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Biomedical subjects

K E Barrett

Publications and source records attributed to K E Barrett.

At least 19 recordsLinked to original sources
PubMed1995

Histamine inhibits prostaglandin E2-stimulated rabbit duodenal bicarbonate secretion via H2 receptors and enteric nerves.

BACKGROUND/AIMS: The gastroduodenal epithelium is protected from acid peptic damage by an adherent mucus-bicarbonate layer. Bicarbonate is secreted by the surface epithelial cells into this mucus layer. Patients with duodenal ulcer disease have impaired proximal duodenal bicarbonate secretion. Mast cells, present in large numbers in the duodenal mucosa, release a number of inflammatory mediators, including histamine. Release of such mast cell mediators has been implicated in ulcer disease. In this study, the ability of histamine to regulate bicarbonate secretion was examined. METHODS: Bicarbonate secretion by rabbit proximal duodenal mucosa was examined in vitro, and the effects of histamine, its agonists, and its antagonists were studied. RESULTS: Histamine essentially eliminated prostaglandin E2-stimulated duodenal mucosal bicarbonate secretion, an effect reversed both by the neurotoxin, tetrodotoxin, and the histamine H2-receptor antagonist, cimetidine, as well as reproduced by the H2-receptor agonist, dimaprit. CONCLUSIONS: In addition to the stimulatory action of histamine on gastric acid secretion, histamine expresses an additional antidefensive action by inhibiting prostaglandin E2-stimulated duodenal epithelial bicarbonate secretion. This effect of histamine is likely mediated via H2 receptors located on enteric nerves.

Animals↗
PubMed1995

Effect of the diglyceride lipase inhibitor, RG80267, on epithelial chloride secretion induced by various agents.

We previously reported that a diglyceride lipase inhibitor, RG80267, inhibits chloride secretion stimulated by adenosine agonists, stimuli whose effects appear unrelated to cAMP, cGMP or cytosolic calcium. Here, the effect of RG80267 on Cl- secretory responses to agents which do utilize these messengers was examined. RG80267 inhibited responses to vasoactive intestinal polypeptide, forskolin (cAMP-dependent) and E. coli heat stable enterotoxin (cGMP-dependent), but not to prostaglandin E1 or cholera toxin (cAMP-dependent). RG80267 enhanced responses to histamine (calcium-dependent). The inhibitory effect of RG80267 was not due to inhibition of cAMP accumulation. Arachidonic acid release may participate in chloride secretion. Vasoactive intestinal polypeptide, but not prostaglandin E1, released radiolabel from cells preloaded with [3H]arachidonic acid. There may thus be differences between mechanisms of various cyclic nucleotide-dependent chloride secretory responses. Arachidonic acid release may modulate the extent of secretion elicited by some secretagogues.

Alprostadil↗
PubMed1995

Mast cells and histamine contribute to bile acid-stimulated secretion in the mouse colon.

Certain dihydroxy bile acids cause secretory diarrhea when present in the colonic lumen at inappropriately high concentrations. However, the mechanism underlying the secretagogue activity has not been fully elucidated. Experiments were performed to test whether mast cells and one of their major mediators, histamine, might contribute to the secretory effect. Chenodeoxycholic acid, a secretory bile acid, and ursodeoxycholic acid, a nonsecretory, hydrophilic bile acid, were compared for their ability to induce chloride secretion across segments of mouse colon mounted in Ussing chambers. Chenodeoxycholic acid, but not ursodeoxycholic acid, induced dose-dependent, biphasic chloride secretion that was greater after serosal than mucosal addition and was greater in distal versus proximal colonic segments. The secretory effect of chenodeoxycholic acid was inhibited by H1 histamine receptor antagonists and modified by the cyclooxygenase inhibitor indomethacin. However, it was unaffected by an H2 histamine receptor antagonist or by atropine. Secretory effects of chenodeoxycholic acid were diminished in magnitude and delayed in colonic tissues from mice with a genetic deficiency of tissue mast cells. Concentrations of chenodeoxycholic acid inducing secretion also released histamine from tissue segments. These data indicate that mast cells and histamine-mediated processes contribute significantly to the secretory effects of dihydroxy bile acids in the murine colon.

Animals↗
PubMed1995

Cross-talk between calcium and cAMP-dependent intracellular signaling pathways. Implications for synergistic secretion in T84 colonic epithelial cells and rat pancreatic acinar cells.

