Bradykinin-stimulated electrolyte secretion in rabbit and guinea pig intestine. Involvement of arachidonic acid metabolites

Neurophysiological mechanisms of bradykinin-evoked mucosal chloride secretion in guinea pig small intestine

AIM: To investigate the mechanism for bradykinin (BK) to stimulate intestinal secretomotor neurons and intestinal chloride secretion.

METHODS: Muscle-stripped guinea pig ileal preparations were mounted in Ussing flux chambers for the recording of short-circuit current (Isc). Basal Isc and Isc stimulated by BK when preincubated with the BK receptors antagonist and other chemicals were recorded using the Ussing chamber system. Prostaglandin E2 (PGE₂) production in the intestine was determined by enzyme immunologic assay (EIA).

RESULTS: Application of BK or B2 receptor (B2R) agonist significantly increased the baseline Isc compared to the control. B2R antagonist, tetrodotoxin and scopolamine (blockade of muscarinic receptors) significantly suppressed the increase in Isc evoked by BK. The BK-evoked Isc was suppressed by cyclooxygenase (COX)-1 or COX-2 specific inhibitor as well as nonselective COX inhibitors. Preincubation of submucosa/mucosa preparations with BK for 10 min significantly increased PGE₂ production and this was abolished by the COX-1 and COX-2 inhibitors. The BK-evoked Isc was suppressed by nonselective EP receptors and EP4 receptor antagonists, but selective EP1 receptor antagonist did not have a significant effect on the BK-evoked Isc. Inhibitors of PLC, PKC, calmodulin or CaMKII failed to suppress BK-induced PGE₂ production.

CONCLUSION: The results suggest that BK stimulates neurogenic chloride secretion in the guinea pig ileum by activating B2R, through COX increasing PGE₂ production. The post-receptor transduction cascade includes activation of PLC, PKC, CaMK, IP₃ and MAPK.

Prostaglandins, not the leukotrienes, regulate Cl(-)/HCO(3)(-) exchange (DRA, SLC26A3) in villus cells in the chronically inflamed rabbit ileum

Previously studies have demonstrated that Cl(-)/HCO(3)(-) exchange was inhibited during chronic intestinal inflammation secondary to decrease in the affinity of the exchanger for Cl(-) rather than the number of transporters. Arachidonic acid metabolites (AAM) are elevated in the mucosa of the chronically inflamed small intestine. However, their role in the alteration of Cl(-)/HCO(3)(-) during chronic enteritis was unknown. Inhibition of AAM formation with arachidonyl trifluoro methylketone (ATMK) in chronically inflamed rabbit intestine reversed the diminished Cl(-)/HCO(3)(-) exchange activity. Kinetics studies showed that the reversal was secondary to restoration of the altered affinity of transporter. Downstream regulation of Cl(-)/HCO(3)(-) inhibition by AAM was determined to be by the cyclooxygenase pathway since only inhibition of cyclooxygenase with piroxicam treatment reversed the inhibited Cl(-)/HCO(3)(-) exchange. Further, DRA was shown to be the primary Cl(-)/HCO(3)(-) exchanger in villus cells. Kinetics and molecular studies indicated that the mechanism of inhibition of Cl(-)/HCO(3)(-) exchange by cyclooxygenase pathway metabolites was secondary to diminished affinity of the transporter for Cl(-) without a change in DRA BBM expression. Thus our data indicated that cyclooxygenase pathway metabolites mediate the inhibition of DRA during chronic intestinal inflammation.

Cysteinyl leukotrienes mediate the response of submucosal ganglia from rat colon to bradykinin

The aim of the present study was to find out the mechanism by which the inflammatory mediator, bradykinin, induces an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](i)) in enteric neurons. For this purpose, ganglia in the isolated submucosa from rat colon were loaded with the Ca(2+)-sensitive dye, fura-2, and were exposed to bradykinin (2·10(-8)mol/l). Under control conditions, the kinin evoked a transient increase in [Ca(2+)](i). Preincubation with quinacrine or arachidonyltrifluoromethylketone (AACOCF(3)), i.e. blockers of cytosolic phospholipase A(2), prevented the raise of [Ca(2+)](i). This inhibition was mimicked by 5,8,11,14-eicosatetrayonic acid (ETYA), an inhibitor of cyclooxygenases as well as lipoxygenases, and by BWA4C, a selective inhibitor of lipoxygenases, whereas indomethacin was ineffective, suggesting the mediation of the kinin response by a lipoxygenase metabolite. Indeed, a leukotriene, leukotriene D(4) (LTD(4)), mimicked the effect of bradykinin. The LTD(4) receptor blocker, MK-571, inhibited the increase in [Ca(2+)](i) evoked by LTD(4) and by bradykinin. Consequently, bradykinin receptors in submucosal ganglia from rat colon are coupled to a stimulation of phospholipase A(2), the release of arachidonic acid and the production of LTD(4), which seems to be finally responsible for the change in the cytosolic Ca(2+) concentration.

Activation of TRPA1 by luminal stimuli induces EP4-mediated anion secretion in human and rat colon

In gastrointestinal (GI) physiology, anion and fluid secretion is an important function for host defense and is induced by changes in the luminal environment. The transient receptor potential A1 (TRPA1) channel is considered to be a chemosensor in several sensory tissues. Although the function of TRPA1 has been studied in GI motility, its contribution to the transepithelial ion transport system has rarely been discussed. In the present study, we investigated the secretory effect of the potential TRPA1 agonist allyl isothiocyanate (AITC) in rat and human colon using an Ussing chamber. The mucosal application of AITC (10(-6)-10(-3) M) induced Cl(-) and HCO(3)(-) secretion in a concentration-dependent manner, whereas the serosal application induced a significantly weaker effect. AITC-evoked anion secretion was attenuated by tissue pretreatment with piroxicam and prostaglandin (PG) E(2); however, this secretion was not affected by TTX, atropine, or extracellular Ca(2+) depletion. These experiments indicate that TRPA1 activation induces anion secretion through PG synthesis, independent of neural pathways in the colon. Further analysis also indicates that AITC-evoked anion secretion is mediated mainly by the EP(4) receptor subtype. The magnitude of the secretory response exhibited segmental heterogeneity in rat colon. Real-time PCR analysis showed the segmental difference was corresponding to the differential expression of EP(4) receptor and cyclooxygenase-1 and -2. In addition, RT-PCR, in situ hybridization, and immunohistochemical studies showed TRPA1 expression in the colonic epithelia. Therefore, we conclude that the activation of TRPA1 in colonic epithelial cells is likely involved in the host defense mechanism through rapid anion secretion.

