuable for diagnostic and evaluation purposes in preclinical models of inflammatory bowel disease.     

Nitric oxide promotes the internalization and passage of viable bacteria through cultured Caco-2 intestinal epithelial cells

Nitric oxide (NO) may play an important role in the pathophysiology of intestinal barrier disruption. Our purpose was to investigate the effects of NO donors on the internalization and passage of bacteria through cultured intestinal epithelial cells. Human intestinal epithelial cell line Caco-2 cells were grown on microtiter plastic plates. The cells were incubated with Escherichia coli and sodium nitroprusside (SNP) or S-nitroso-N-acetyl-penicillamine (SNAP), as NO donors, at several concentrations. The numbers of viable bacteria internalized into the epithelial cells were measured. Caco-2 cells were also grown to confluency on membranes of bicameral systems. The cells were incubated with E. coli and SNP. The numbers of viable bacteria passed through the epithelial layer were determined. Viability of the bacteria and the intestinal epithelial cells after culture with SNP or SNAP were also determined. Both SNP and SNAP at .1 or 1 mmol/L increased the number of viable bacteria internalized into the enterocytes. Both 1 or 10 mmol/L SNP promoted bacterial passage through the intestinal epithelial layer. However, 10 mmol/L SNP decreased the number of viable Caco-2 cells and failed to increase the bacterial internalization into Caco-2 cells. Incubation of E. coli with SNAP at 10 mmol/L slightly decreased the number of viable bacteria and failed to increase the bacterial internalization into Caco-2 cells. We conclude that NO donors promote both the viable bacterial uptake and passage through the intestinal epithelial layer.     

Effects of pro- and anti-inflammatory cytokines and nitric oxide donors on hyaluronic acid synthesis by synovial cells from patients with rheumatoid arthritis

The aim of the present study was to investigate the effects of (i) the pro-inflammatory cytokines IL (interleukin)-1beta, TNF-alpha (tumour necrosis factor-alpha), IFN-gamma (interferon-gamma) and anti-inflammatory cytokines IL-4 and IL-13, and (ii) NO (nitric oxide) donors on HA (hyaluronic acid) production by synovial cells from patients with rheumatoid arthritis. Synovial cells obtained from five patients with rheumatoid arthritis were incubated for 24 h without or with IL-1beta, TNF-alpha, IFN-gamma, or with this mixture for 24 h plus IL-4 or IL-13 for the last 6 h. The same cells were also incubated for 3-24 h without or with SNP (sodium nitroprusside) or SNAP (S-nitroso-N-acetyl-DL-penicillamine). HA secretion was determined by an immunoenzymic assay based on HA-specific binding by proteoglycan isolated from bovine cartilage. IL-1beta, TNF-alpha and IFN-gamma alone or in combination stimulated HA synthesis, whereas IL-4 and IL-13 dose-dependently inhibited HA production induced by Th1 cytokines. HA production was significantly increased by the presence of 1 mM SNP after 6 and 12 h (maximal effect). HA production was significantly increased by the presence of 0.01 and 0.1 mM SNAP after 12 h of incubation, and cells treated with 1 mM SNAP showed a maximal HA production after 24 h of incubation. In conclusion, the present study provides data concerning the regulatory role of pro- and anti-inflammatory cytokines and NO donors on HA metabolism in rheumatoid synovial cells and may help in understanding the pathophysiology of rheumatoid arthritis.     

Epinephrine is an enhancer of rat intestinal absorption.

Some physiological substances, including acetylcholine and nitric oxide, are useful candidates for stimulation of intestinal absorption of drugs. In the present study, we elucidated the ability of epinephrine (Epi) to stimulate the intestinal absorption of drugs. We evaluated the ability of Epi to enhance absorption of macromolecules using dextran (Mw 4000 Da), which is poorly absorbed from the intestine, as a model compound in situ in a closed loop of the rat jejunum. Treatment of the jejunum with Epi resulted in significant increase in absorption of dextran in a dose-dependent fashion. The area under the curve (AUC) from 0 to 4 h in the Epi-treated jejunum was 13-fold higher than that in the vehicle-treated jejunum. The absorption-enhancing activity of Epi was 40-fold higher than that of caprate, a clinically used absorption-enhancer of drugs. In the experimental conditions used in this study, histological injury of the mucosa and perturbation of the mucosal membrane were not observed in the Epi-treated jejunum. Treatment with an antagonist of alpha-adrenergic receptors attenuated the stimulation of intestinal absorption by Epi, and treatment with an agonist of alpha-adrenergic receptors resulted in enhancement of intestinal absorption. While an antagonist of beta-adrenergic receptors enhanced the absorption-enhancing effect of Epi, an agonist of beta-adrenergic receptors stimulated intestinal absorption. These results indicate that stimulation of adrenergic receptors may be a novel strategy for intestinal absorption of drugs.     