Treatment of various cells with combinations of agents that increase either cAMP or cytosolic calcium can lead to synergistic responses. This study examined interactions, or cross-talk, between these two intracellular messengers and its implication for signaling in two secretory cell types, T84 human colonic epithelial cells and rat pancreatic acinar cells. T84 cell chloride secretion was measured in Ussing chambers. Acinar cell activation was monitored as amylase secretion. Cytosolic calcium was assessed via fura-2 microfluorimetry. A cell-permeant analogue of cAMP synergistically enhanced secretory responses to calcium-mobilizing hormones in both cell types, but paradoxically reduced overall calcium mobilization. The reduction in calcium mobilization could be attributed to an inhibition of calcium influx in T84 cells, although a different mechanism likely operates in acinar cells. The effects of the cAMP analogue were reproduced by other agents that increase cAMP. Furthermore, econazole, an inhibitor of calcium influx, potentiated secretory responses to calcium-dependent stimulation in T84 cells without itself inducing secretion. We conclude that there is cross-talk between calcium and cAMP-dependent signaling pathways at the level of second messenger generation in two secretory cell types. This cross-talk appears to regulate the extent of secretory responses.

Amylases↗
PubMed1994

Long-term uncoupling of chloride secretion from intracellular calcium levels by Ins(3,4,5,6)P4.

Osmoregulation, inhibitory neurotransmission and pH balance depend on chloride ion (Cl-) flux. In intestinal epithelial cells, apical Cl- channels control salt and fluid secretion and are, in turn, regulated by agonists acting through cyclic nucleotides and internal calcium ion concentration ([Ca2+]i). Recently, we found that muscarinic pretreatment prevents [Ca2+]i increases from eliciting Cl- secretion in T84 colonic epithelial cells. By studying concomitant inositol phosphate metabolism, we have now identified D-myo-inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P4), as the inositol phosphate most likely to mediate this uncoupling. A novel, membrane-permeant ester prepared by total synthesis delivers Ins(3,4,5,6)P4 intracellularly and confirms that this emerging messenger does inhibit Cl- flux resulting from thapsigargin- or histamine-induced [Ca2+]i elevations.

Atropine↗
PubMed1994

Inhibition of human peripheral blood lymphocyte function by protoporphyrin and longwave ultraviolet light.

Modulation of immunologic effector cells by exogenous photoactive substances has been advanced as an underlying mechanism for the efficacy of various photochemotherapeutic regimens. It is also possible that endogenous photosensitizers, such as protoporphyrin, could similarly modify the function of immune cell types. We examined the effects of protoporphyrin plus longwave UV light on the ability of human PBL to proliferate in response to mitogens. Noncytotoxic dosages of protoporphyrin plus UV light suppressed PHA-stimulated proliferation of both PBMC and enriched T cells. CD8+ cells were more sensitive to this inhibitory effect than CD4+ cells. The inhibitory effect was also observed when proliferation was induced by the combination of a phorbol ester and ionomycin. Inhibition of PBMC proliferation was associated with inhibition of IL-2 secretion but proliferation was not restored with exogenous IL-2. Instead, the effect of protoporphyrin plus UV light may be on IL-2R. Cells treated with protoporphyrin and UV light did not display the increase in CD25 and the beta-chain of the IL-2R induced by PHA in control cells. In contrast to the effects of protoporphyrin and UV light on IL-2 and IL-2R alpha-chain protein expression, the accumulation of mRNA for these proteins induced by PHA was unaffected. None of the effects of protoporphyrin plus UV light on lymphocytes were observed in control experiments where cells were treated with either protoporphyrin or UV light alone. We conclude that biologically relevant dosages of protoporphyrin and UV light modify the function of circulating lymphocytes.

Base Sequence↗
PubMed1994

Enhanced tissue responsiveness in colonic ion transport of cow's milk-sensitized guinea pigs.

The effects of leukotriene D4 (LTD4) on ion transport were investigated in submucosa/mucosa colonic segments from guinea pigs sensitized to cow's milk and in age-matched, non-immune animals. Mediators released from mast cells in immune animals challenged with beta-lactoglobulin evoked an increase in short-circuit current that was reduced by SK&F 102922, a peptidoleukotriene antagonist. Serosal addition of LTD4 (0.15-1 microM) evoked a concentration-dependent, bumetanide-sensitive increase in short-circuit current which was greater in immune than non-immune controls. In the absence of ongoing neural activity, 1 microM LTD4 evoked an 8-20 microA/cm2 increase in short-circuit current which was increased 8-13-fold when ongoing neural activity was present. In tissues with ongoing activity, the response to 0.15 microM LTD4 was reduced by SK&F 102922, tetrodotoxin and atropine. LTD4 enhanced the responsiveness of the tissue to carbachol by a factor of two, but did not affect responses of T84 colonic epithelial cell monolayers to this agent. These results show enhanced secretory function for LTD4 in animals with allergy to cow's milk. They suggest that the level of ongoing neural activity in the enteric neural microcircuits is one of the major determinants of colonic secretory capacity.

Animals↗
PubMed1994

Mast cells are not essential to inflammation in murine model of colitis.