The pharmacological modification of secretory responses

Electrolyte transport across the intestinal mucosa can be modulated by several neurotransmitters, hormones and drugs. Opiate agonists and endogenous opioid peptides inhibit electrolyte secretion both in vitro and in vivo. These drugs appear to act at several levels. Thus, opioid effects can be elicited at the local mucosal level. Secondly, antisecretory effects can be demonstrated when opioids are administered into the brain. These central effects appear to involve activation of the sympathetic innervation of the intestine. Thirdly, some antidiarrhoeal drugs such as loperamide may have ancillary non-opiate-like actions that contribute to their effectiveness. In cases of inflammatory bowel disease where local concentrations of inflammatory mediators such as kinins and eicosanoids may be high, non-steroidal anti-inflammatory drugs may be effective in treating diarrhoeal symptoms. The existence of many types of receptors on mucosal cells indicates that several pharmacological approaches exist for the potential modulation of electrolyte transport.

Muscarinic acetylcholine receptor activation increases transcellular transport of macromolecules across mouse and human intestinal epithelium in vitro

The intestinal epithelium acts as a barrier restricting uptake of luminal macromolecules such as dietary antigens and microbes. Here, we examined the role of cholinergic signalling in the regulation of permeability to macromolecules. Mouse jejunum was mounted in Ussing chambers and permeability was determined by measuring the flux of the antigen-sized protein, horseradish peroxidase (HRP), across the tissue. Baseline HRP permeability was significantly reduced by neural blockade with tetrodotoxin or cholinergic muscarinic antagonism with atropine, suggesting that ongoing release of endogenous acetylcholine from enteric nerves regulates barrier function. Exogenous addition of the muscarinic agonist bethanechol caused significant increases in both HRP flux and the area of HRP-containing endosomes in enterocytes. Bethanechol-enhanced HRP flux was abrogated by the M3 receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), the phospholipase A(2) inhibitor quinacrine, and the cyclooxygenase inhibitor indomethacin. Complementary in vitro studies showed direct effects of bethanechol on T84 epithelial cells, where increased HRP uptake was associated with increased F-actin, and increased cytosolic phospholipase A(2) (cPLA(2)) phosphorylation. Taken together, these results provide evidence for cholinergic regulation of transepithelial transport of macromolecules, mainly mediated by activation of M3 receptors with subsequent involvement of phospholipase A(2) and cyclooxygenase products.

Metabotropic signal transduction for bradykinin in submucosal neurons of guinea pig small intestine

Intracellular recording methods with "sharp" microelectrodes were used to study signal transduction mechanisms underlying the excitatory action of bradykinin (BK) in morphologically identified neurons in the small intestinal submucosal plexus. Exposure to BK evoked slowly activating membrane depolarization and enhanced excitability associated with increased input resistance in AH-type and decreased input resistance in S-type neurons. Preincubation with pertussis toxin did not affect the BK-evoked responses. Pretreatment with the cyclooxygenase inhibitors indomethacin or piroxicam suppressed or abolished the BK-evoked responses. Application of prostaglandin (PG) E(2) or PG analogs evoked BK-like depolarizing responses in the submucosal plexus with a potency order of PGE(2) > PGE(1) > 17-phenyl trinor-PGE(2) > PGI(2) > sulprostone > PGF(2alpha). Depolarizing responses to bradykinin or PGE(2) in S-type neurons were suppressed in the presence of the phospholipase C inhibitor U73122 [(1-6-[([17beta]-3-methoxyestra-1,3,5[10]-tren-17-71)amino]hexyl)-1H-pyrrole-2,5-dione)], but not the inactive analog U73343 [(1-6-[([17beta]-3-methoxyestra-1,3,5[10]trien-17yl)amino]hexyl)-2,5-pyrrolidinedione)]. The inositol-1,4,5-trisphosphate receptor antagonist 2-aminoethoxy-diphenylborane and the calmodulin inhibitor W-7, but not ryanodine, suppressed both bradykinin- and PGE(2)-evoked responses. KN-62, an inhibitor of calmodulin kinases, or GF109203X, a specific protein kinase C inhibitor, suppressed both BK- and PGE(2)-evoked depolarizing responses. Selective protein kinase A inhibitors did not alter BK- or PGE(2)-evoked depolarizing responses in S neurons. The results suggest that BK stimulates synthesis and release of PGE(2), which acts at EP(1) receptors to evoke depolarizing responses in submucosal neurons. The postreceptor transduction cascade includes activation of phospholipase C, inositol-1,4,5-trisphosphate production, intraneuronal Ca2+ mobilization, activation of protein kinase C and/or calmodulin kinases, and phosphorylation of cationic channels.

Impact on the bowel of amtolmetin guacyl, a new gastroprotective non-steroidal anti-inflammatory drug

Amtolmetin guacyl (MED15) is a new non-steroidal anti-inflammatory drug (NSAID) which shares anti-inflammatory, analgesic and antipyretic activity with the other drugs of the NSAID family but which shows, unexpectedly, strong gastroprotective activity similar to misoprostol. This effect has been attributed to the presence in its molecule of a vanillic moiety responsible for stimulation of capsaicin receptors present throughout the length of the gastrointestinal tract. MED15 shows antispasmodic activity in the bowel against a number of agonists and compares favourably with reference compounds. In in vivo indomethacin-induced rat ileitis, MED15 heals better than 5-aminosalicylic acid and sulfasalazine, as well as down-regulating intestinal wall myeloperoxidase content. In acetic acid-induced colitis in the rat, levels of malondialdehyde were found to be more markedly reduced with MED15 than with 5-aminosalicylic acid. In contrast with the effect in the stomach, MED15 protective effect in the bowel appears to be unrelated to nitric oxide (NO) production. The MED15 enteroprotective effect is related to stimulation of intestinal capsaicin receptors as demonstrated by the loss of protective effect in the presence of capsazepine, a specific receptor antagonist of capsaicin. In conclusion, following the favourable results obtained in animal models and notwithstanding the pharmacological effects typical of an NSAID, MED15 may rationally be proposed for the treatment of various human colitis conditions and Crohn's disease.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Comparison of tissue kallikrein and kinin receptor expression in gastric ulcers and neoplasms

Recent studies have suggested the possible involvement of the kallikrein-kinin cascade in gastric inflammatory diseases and malignant transformation in peptic ulcers. Germ-line mutations of genes responsible for repairing DNA mismatch may cause the transformation of chronic peptic ulceration to malignancy. The aim of the study was to compare the expression of the serine protease, tissue kallikrein (TK) and kinin peptide receptors, B1 and B2, in human gastric carcinoma and ulcers. Furthermore, experiments were designed to ask the question whether holistic changes known to occur in carcinoma would be reflected in the expression of the neutrophil kallikrein-kinin cascade. Expression of tissue kallikrein and kinin receptors in both tumour and ulcer tissue and circulating neutrophils of cancer and ulcer patients was determined by immunolabelling techniques, using specific antibodies. The immunolabelled images were visualised by light, confocal and electron microscopy. This is a first study that provides strong evidence for enhanced expression of TK in peptic ulceration and gastric malignancy, suggestive of a crucial role for serine proteases in cancer.