Isocal and Portagen alter intestinal uptake of sugars but not mucosal surface area in rabbits with an intact intestinal tract and following ileal resection

This study was undertaken to examine the effect of feeding 2 defined-formula diets, Isocal and Portagen, on intestinal morphology and transport function in rabbits subjected to an ileal resection, and in control animals with an intact intestinal tract. Neither ileal resection nor feeding Isocal or Portagen significantly altered jejunal mucosal surface area. In animals with an intact intestinal tract, feeding Isocal was associated with decreased jejunal uptake of 2 mM glucose, whereas both Isocal and Portagen were associated with increased jejunal uptake of 40 mM glucose, and increased ileal uptake of 2 mM glucose, as compared with rabbits fed chow. In animals with an ileal resection, feeding Isocal was associated with increased jejunal and colonic uptake of galactose, whereas jejunal uptake of glucose was unaffected. Thus, (1) feeding defined-formula diets alters the intestinal active uptake of sugars without altering intestinal mucosal surface area; (2) sugar uptake rates were generally higher in animals fed Isocal than in those fed Portagen; (3) ileal resection obscured the differences in the jejunal uptake of higher concentrations of glucose observed in control rabbits (with an intact intestinal tract) fed these defined-formula diets; and (4) the differences in the effects of defined-formula diets on sugar uptake in control and resected animals was likely due to a direct effect on the glucose and galactose carrier(s), rather than to an indirect effect on food consumption, body weight gain, intestinal morphology, or the effective resistance of the intestinal unstirred water layer. It is suggested that the functional differences observed between animals fed Isocal versus Portagen may be due to the varying source of lipids and proteins in these diets, their different ratios of saturated-to-polyunsaturated fatty acids, and their different percent content of carbohydrate.     

Ethyl pyruvate ameliorates intestinal epithelial barrier dysfunction in endotoxemic mice and immunostimulated caco-2 enterocytic monolayers

Ethyl pyruvate (EP) solution ameliorates ileal mucosal hyperpermeability and decreases the expression of several proinflammatory genes in ileal and/or colonic mucosa when it is used instead of Ringer's lactate solution (RLS) to resuscitate mice from hemorrhagic shock. To test the hypothesis that EP can ameliorate gut barrier dysfunction induced by other forms of inflammation, we incubated Caco-2 monolayers for 24 to 48 h with cytomix (a mixture of interferon-gamma, tumor necrosis factor-alpha, and interleukin-1beta) in the presence or absence of graded concentrations of EP or sodium pyruvate. Cytomix increased the permeability of Caco-2 monolayers to fluorescein isothiocyanate-labeled dextran (FD4; average molecular mass 4 kDa), but this effect was inhibited by adding 0.1 to 10 mM EP (but not similar concentrations of sodium pyruvate) to the culture medium. EP inhibited several other cytomix-induced phenomena, including nuclear factor-kappaB activation, inducible nitric oxide synthase mRNA expression, and nitric oxide production. Cytomix altered the expression and localization of the tight junctional proteins, ZO-1 and occludin, but this effect was prevented by EP. Delayed treatment with EP solution instead of RLS ameliorated ileal mucosal hyperpermeability to FD4 and bacterial translocation to mesenteric lymph nodes in mice challenged with lipopolysaccharide (LPS). These data support the view that EP ameliorates cytokine- and/or LPS-induced derangements in intestinal epithelial barrier function.     