A simple rat model of chronic intestinal inflammation was adapted to mice in order to ascertain whether mast cells play an essential role in its induction or perpetuation. Colitis was induced in C57BL mice by intrarectal administration of trinitrobenzene sulfonic acid in 50% ethanol. Higher doses of trinitrobenzene sulfonic acid per gram of body weight were required in mice than rats, with a narrower effective dose range (the upper dose limited by unacceptable mortality and the lower by decreased inflammation). Colons of treated mice were macroscopically inflamed, with transmural damage, adhesions to adjacent structures, and ulcerations. Inflammation was scored subjectively and by tissue weight and myeloperoxidase content; each index was increased dose-dependently by trinitrobenzene sulfonic acid doses of 0.3-10 mg. Six milligrams of trinitrobenzene sulfonic acid induced reproducible inflammation for up to four weeks. Trinitrobenzene sulfonic acid could induce inflammation in both mast-cell-deficient W/Wv mice and their normal +/+ littermates in a similar fashion. Thus it is possible to induce chronic colitis in the mouse. Mast cells are not essential participants in this process.

Animals↗
PubMed1994

Inhibition of mast cell mediator secretion induced by protoporphyrin plus long-wave ultraviolet light: a morphometric and ultrastructural analysis.

It has been shown that treatment of rat peritoneal mast cells with protoporphyrin plus long-wave ultraviolet light (UVA) irradiation can suppress mediator secretion. In this study we conducted a morphometric and ultrastructural analysis of rat peritoneal mast cells to investigate possible alterations produced by this treatment before or after stimulation with calcium ionophore. Protoporphyrin plus UVA, at doses causing inhibition of mediator release, had no effect on either cell size or viability but increased cellular sphericity. There was a 43% reduction of the cell surface area, and qualitative inspection of the cells revealed that this change was associated with a reduction in microfolds on the cell surface. After 1 minute of incubation with calcium ionophore A23187 (1 mumol/L), both cells that were pretreated with protoporphyrin plus UVA and control cells showed dramatic changes in granule structure. Although treated cells had an unchanged tendency to have granules that closely approached the plasma membrane, there was an inhibition of granule extrusion in response to ionophore stimulation. These observations may be relevant to the inhibitory effect of protoporphyrin plus UVA on the generation and release of mast cell mediators.

Animals↗
PubMed1994

Protein kinase C activity does not mediate the inhibitory effect of carbachol on chloride secretion by T84 cells.

Carbachol induces calcium-dependent chloride secretion and activates protein kinase C in T84 cells. However, prolonged stimulation with carbachol or direct activation of protein kinase C inhibits subsequent calcium-dependent chloride secretion. Furthermore, the ability of carbachol to elevate inositol tetrakisphosphate levels may be linked to inhibition of chloride secretion. Here we demonstrate that protein kinase C activation increases levels of inositol tetrakisphosphates (1,3,4,6- and 3,4,5,6-isomers) in T84 colonic epithelia. Furthermore, this corresponds to an inhibition of chloride secretion. However, protein kinase C is unlikely to mediate the analogous effects of carbachol. Neither the ability of carbachol to inhibit calcium-dependent chloride secretion nor its effects on inositol 3,4,5,6-tetrakisphosphate levels were reversed by staurosporine. Carbachol also has quantitatively and qualitatively different effects on inositol tetrakisphosphate isomers than protein kinase C activators. Thus protein kinase C activity can increase levels of various inositol tetrakisphosphate isomers within T84 cells but does not mediate carbachol-induced increases in these putative messengers. These data further support the hypothesis that inositol 3,4,5,6-tetrakisphosphate is a negative second messenger, uncoupling epithelial chloride secretion from changes in intracellular calcium.

Alkaloids↗
PubMed1994

Membrane-permeant derivatives of cyclic AMP optimized for high potency, prolonged activity, or rapid reversibility.