CFTR-mediated inhibition of epithelial Na+ conductance in human colon is defective in cystic fibrosis

Cystic fibrosis (CF) patients show characteristic defects in epithelial ion transport, such as failure in cAMP-dependent Cl- secretion. Because the cystic fibrosis transmembrane conductance regulator (CFTR) also functions as a downregulator of epithelial Na+ channels (ENaC), enhanced Na+ conductance was found in the airways of CF patients. Here, we examined whether enhanced epithelial Na+ conductance is also present in the colonic epithelium of CF patients and examined the underlying mechanisms. Thus transepithelial voltages were measured, and equivalent short-circuit currents (I(sc-eq)) were determined by means of a novel type of Ussing chamber. Non-CF tissues demonstrated cAMP-dependent Cl- secretion that was absent in biopsies of CF patients. Correspondingly, Isc-eq was inhibited in non-CF but not in CF epithelia when synthesis of endogenous prostaglandins was blocked by indomethacin. In the presence of indomethacin, a larger portion of amiloride-sensitive Isc-eq was detected in CF tissues, suggesting enhanced ENaC conductance in colonic mucosa of CF patients. Increase of intracellular cAMP by forskolin and IBMX inhibited amiloride-sensitive ENaC currents in non-CF tissues but not in CF biopsies. Therefore, enhanced epithelial Na+ conductance is present in the CF colon and is probably due to missing downregulation by CFTR.

ACE inhibition by enalaprilate stimulates duodenal mucosal alkaline secretion via a bradykinin pathway in the rat

The effects of enalaprilate on duodenal mucosal alkaline secretion (in situ titration) and mean arterial blood pressure were investigated in chloralose-anesthetized male rats. A bolus injection of enalaprilate (0.7 mg/kg intravenously) increased alkaline secretion by about 60%, and this response was resistant to guanethidine (5 mg/kg intravenously), splanchnicotomy, and vagotomy. Furthermore, angiotensin II infusion (0.25-2.5 microg/kg/hr intravenously) following the administration of enalaprilate failed to influence this response. Bradykinin (10(-6)-10(-4) M) applied topically to the serosal surface of the duodenal segment under study increased dose-dependently the duodenal mucosal alkaline secretion, an effect that could be blocked by the selective bradykinin receptor subtype-2 antagonist HOE140 (100 nmol/kg intravenously). HOE140 also antagonized the response to enalaprilate. These data suggest that enalaprilate increases duodenal mucosal alkaline secretion via a local bradykinin pathway involving receptors of the bradykinin receptor subtype-2 antagonist, rather than by blockade of endogenous angiotensin II or by central autonomic neural regulation.

Distribution of tissue kallikreins in lower vertebrates: potential physiological roles for fish kallikreins

Fish skeletal muscle prokallikrein was purified from black sea bass, Centropritis striata, and used for the production of polyclonal antiserum. Tissue proteins from primitive fish and teleosts, an alligator, and an insectivore were resolved by sodium dodecylsulfate-polyacrylamide gel electrophoresis, Western blotted, and probed with fish muscle prokallikrein antiserum. A recurring theme was the presence of approximately 36 and 72 kDa kallikrein-like proteins in skeletal muscle, heart, gill, kidney, and spleen of higher teleosts and in selected tissues of sturgeon, shark, alligator, and mole. The presence of immunoreactive kallikreins in osmoregulatory organs such as the gills of teleosts and the rectal gland of sharks signifies a potential role for these proteins in osmoregulation. Black sea bass, rock bass, and sturgeon contained many immunoreactive kallikreins in their swimbladders, which implicates a role for kallikreins in the regulation of blood flow and vascular permeability to facilitate gas exchange within the bladder. Kallikreins were consistently identified in skeletal muscle and heart of all the species evaluated and may regulate local blood flow, muscle contraction or relaxation, or participate in various transport processes. The antiserum to fish prokallikrein recognized immunoreactive kallikreins from pancreatic tissues from fish and lower vertebrates, but not from the pyloric caecum of sea bass. The wide distribution of tissue kallikrein in lower vertebrates suggests that it may participate in a variety of physiological functions.

Kinin receptor status in normal and inflammed gastric mucosa

No documented studies have been reported on the presence of B1 and B2 kinin receptors in the mammalian gastric mucosa. This first study aimed to immunolocalise sites of B1 and B2 kinin receptors in the human pyloric gastric mucosa and to evaluate its role in gastritis. Biopsies were obtained from patients with dyspepsia during endoscopic examination of the patient. The diagnosis and grading of the gastritis was performed on histological examination. Sections were immunostained for both B1 and B2 receptors using rabbit anti-human B1 and B2 kinin receptor antibodies. Control tissue was obtained from partial gastrectomy specimens, following surgical excision of the antrum for duodenal ulcers. The control antrum tissue showed strong immunoreactivity for kinin B2 receptors with positivity noted along the luminal border, at the base of the mucous and stem cells. The B1 receptor was not immunolocalised. Biopsies of all five patients with gastritis showed a decrease in immunolabelling of the B2 receptor and an induction of the B1 receptor especially in regenerating cells. In gastritis there is destruction of the normal mucosal glandular architecture with subsequent regeneration of the epithelial cells. The pyloric glands are infiltrated by acute inflammatory cells that cause crypt abscesses with loss of the epithelial cell membranes. This may explain the reduction in the immunolocalisation of the B2 kinin receptors and the induction of the B1 receptors in active gastritis. Follow up studies after treatment of the inflammation with a combination of B1/B2 kinin receptor antagonists are indicated.