An in vitro evaluation of metabolism and poor membrane permeation impeding intestinal absorption of leucine enkephalin, and methods to increase absorption

The goals of this study were to evaluate how metabolism and poor membrane permeability act as barriers to absorption of a model peptide, leucine enkephalin (YGGFL), and how those barriers can be overcome. The in vitro everted rat intestine method was used. YGGFL was rapidly metabolized when exposed to the mucosa of the jejunum, and destyrosyl leucine enkephalin was formed. Metabolism was slower for ileal intestinal segments, however, and was almost absent when colonic segments were used. The aminopeptidase inhibitor boroleucine, an aminoboronic acid derivative, reduced the YGGFL metabolism rate 2-fold at 1/100th the concentration of substrate when the intestine was simultaneously exposed to substrate and inhibitor. Pretreatment of the intestine with boroleucine further reduced the metabolism rate. Thiorphan, an inhibitor of endopeptidase 24.11, had additive inhibitory effects with boroleucine. Inhibition of metabolism alone did not enable YGGFL to permeate the membrane. The permeability enhancer, sodium glycocholate, also did not alone enable membrane permeation. Substantial YGGFL permeated the membrane only when metabolism was inhibited and permeability was enhanced. EDTA had both these effects.     

Nitric oxide mediates lung vascular permeability and lymph-borne IL-6 after an intestinal ischemic insult

Acute lung injury following intestinal I/R depends on neutrophil-endothelial cell interactions and on cytokines drained from the gut through the lymph. Among the mediators generated during I/R, increased serum levels of IL-6 and NO are also found and might be involved in acute lung injury. Once intestinal ischemia itself may be a factor of tissue injury, in this study, we investigated the presence of IL-6 in lymph after intestinal ischemia and its effects on human umbilical vein endothelial cells (HUVECs) detachment. The involvement of NO on the increase of lung and intestinal microvascular permeability and the lymph effects on HUVEC detachment were also studied. Upon anesthesia, male Wistar rats were subjected to occlusion of the superior mesenteric artery during 45 min, followed by 2-h intestinal reperfusion. Rats were treated with the nonselective NO synthase (NOS) inhibitor L-NAME (N(omega)-nitro-L-arginine methyl ester) or with the selective inhibitor of iNOS aminoguanidine 1 h before superior mesenteric artery occlusion. Whereas treatment with L-NAME during ischemia increased both IL-6 levels in lymph and lung microvascular permeability, aminoguanidine restored the augmented intestinal plasma extravasation due to ischemia and did not induce IL-6 in lymph. On the other hand, IL-6 and lymph of intestinal I/R detached the HUVECs, whereas lymph of ischemic rats upon L-NAME treatment when incubated with anti-IL-6 prevented HUVEC detachment. It is shown that the intestinal ischemia itself is sufficient to increase intestinal microvascular permeability with involvement of iNOS activation. Intestinal ischemia and absence of constitutive NOS activity leading to additional intestinal stress both cause release of IL-6 and increase of lung microvascular permeability. Because anti-IL-6 prevented the endothelial cell injury caused by lymph at the ischemia period, the lymph-borne IL-6 might be involved with endothelial cell activation. At the reperfusion period, this cytokine does not seem to be modulated by NO.     

Interactions of the dipeptide ester prodrugs of acyclovir with the intestinal oligopeptide transporter: competitive inhibition of glycylsarcosine transport in human intestinal cell line-Caco-2

The oligopeptide transporter may be exploited to enhance the absorption of drugs by synthesizing their dipeptide ester prodrugs, which may be recognized as its substrates. Various dipeptide esters of acyclovir (ACV), an antiviral nucleoside analog, were synthesized. Enzymatic hydrolysis and affinity of the prodrugs toward the human intestinal peptide transporter hPEPT1 were studied using the human intestinal Caco-2 cell line. Affinity studies were performed by inhibiting the uptake of [(3)H]glycylsarcosine by the prodrugs. The uptake of glycylsarcosine was found to be saturable at higher concentrations and was competitively inhibited by the prodrugs of ACV. All prodrugs except Tyr-Gly-ACV demonstrated a higher affinity (1.41-4.96 mM) toward hPEPT1 than cephalexin (8.19 +/- 2.12 mM), which was used as a positive control. Two prodrugs, Gly-Val-ACV and Val-Val-ACV, showed comparable affinity to Val-ACV, an amino acid prodrug of ACV recognized by PEPT1/PEPT2. The permeability of Gly-Val-ACV (2.99 +/- 0.59 x 10(-6) cm/s) across Caco-2 was comparable with that of Val-ACV (3.01 +/- 0.21 x 10(-6) cm/s) and was significantly inhibited (63%) in presence of glycylsarcosine. The transport of GVACV across Caco-2 was saturable at higher concentrations, and the parameters were calculated as K(m) 3.16 +/- 0.31 mM and V(max) 0.014 +/- 0.00058 nmol cm(-2) min(-1). Overall, the results suggest that the dipeptide prodrugs of ACV have a high affinity toward the intestinal oligopeptide transporter hPEPT1 and therefore seem to be promising candidates in the treatment of ocular and oral herpesvirus infections, because cornea and intestinal epithelia seem to express the oligopeptide transporters.     