A novel membrane-permeant derivative of cAMP, cAMP acetoxymethyl ester (cAMP/AM), was synthesized via silylated intermediates. Its ability to induce Cl- secretion by T84 cells, a human colon cancer cell line, was compared with that of two other membrane-permeant cAMP derivatives that were recently introduced, N6,O2'-dibutyryl-cAMP acetoxymethyl ester (bt2cAMP/AM) and Sp-5,6-dichlorobenzimidazole-1-beta-D-ribofuranoside 3',5'-cyclic phosphorothioate (Sp-5,6-DCl-cBIMPS). All of these compounds are powerful activators of Cl- secretion when applied extracellularly, with EC50 values of 60 microM, 0.7 microM, and 3 microM, respectively. However, cAMP/AM was expected to be readily degraded inside cells, in contrast to the cyclophosphodiesterase-resistant Sp-5,6-DCI-cBIMPS or the only slowly metabolizable N6-butyryl-cAMP derived from bt2cAMP/AM. Reversibility of cAMP/AM action was demonstrated by wash-out experiments; Cl- secretion induced by high doses of cAMP/AM (100 microM) could be quickly abolished by rinsing of the cells, whereas similar experiments with bt2cAMP/AM and Sp-5,6-DCI-cBIMPS showed much slower decreases. Even more sensitive to residual cAMP derivatives was the synergistic effect of carbachol, which was applied after the incubation with membrane-permeant derivatives and their subsequent wash-out. Although doses of cAMP derivatives that barely activated Cl- secretion were readily capable of inducing a synergistic response with carbachol, cells incubated with high doses of cAMP/AM (100 microM) and subsequently washed showed only a nonsynergistic carbachol response, in contrast to cells incubated with bt2cAMP/AM or Sp-5,6-DCI-cBIMPS. We therefore characterize cAMP/AM as a membrane-permeant derivative of cAMP that is easily metabolizable inside cells and hence is most useful for applications where a transient intracellular cAMP signal is desired. In contrast, completely nonmetabolizable Sp-5,6-DCI-cBIMPS seems to be more useful in longer incubations that require steady levels of cAMP-dependent protein kinase activation. bt2cAMP/AM combines the advantages of intracellular trapping by ester hydrolysis and reduced cyclophosphodiesterase sensitivity of its active intracellular product, which probably lead to its particularly high potency.

Cell Membrane↗
PubMed1993

Alterations in Fc epsilon RI induced by protoporphyrin plus long-wavelength ultraviolet light in mouse bone marrow-derived mast cells.

As previously reported, protoporphyrin plus long-wavelength UV light (PP/UVA) inhibits IgE-mediated degranulation of mouse bone marrow-derived mast cells, as assessed by measurement of the release of beta-hexosaminidase. This inhibitory effect was seen with cells sensitized with IgE either before or after PP/UVA treatment (57.8 and 55.3% inhibition, respectively). PP/UVA did not dissociate IgE already bound to cells as assessed either by measuring release of bound 125I-IgE or by flow cytometric analysis. Results from immunoadsorption followed by SDS-PAGE analysis suggested that PP/UVA treatment may cause stable conjugation of IgE to its receptor. In unsensitized cells, PP/UVA did not cause conjugation of the unoccupied Fc epsilon RI to other proteins in the plasma membrane. Nevertheless, Scatchard analysis revealed that PP/UVA decreased the number of Fc epsilon RI per cell by 37% (0.95 x 10(5) vs 1.51 x 10(5)/cell), whereas affinity of the receptor for IgE was comparable between PP/UVA-treated and untreated cells (3.40 nM vs 3.27 nM). Flow cytometric analysis also confirmed the decrease in Fc epsilon RI number in PP/UVA-treated unsensitized mouse bone marrow-derived mast cells. Although 84% of PP/UVA-treated and 82% of untreated cells expressed positive fluorescence when stained with FITC-conjugated IgE, fluorescence intensity was reduced by 40% after PP/UVA treatment. We conclude that PP/UVA alters the conformational structure and/or number of Fc epsilon RI expressed on the mast cell surface. This effect could potentially explain the ability of PP/UVA to inhibit mast cell secretory function and may be related to an ability of PP/UVA to alter the properties of the plasma membrane.

Animals↗
PubMed1993

Acetoxymethyl esters of phosphates, enhancement of the permeability and potency of cAMP.

Acetoxymethyl esters of alkyl or aryl phosphates can be prepared by reacting their trialkylammonium or silver salts with acetoxymethyl bromide. Because acetoxymethyl esters are rapidly cleaved intracellularly, they facilitate the delivery of organophosphates into the cytoplasm without puncturing or disruption of the plasma membrane. In addition, acylation of free hydroxyls, for example with butyryl groups, is useful both for synthetic convenience and increased hydrophobicity of the permeant derivatives. The highly polar intracellular messengers cAMP and cGMP were thus converted into uncharged membrane-permeant derivatives. Extracellularly applied N6,2'-O-dibutyryl cAMP acetoxymethyl ester (Bt2cAMP/AM) is shown to simulate intracellular cAMP in three model systems, namely dissociation of cAMP-dependent protein kinase in fibroblasts, activation of Cl- secretion of monolayers of the human colon epithelial cell line T84, and dispersion of pigment granules in angel fish melanophores. Bt2cAMP/AM is effective at concentrations two or three orders of magnitude less than those required for commonly used membrane-permeant cAMP derivatives such as Bt2cAMP, 8-Br-cAMP, and 8-pCPT-cAMP lacking the acetoxymethyl ester. This methodology should be of general utility for the intracellular delivery of phosphate-containing second messengers.

Animals↗