Production and localization of cGMP and PGE2 in nitroprusside-stimulated rat colonic ion transport

Nitrovasodilators, such as sodium nitroprusside (SNP), release nitric oxide (NO) and stimulate intestinal electrolyte transport. However, the second messengers involved in this process are unknown. NO stimulates soluble guanylate cyclase activity in other tissues, but stimulation of this enzyme has not previously been described for intestine. We report a 20-fold increase in guanosine 3',5'-cyclic monophosphate (cGMP) production by radioimmunoassay in colonic mucosal strips stimulated with SNP. SNP also caused a significant increase in prostaglandin (PG) E2 release but did not stimulate release of the prostanoids thromboxane B2 or 6-keto-PGF1alpha. Stimulation of isolated colonic crypts and the remaining subepithelial mucosa demonstrated that the latter was the major source of the increases in cGMP and PGE2. Immunostaining of colonic mucosa revealed minimal basal cGMP immunoreactivity but large increases in abundance, localizing to the subepithelium, after SNP treatment. Under basal conditions, there was diffuse immunostaining for constitutive NO synthase in both the epithelial and subepithelial compartments, which was corroborated with NADPH diaphorase staining. In conclusion, SNP was an NO donor stimulates production of cGMP and PGE2 from the subepithelium. NO may be an important mediator of colonic secretion and other processes predominantly via its direct effects on cells of the lamina propria.

Eicosanoids and histamine mediate C5a-induced electrolyte secretion in guinea pig ileal mucosa

C5a is a biologically active polypeptide formed during the course of complement activation and is known to possess histamine-releasing and neutrophil chemotactic properties. In the present study, we have demonstrated that C5a can regulate electrolyte transport across guinea pig ileum, and we have investigated its mechanism of action. Segments of ileum stripped of longitudinal muscle were mounted in Ussing chambers (Krebs' buffer, 37 degrees C, 95% O2/5% CO2) for monitoring short-circuit current (Isc). Serosal application of C5a evoked a transient increase in Isc with an EC50 value of 5.0 nM indicating a potent effect. The C5a-induced increase in Isc was abolished by elimination of both Cl- and HCO3- from the Krebs' solution. Pretreatment with the cyclooxygenase inhibitor indomethacin (5 microM), the neurotoxin tetrodotoxin (0.5 microM) and the H1 receptor antagonist pyrilamine (0.5 microM) reduced the effect of C5a, but the muscarinic antagonist atropine (0.5 microM) was without effect. C5a (100 nM) also evoked the release of histamine (measured by radioimmunoassay in the serosal bathing fluid) by 282% of the control value. In conclusion, in the guinea pig ileum C5a stimulates mucosal anion secretion by releasing histamine and cyclooxygenase products of arachidonic acid. The response is also mediated, in part, via non-chloinergic enteric nerves.

Bradykinin modulates mucin secretion but not synthesis from an intestinal goblet cell line

The effect of the inflammatory mediator bradykinin on glycoprotein synthesis and mucin secretion in the human colonic adenocarcinoma cell line HT29-18N2 was examined. Bradykinin, at a threshold of 0.01 microM, accelerated the rate of mucin discharge as assessed by a mucin-specific ELISA. Using immunofluorescence microscopy, a thick meshwork of extracellular mucus was observed over bradykinin-treated monolayers but not mock-treated controls. Morphometric analysis of bradykinin-treated monolayers revealed no decreases in intracellular mucin stores or any other easily discernable morphological alteration. The ability of the cyclooxygenase inhibitors indomethacin and naproxen to decrease the response to bradykinin by approximately 68% indicates the effect is mediated, at least partially, through the generation of prostaglandins. Bradykinin did not alter the rate of incorporation of 3H-glucosamine into newly synthesized glycoproteins. Bradykinin-accelerated mucin secretion may be linked to the depletion of intracellular mucin stores in the inflammatory bowel disease ulcerative colitis.

Effect of nordihydroguaiaretic acid on glucose absorption, metabolism and (Na(+)+K+)-ATPase activity in rat jejunum

A regulatory role of endogenously synthesized eicosanoids on the absorption, transmural transport and metabolism of glucose in perfused, isolated loops of jejunum in vitro was investigated using the lipoxygenase/cyclooxygenase inhibitor, nordihydroguaiaretic acid (NDGA). NDGA diminished glucose absorption over the range 100-500 microM: maximal inhibition at 500 microM NDGA was 52 +/- 9 and 64 +/- 9% (mean +/- SE, P < 0.001) for jejuna from fed rats and rats maintained on glucose water for 48 hr, respectively. In each instance, transmural transport was effectively abolished. The vectorial disposition of lactate release was also changed such that the ratio of luminal to serosal production was increased from 0.19 +/- 0.02 to 1.72 +/- 0.12 (P < 0.001) in fed rats, indicating inhibition of the Na+ pump. NDGA inhibited (Na(+)+K+)-ATPase activity in whole mucosal homogenates with a concentration dependence similar to that observed for glucose absorption. However, NDGA also inhibited Mg(2+)-ATPase activity in whole homogenates and purified rabbit skeletal muscle phosphofructokinase under the same conditions. The results are discussed in terms of the dissipation of the transmembrane Na+ gradient via direct inhibition of the (Na(+)+K+)-ATPase by NDGA. Inhibition of the ATPase precludes the use of NDGA as a suitable drug with which to investigate the role of endogenously synthesized eicosanoids in the regulation of intestinal function.

Use of coculture of colonic mucosal biopsies to investigate the release of eicosanoids by inflamed and uninflamed mucosa from patients with inflammatory bowel disease

Eicosanoid production was measured in cultured biopsies of colonic mucosa from control patients, with the irritable bowel syndrome, and from patients with proctosigmoiditis and with colonic Crohn's disease. Cultured inflamed colonic mucosa from patients with proctosigmoiditis and Crohn's disease produced more prostaglandin E2 and leukotrienes C4 than control tissues. In addition, eicosanoid production by macroscopically uninflamed or 'quiescent' mucosa from the right colon was examined in patients with proctosigmoiditis and between skip lesions in Crohn's disease patients. In the proctosigmoiditis group quiescent mucosa produced eicosanoids in similar quantities to control tissue. Coculture of quiescent plus inflamed tissue however, generated a marked increase in eicosanoid output in 12 of 20 of the patients and this was similar to the quantity obtained from two pieces of inflamed tissue. In the Crohn's disease group, quiescent mucosa produced more eicosanoids than control mucosa but production was markedly stimulated by coculture with inflamed mucosa in all patients. These findings suggest that in some patients with proctosigmoiditis and in all patients with Crohn's disease quiescent mucosa appears to be sensitised. A small but significant increase in the macrophage population may be partly responsible but it is likely that these and other cells are primed to release eicosanoids, and may be induced to do so by soluble mediators produced by actively inflamed tissue.