Nitric oxide and its metabolites mediate ethanol-induced microtubule disruption and intestinal barrier dysfunction

Loss of gastrointestinal (GI) barrier integrity has been implicated in a wide range of inflammatory illnesses, including alcoholic cirrhosis. Using monolayers of Caco-2 (intestinal) cells as a model, we showed that the ability of ethanol (EtOH) to disrupt intestinal barrier integrity depends on damage to the microtubule (MT) cytoskeleton, especially oxidative injury. One drug that prevented both the MT damage and barrier disruption was L-N(6)-1-iminoethyl-lysine, a selective inhibitor of the inducible form of nitric-oxide synthase (iNOS). Because of this finding and because overproduction of nitric oxide (NO) and generation of peroxynitrite (ONOO(-)) have been proposed to be responsible for mucosal injury in other GI disorders, we sought to determine whether NO overproduction and ONOO(-) formation mediates EtOH-induced MT damage and loss of intestinal barrier function. To this end, Caco-2 monolayers were exposed to EtOH or to authentic ONOO(-) or ONOO(-) generators with or without pretreatment with iNOS inhibitors or antioxidants. We found that EtOH caused 1) iNOS activation, 2) NO overproduction, 3) increases in oxidative stress and superoxide anion production (superoxide dismutase quenchable fluorescence of dichlorofluorescein), 4) nitration and oxidation of tubulin (immunoblotting), 5) decreased levels of stable polymerized tubulin, and 6) increased levels of disassembled tubulin. EtOH also 7) extensively damaged the MT cytoskeleton and 8) disrupted barrier function. Authentic ONOO(-) or ONOO(-) donors had similar effects. Pretreatment with a selective iNOS inhibitor, L-N(6)-1-iminoethyl-lysine, or with antioxidants (ONOO(-) scavengers urate or L-cysteine; superoxide anion scavenger superoxide dismutase) attenuated damage due to EtOH or to ONOO(-) generators. We conclude that EtOH-induced MT damage and intestinal barrier dysfunction require iNOS activation followed by NO overproduction and ONOO(-) formation. These findings provide a rationale for the development of novel therapeutic agents for alcohol-induced GI disorders that inhibit this mechanism.     

Conjugated primary bile salts reduce permeability of endotoxin through intestinal epithelial cells and synergize with phosphatidylcholine in suppression of inflammatory cytokine production

OBJECTIVE: Endotoxemia was shown to be integral in the pathophysiology of obstructive jaundice. In the current study, the role of conjugated primary bile salts (CPBS) and phosphatidylcholine on the permeability of endotoxin through a layer of intestinal epithelial cells and the consequent activation of basolaterally cocultured human mononuclear leukocytes were measured. DESIGN: In a coculture model, a layer of differentiated, confluent Caco-2 cells was apically stimulated with growth-arrested, nonpathogenic Escherichia coli. SETTING: Basic human cell culture laboratory. INTERVENTIONS: The effect of CPBS (0.5 mM and 1.5 mM), phosphatidylcholine (0.38 mM), and human bile (0.5% vol/vol) on the barrier function was assessed by the measurement of transepithelial electrical resistance, by endotoxin permeability through the intestinal epithelial cell layer, and by basolateral cytokine enzyme-linked immunosorbent assay measurement (tumor necrosis factor-[alpha], interleukins-6, -8, and -10). Micelles formed by CPBS were detected by dynamic light scattering. The association of endotoxin with CPBS micelles was tested by fluorescence resonance energy transfer. MEASUREMENTS AND MAIN RESULTS: Apical addition of CPBS suppressed the permeability of endotoxins through the intestinal epithelial cell layer significantly. In parallel, apical supplementation of CPBS dose-dependently reduced the basolateral production of all cytokines measured. Apical phosphatidylcholine supplementation enhanced this effect significantly. CPBS formed micelles (diameter, 134 +/- 7 nm), which were able to bind endotoxin to their surface. CONCLUSIONS: CPBS can reduce the permeation of endotoxin through intestinal epithelial cell layers by binding it to micelles. Thereby, the inflammatory processes beyond the mucosal surface are suppressed, an effect that is enhanced by phosphatidylcholine.     