Bradykinin receptors: pharmacological properties and biological roles

Kinins contribute to the acute inflammatory response and are implicated in the pathophysiology of inflammatory disease. The development of therapeutically viable agents that counteract the effects of kinins is, therefore, potentially very rewarding. Since kinin actions are generally mediated via an interaction with cell-surface receptors, one approach is the development of site-specific receptor antagonists. The emphasis in this review is to outline our current understanding of the properties of bradykinin receptors and the potential therapeutic applications for drugs acting at these sites. As a result of the recent introduction of potent bradykinin receptor antagonists and the cloning of bradykinin receptor genes, considerable advances in kinin research can now be confidently anticipated.

Epithelial secretory response to inflammation

As suggested by this and previous reviews, the neuroimmunoregulation of intestinal secretion is a complex series of endocrine, neurocrine, paracrine and autocrine interactions between the underlying cells in the mucosa and submucosa and the intestinal enterocyte. Under normal conditions, the balance of each of these systems is delicately controlled, thus allowing for normal, consistent intestinal function. However, when this finely-tuned system is altered, such as in a diseased state, the resultant effect is an amplification of the host defense response. Initially thought to be protective against further insult, this local immune response, if allowed to continue uncontrollably, can exacerbate the disease process.

Endothelin-1 stimulates chloride and potassium secretion in rabbit descending colon

The vasoactive peptide endothelin-1 (ET-1) which is present in high concentrations in the colon, causes concentration-dependent electrogenic Cl- secretion in rabbit descending colon. This effect is half-maximal at 0.11 mumol/l. Like other secretagogues, ET-1 also stimulates K+ secretion. The secretory effect of ET-1 is associated with increased release of prostaglandin E2 from the serosal surface of the mucosa. ET-1-induced Cl- secretion is completely inhibited by the loop diuretic bumetanide and by indomethacin and quinacrine, inhibitors of prostaglandin synthesis. Neuronal mechanisms do not seem to be involved, as tetrodotoxin did not affect the secretory response to ET-1 significantly. On the other hand, neither the catalytic activity nor the transport function of the Na+/K(+)-ATPase of rabbit colon epithelium is affected by endothelin-1 (ET-1) in concentrations up to 10 mumol/l. It is concluded that ET-1 causes Cl- and K+ secretion by stimulating phospholipase A2 and release of prostaglandins, whereas Na+ transport is not altered.

Characterization of a novel binding site for 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]bradykinin on epithelial membranes of guinea pig ileum

We have recently shown that (a) [125I-Tyr8]bradykinin (BK) recognized bradykinin binding sites in guinea pig epithelium membranes with a Kd value of 1.6 nM and a Bmax of 156 fmol/mg protein, and (b) B2 agonists and some B2 antagonists, such as D-Arg-[Hyp3,D-Phe7,Leu8]BK, inhibited this specific binding with a Ki value of 32 nM. In the present study, we have radioiodinated the B2 antagonist Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK and have performed a full characterization of the binding properties of this tracer in the same membrane preparation. Equilibrium experiments performed in the absence or presence of an excess of BK (10(-5) M) showed that 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK specifically labelled two different sites. One of these is the same as the site labelled by [125I-Tyr8]BK, and this indicates that 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK interacts specifically with kinin B2 receptors. Equilibrium experiment performed in the presence of an excess of BK (10(-5) M) indicated that specific binding of 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK to the second site is also saturable and Scatchard analysis showed that the site is of high affinity with a Kd of 16.8 nM and a Bmax of 2.08 pmol/mg protein. Surprisingly, unlabelled B2 agonists such as bradykinin, [Tyr8]BK, [Leu8]BK, [Hyp3,Tyr8(OMe)]BK, D-Arg-[Hyp3]BK and kallidin were found to be inactive on this second site. A series of B2 receptor antagonists, Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK, D-Arg-[Hyp3,D-Phe7,Leu8]BK, D-Arg-[Hyp3,Leu5,8,D-Phe7]BK, D-Arg-[Hyp3,Gly6,D-Phe7,Leu8]BK and D-Arg-[Hyp3,Thi5,8,D-Phe7]BK inhibited 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK binding with Ki values of 25.0, 20.9, 15.8, 64.6 and 6606.9 nM respectively. On the other hand, [Thi5,8,D-Phe7]BK did not interfere with 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK but was found to be a potent inhibitor of [125I-Tyr8]BK binding (Ki = 53.7 nM). As expected, B1 receptor agonists, antagonists and peptides non-related to BK such as substance P, neurokinin A, neurokinin B, angiotensin II, bombesin, vasopressin and the calcitonin gene related peptide were unable to compete with 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK. The results show that 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK is interacting with two distinct binding sites in the guinea pig epithelium: one is the well known bradykinin B2 receptor and the other is a new, non-characterized binding site that interacts exclusively with some bradykinin receptor antagonists.

125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK, a radiolabelled B2 antagonist specifically interacts with two distinct binding sites on epithelial membranes of guinea pig ileum

Kinins are endogenously formed peptides that have diverse biological actions, including effects on the gastrointestinal tract. In the search of selective ligands, we studied the binding properties of a selective B2 radioiodinated antagonist (Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK) on epithelial membranes of guinea pig ileum. Equilibrium binding experiments showed that 125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK specifically labels two different sites. One of these sites is the conventional B2 receptor. The new tracer recognized this site with a Kd of 34.7 nM and revealed a Bmax of 156 fmol/mg protein. In equilibrium binding experiments 125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK also recognized a second specific site. Scatchard analysis showed that this second site was of high affinity (Kd of 16.8 nM) and very abundant (Bmax of 2.08 pmol/mg protein). Surprisingly, the natural B2 agonists bradykinin and kallidin were unable to inhibit the specific binding of 125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK to the second site. A series of B2 antagonists failed to inhibit the specific binding of the new radiolabelled peptide. As expected, non related peptides such as angiotensin II, neurokinin A and B, substance P, vasopressin, calcitonin gene related peptide and bombesin were also inactive. These results show that the new tracer is interacting with two distinct binding sites in epithelial membranes of guinea pig ileum. One is the well known bradykinin B2 receptor and the other is a new, non characterized binding site that interacts exclusively with bradykinin receptor antagonists.

Mechanisms of platelet-activating factor-induced electrolyte transport in the rat jejunum

Platelet-activating factor (PAF) is known to cause chloride secretion in the small and large intestine in vitro. The present study investigated the mechanism of action of PAF-induced electrolyte transport in stripped rat jejunal segments mounted in standard Ussing chambers. Short circuit current was monitored as the indicator of active transport. PAF caused a concentration-dependent increase in short circuit current, whereas its precursor, lyso-PAF, did not. The response to 0.2 microM PAF was inhibited by 92% when chloride ion in the bathing solution was replaced. The response was also significantly inhibited by the PAF receptor antagonist, WEB 2170, the cyclooxygenase inhibitor, indomethacin, the phospholipase A2 inhibitor, mepacrine, and the calcium channel blocker, verapamil. In other in vitro experiments, PAF was shown to stimulate jejunal synthesis of prostaglandin E2, but not 6-keto-prostaglandin F1 alpha or the peptidoleukotrienes C4, D4 or E4. In similar experiments, 0.2 microM PAF enhanced the depletion of jejunal mucosal free arachidonic acid as measured by gas chromatography. These results show that platelet-activating factor stimulates chloride transport in the rat jejunum in vitro, and that the response is dependent upon extracellular calcium, the stimulation of phospholipase A2 and the cyclo-oxygenase catalyzed metabolism of arachidonic acid to prostaglandin E2.

Regulation of ion transport in porcine distal colon: effects of putative neurotransmitters

Porcine distal colon epithelium was mounted in Ussing chambers and bathed with porcine Ringer's solution. The effects of vasoactive intestinal polypeptide, norepinephrine, and carbamylcholine on Na and Cl fluxes and transepithelial electrical parameters were determined after their serosal administration. Vasoactive intestinal peptide increased the Cl-dependent component of the short-circuit current with a half-maximal effect at 115 nmol/L. Transepithelial Na and Cl flux studies demonstrated that the increase in current was caused by stimulation of Cl secretion. Norepinephrine also stimulated Cl secretion and increased the serosal-to-mucosal Na flux, producing a half-maximal effect at 1.6 mumol/L. Selective blockade of alpha 1 adrenoceptors by 0.5 mumol/L prazosin produced an eightfold decrease in norepinephrine potency. Carbamylcholine produced a significant increase in Cl secretion and decreased absorption of both Na and Cl with a concentration of 10 mumol/L producing a half-maximal effect. The muscarinic cholinoceptor blocker atropine (0.1 mumol/L) produced a 22-fold decrease in carbamylcholine potency. The effects of all three transmitter substances were unaffected after pretreatment of tissues with the neuronal conduction-blocker tetrodotoxin or an inhibitor of arachidonic acid metabolism. These results indicate that (a) vasoactive intestinal polypeptide stimulates Cl secretion without affecting Na absorption; (b) norepinephrine acting through alpha 1 adrenoceptors stimulates net Cl secretion and activates a serosal-to-mucosal Na transport mechanism; and (c) carbamylcholine acting through muscarinic receptors stimulates Cl secretion and inhibits Na and Cl absorption.

Phorbol ester-stimulated secretion in chicken ileum: role of arachidonic acid metabolism

Phorbol 12,13-dibutyrate, an activator of protein kinase C, stimulates electrogenic anion secretion in chicken ileum. This effect can be inhibited by the cyclooxygenase inhibitor piroxicam, suggesting arachidonic acid metabolites as mediators. Phorbol 12,13-dibutyrate stimulates increases in prostaglandin E2 release and mucosal 3',5'-cyclic adenosine monophosphate content in intact mucosa. Stimulation of prostaglandin E2 release appears to come mainly from the subepithelial compartment and not from epithelial cells. Unlike its effect on anion secretion, phorbol 12,13-dibutyrate inhibition of sodium uptake by sodium-hydrogen exchange in isolated mature villus enterocytes is not inhibited by piroxicam. This study concludes that phorbol ester effects on anion secretion in chickens appear to be mediated by arachidonic acid metabolites most likely produced by cells of the lamina propria and submucosa, whereas phorbol ester effects on inhibiting upper villus cell sodium-hydrogen exchange may be a direct effect of phorbol esters.

Role of prostaglandins and calcium in the effects of Entamoeba histolytica on colonic electrolyte transport

We have previously shown that Entamoeba histolytica lysates contain the neurohormones serotonin, neurotensin, immunoreactive substance P, and probably acetylcholine, and that amebic lysates inhibit sodium and chloride absorption and stimulate chloride secretion in the rat descending colon as measured by the Ussing chamber-voltage clamp technique. We now demonstrate that these transport effects have both calcium-dependent and calcium-independent components. In addition, arachidonic acid metabolites of the cyclooxygenase pathway are probably involved in the Entamoeba histolytica-induced changes in colonic transport that are not dependent on Ca++ entry. Prostaglandin E2 (10(-5) M), indomethacin (10(-6) M), piroxicam (5 x 10(-5) M), and mepacrine (10(-4) M) partially inhibited the amebic lysate effect on active transport in the rat descending colon. In addition, verapamil (10(-4) M) partially inhibited the effect of amebic lysates. The effect of verapamil was additive with that of indomethacin, totally blocking the effect of amebic lysate on short-circuit current. However, amebic lysates do not contain prostaglandin E2 as measured by sensitive radioimmunoassay. Amebic lysates stimulated prostaglandin E2 release from rat colonic mucosal strips. Amebic lysate significantly increased colonic cyclic adenosine monophosphate content. Piroxicam inhibited the lysate-induced increase in colonic cyclic adenosine monophosphate content. These results indicate that although amebic lysate does not contain prostaglandin E2, it caused arachidonic acid metabolites to be produced by the cyclooxygenase pathway, and these are probably involved in the Entamoeba histolytica-induced changes in colonic transport. Neurohormones in Entamoeba histolytica may act directly on colonic tissue to stimulate intestinal secretion, probably via a Ca+(+)-dependent mechanism that is blockable by verapamil, or indirectly via stimulation of prostaglandin E2 generation and release from the rat colon via a cyclic adenosine monophosphate-dependent mechanism. These effects appear separate. The cyclic adenosine monophosphate-dependent secretion is the predominant mechanism in this model of colonic amebic diarrhea.