In vitro behavior of human intestinal mucosa. The influence of acetyl choline on ion transport.

The possibility that the autonomic nervous system may influence the function of intestinal mucosa was investigated by assessing the effect of acetyl choline on ion transport in human intestine. Isolated pieces of stripped ileal mucosa were mounted in Perspex flux-chambers and bathed in isotonic glucose Ringer's solution. Acetyl choline caused a rise in mean potential difference (8.8-12.3 mV, P less than 0.002) and short circuit current (287.7-417.2 muA-cm-2, P less than 0.01) (n = 12), observable at a concentration of 0.01 mM and maximal at 0.1 mM. This effect was enhanced by neostigmine and blocked by atropine. Isotopic flux determinations revealed a change from a small mean net Cl absorption (58) to a net Cl secretion (-4.3mueq-cm-2-h-1P less than 0.001) due predominantly to an increase in the serosal to mucosal unidirectional flux of Cl (10.63-14.35 mueq-cm-2-h-1P less than 0.05) and a smaller reduction in the mucosal to serosal flux (11.22 to 10.02 mueq-cm-2-h-1P less than 0.05). Unidirectional and net Na transport was unaffected. A similar electrical and ion transport response was observed in a single study of two pieces of jejunal mucosa. In the absence of glucose net chloride secretion was produced and again an insignificant effect on net sodium transport was noted. Acetyl choline did not provoke a sustained effect on mucosal cyclic adenine nucleotide levels although a short-lived cyclic adenine nucleotide response was seen in some tissues 20-30 s after drug addition. These studies demonstrate that acetyl choline does influence human intestinal ion transport by stimulating chloride secretion and suggest a possible mechanism by which the parasympathetic nervous system could be concerned in the control of ion transport.     

Decreased permeation of cephalosporins through the outer membrane of Escherichia coli grown in salicylates.

Escherichia coli K-12 cells grown in 1 to 5 mM sodium salicylate (SAL) or acetylsalicylate show increased phenotypic resistance to various antibiotics (J. L. Rosner, Proc. Natl. Acad. Sci. USA 82:8771-8774, 1985), including cephalosporins (this study). To determine whether these effects are caused by a decreased uptake of the antibiotics, the permeation of several cephalosporins through the outer membrane was measured. For E. coli K-12 grown in LB broth containing 5 mM SAL or acetylsalicylate, permeation of the outer membrane by the five cephalosporins tested decreased three- to fivefold compared with that in cells not grown in salicylates. Permeation of the outer membrane by cephaloridine decreased within 15 min of the addition of SAL to cells grown in broth and reached a minimum in 1 to 2 h. When cells were transferred from broth with SAL to broth without SAL, their permeability to cephaloridine increased slowly for the first 45 min and more rapidly over the next 1.5 h; the permeability then attained normal levels by 3 h. The permeability changes that occurred after media shifts, either to or from SAL, were prevented by concentrations of chloramphenicol that inhibited protein synthesis. These effects of SAL on outer membrane permeability are fully consistent with their effects on antibiotic resistance and with the report (T. Sawai, S. Hirano, and A. Yamaguchi, FEMS Microbiol. Lett. 40:233-237, 1987) that the outer membranes of SAL-treated cells are deficient in certain porins. Permeation of cephaloridine through the outer membrane also decreased when a virulent strain of E. coli K1 was grown in the presence of as little as 1 to 2 mM SAL. This raises the concern that high levels of salicylates in patients night interfere with cephalosporin or other antibiotic therapies.