Chemotactic peptide-induced acute colitis in rabbits

Bacterial chemotactic peptides from the intestinal lumen could potentially induce inflammation if they reached the mucosa. We tested several peptides chemotactic for different inflammatory cells, as well as a nonchemotactic peptide, bradykinin, for their ability to induce colitis in vivo in rabbits. These peptides were also assessed for their ability to stimulate release of the eicosanoids leukotrienes B4 and C4 and prostaglandin E2 from normal rabbit colons perfused ex vivo. Intracolonic administration of n-formyl-methionyl-leucyl-phenylalanine (chemotactic for neutrophils); its methyl ester (chemotactic for monocytes), and alanyl-glycyl-seryl-glutamic acid (chemotactic for eosinophils) all produced colitis (assessed grossly and histologically) within 4 days. Bradykinin did not induce colitis although it did release prostaglandin E2. n-Formyl-methionyl-leucyl-phenylalanine methyl ester induced the greatest degree of colitis in vivo and released prostaglandin E2 and leukotrienes ex vivo. n-Formyl-methionyl-leucyl-phenylalanine and alanyl-glycyl-seryl-glutamic acid induced comparable degrees of inflammation, but alanyl-glycyl-seryl-glutamic acid produced no eicosanoid release while n-formyl-methionyl-leucyl-phenylalanine released both prostaglandin E2 and leukotriene B4 and leukotriene C4 products from normal ex vivo perfused colons. Thus alanyl-glycyl-seryl-glutamic acid produces colitis independent of proinflammatory eicosanoids while eicosanoid release could contribute to colitis produced by n-formyl-methionyl-leucyl-phenylalanine methyl ester. This experimental model of colitis may reflect one possible etiology of inflammatory bowel disease in humans, when bacterial chemotactic peptides breach mucosal defenses in susceptible individuals.

Role of eicosanoids as mediators of inflammation in inflammatory bowel disease

An unknown initiating event in inflammatory bowel disease (IBD) activates the immune system, which is followed by infiltration of the intestinal mucosa with inflammatory cells and the production of soluble mediators of inflammation. These mediators of inflammation include the metabolites of arachidonic acid. The results of research on arachidonic acid metabolites are reviewed, and it is concluded that the major arachidonic metabolites in human IBD mucosa are the lipoxygenase products leukotriene B4 (LTB4) and 5-hydroxy-6,8,11,14-eicosatetraenoic acid. These metabolites are found in much higher concentrations in mucosa from patients with IBD than from healthy controls. Significantly more chemotactic activity is found in IBD mucosa than in healthy mucosa, and most of this activity is attributable to LTB4. Enhanced synthesis of LTB4 could account for much of the inflammatory response in IBD. Inhibition of the lipoxygenase pathway could be the mechanism that accounts for the therapeutic efficacy of mesalazine.

Kallikrein-gene expression in the rat gastrointestinal tract

The serine proteinase glandular kallikrein has been demonstrated in the gastrointestinal tract, although there is some doubt as to whether it is synthesized there or derives from exocrine-gland secretions. Using a rat pancreatic kallikrein cRNA probe we have demonstrated kallikrein-like gene expression in the corpus, duodenum, jejunum, ileum, caecum and colon, and compared the pattern of expression with that of the gastrointestinal peptides somatostatin, gastrin and glucagon. In addition, using a panel of oligonucleotide probes specific for various members of the rat kallikrein-gene family, we have shown that the kallikrein-like gene expressed appears to be expressed as true kallikrein.

Effect of bradykinin on feline gallbladder water transport and prostanoid formation

Continuing evaluation of the pathophysiology of gallbladder disease has demonstrated significant relationships between gallbladder mucosal fluid transport, gallbladder inflammation, and prostanoid formation. Inflamed gallbladder mucosa secretes, rather than absorbs, fluid, a process associated with prostaglandin formation. Bradykinin has been previously implicated in the pathogenesis of cholecystitis and, in the intestine, bradykinin stimulates mucosal fluid secretion by a prostaglandin-mediated mechanism. Bradykinin was infused into the gallbladder lumen and administered intraarterially into the hepatic artery of perfused cat gallbladders. Both methods of bradykinin administration reversed the mucosal absorption present during control experiments as measured by concentration changes in a nonabsorbable marker. Perfusate and gallbladder tissue prostaglandin E concentrations were significantly increased by bradykinin when compared to control values. Concentrations of 6-keto PGF1 alpha in perfusate solutions and in gallbladder tissue were significantly increased, suggesting bradykinin increased prostacyclin formation. Bradykinin administration significantly increased inflammation, as evaluated by a histologic scoring system. Indomethacin was administered intravenously along with luminal perfusion of the gallbladder with bradykinin. Indomethacin significantly decreased gallbladder fluid secretion and prostanoid formation, but not histologic inflammation, when compared to values produced by bradykinin alone. An increase in systemic vascular and bile kinin concentrations produces gallbladder mucosal water secretion, a process which may be mediated by prostanoids. Histologic inflammation produced by bradykinin was not prevented by indomethacin.

Stimulation of chloride secretion by N-formyl-methionylleucylphenylalanine (FMLP) in rabbit ileal mucosa

1. Formyl-methionyl peptides are potent neutrophil chemoattractants which may be involved in inflammatory responses in the intestine. Effects of formyl-methionylleucylphenylalanine (FMLP) on electrical properties and Cl- fluxes were examined with the Ussing chamber technique employing stripped segments of rabbit ileal mucosa. 2. Serosal but not mucosal addition of FMLP elicited a transient (peak effect within 2 min), concentration-dependent (maximal effect at 30 nM and half-maximal effect at 3 nM) increase in short-circuit current (Isc) which was not inhibited by pretreatment of the tissue with mepyramine (10 microM), tetrodotoxin (0.1 microM) or atropine (10 microM). The related peptides FMLP-benzylamide (FMLP-benz) and methionyl-leucylphenylalanine (MLP) produced concentration-dependent increases in Isc which were qualitatively similar to FMLP. The order of potencies of these peptides was FMLP-benz greater than FMLP greater than MLP. 3. The lack of an effect of mucosal FMLP (300 nM) on Isc does not appear to be due to metabolism since addition of an aliquot of this bathing solution to the serosal bathing solution of a naive tissue increased Isc as expected. Addition of FMLP (30 nM) to the serosal bathing solution of a tissue pre-stimulated with serosal FMLP (30 nM) failed to elicit a response. 4. The increase in Isc produced by FMLP (30 nM) was inhibited by removal of Ca2+ from the serosal bathing solution and by removal of Cl- from both bathing solutions. FMLP (30 nM) increased the serosal-to-mucosal flux of Cl- and decreased the transepithelial conductance (Gt). 5. The cyclo-oxygenase inhibitors indomethacin (1 microM), mefenamate (10 microM) or piroxicam (30 microM) added to both the serosal and mucosal bathing solutions inhibited the increase in Isc elicited by FMLP (30 nM). 6. Measurement of release of immunoreactive thromboxane B2, 6-keto-PGF1 alpha and PGE2 revealed that FMLP (30 nM) selectively increased PGE2 release by an indomethacin-sensitive pathway. 7. Thus, in addition to being potent chemoattractants, formyl-methionyl peptides stimulate electrogenic Cl- secretion and also increase prostaglandin production.

The stimulation of glucose absorption and metabolism in rat jejunum by bradykinin: dependence on the composition of commercial diets

Rats were maintained on one of two standard commercial chow diets, Oxoid modified 41B or Bantin & Kingman rat and mouse diet, which differ in that linoleic acid comprises 27% and 44% of their total fatty acid content, respectively: the effects of bradykinin on the absorption, transmural transport and metabolism of glucose (5 mM) were then measured by the perfusion of isolated jejunal loops in vitro. With intestine from rats fed the Oxoid diet, bradykinin (100 nM in the serosal medium) caused significant increases in the rates of glucose absorption (34%, P less than 0.01) and lactate production (69%, P less than 0.01). These bradykinin-stimulated rates were the same, within experimental error, as those observed in the absence of bradykinin with intestine taken from rats fed the Bantin & Kingman diet and on which bradykinin had no effect. It is concluded that feeding rats with different commercial brands of apparently similar laboratory chow diets may result in significantly altered steady-states of glucose homeostasis in rat small intestine and in quite different sensitivities of glucose homeostasis to bradykinin. The possibility is considered that the differences in absorption might result in part from differences in the proportion of linoleic acid, which is known to enhance glucose absorption.

Platelet activating factor is a potent colonic secretagogue with actions independent of specific PAF receptors

Several candidate mediators of acute inflammation such as E-type prostaglandins, histamine and bradykinin are potent pro-diarrhoeal colonic secretagogues. They act to increase serosal to mucosal transport of chloride and passive water efflux. We investigated the effects of platelet activating factor (PAF) on muscle-stripped rat colon, measuring transepithelial potential difference (p.d.) and, under voltage clamp conditions, short circuit current (Isc). PAF induced dose-dependent increases in p.d. and Isc, with an approximate EC50 of 1.5 X 10(-10) M; similar concentrations of lyso-PAF had a much smaller but discernible effect. PAF and lyso-PAF both displayed 'sidedness' with serosal application effective and mucosal application ineffective. Inhibitor studies suggest that chloride is the principal ion carrier, but the specific PAF receptor antagonists kadsurenone, L652731, CV 3988 and WEB 2086 did not block the response. Unlike bradykinin, PAF did not cause the release of PGE2 into the serosal bathing fluid, and its action was not attenuated by the cyclo-oxygenase inhibitors piroxicam, mefenamic acid or flurbiprofen. We conclude that PAF has a powerful pro-diarrhoeal secretory action on colonic epithelium which is not mediated by the previously defined PAF receptor(s) and is independent of prostanoid generation.

Secretory effects of kinins on colonic epithelium in relation to prostaglandins released from cells of the lamina propria

1. Sheets of muscle-stripped rat and rabbit colon with epithelium intact or removed were mounted in Ussing-type chambers for recording of transepithelial p.d., resistance and short circuit current (Isc), and measurement by radioimmunoassay (RIA) of the release of prostaglandins into serosal and mucosal bathing solutions. 2. In epithelial-intact preparations prostaglandin E2 (PGE2), PGE1, PGF2 alpha, U46619 and prostacyclin (10(-7)-10(-6) M) caused increases in Isc and transepithelial p.d., in (approximate) descending order of potency. Epithelial-removed preparations did not exhibit any transepithelial p.d. 3. In epithelial-intact preparations, lysyl-bradykinin (LBk) applied serosally but not mucosally caused increased p.d. and release of PGE2 (and to a lesser extent other prostaglandins) into serosal but not mucosal bathing solutions. In epithelial-removed tissues, responsiveness to LBk was maintained, but it did not exhibit 'sidedness', i.e. LBk was effective when applied on either side and PGE2 release occurred into both compartments. 4. Indomethacin and other non steroidal anti-inflammatory drugs (NSAIDs) abolished the LBk-induced p.d. and reduced PGE2 release if applied serosally but not mucosally in epithelial-intact preparations. In epithelial-removed tissues, indomethacin added to either side abolished prostaglandin release into both compartments. 5. Calcium removal from serosal but not mucosal bathing solution (Ca2+-free EGTA Krebs) abolished p.d. generation by LBk in epithelial-intact preparations, and reduced PGE2 release in rabbit but not rat colon. Similarly, in epithelial-removed preparations, calcium removal did not affect kinin-induced PGE2 generation in rat but strongly attenuated it in rabbit colon. 6. We conclude that (i) kinins activate the arachidonate cascade principally by interactions with cells in the subepithelial (lamina propria) layer, rather than with the epithelial cells themselves, (ii) PGE2 contributes substantially to the kinin-induced increase of transepithelial p.d. as a messenger released from kinin-responsive subepithelial cells and acting on the basolateral pole of the epithelial cells, (iii) the apparent sidedness of colonic epithelium in terms of responses to kinins, NSAIDs and calcium removal is due to the barrier properties of the epithelial cell layer, and (iv) there are differences in calcium sequestration and apparent calcium dependence of prostaglandin biosynthesis between rat and rabbit colonic subepithelial cells.

Hypertrophic gastritis associated with increased gastric mucosal prostaglandin E2 concentrations in a patient with the carcinoid syndrome

A case of a 69 year old man in whom hypertrophic gastritis was associated with the carcinoid syndrome is reported. Concentrations of prostaglandin E2 were increased in plasma, gastric juice, gastric mucosa and urine. He had marked hypochlorhydria in response to pentagastrin stimulation (Peak acid output (PAO) pg:0.2 mmol/h). After successful hepatic arterial embolisation of the metastases (as indicated by an 85% decrease in 24 h urinary 5-HIAA) the concentrations of prostaglandin E2 decreased in the plasma, gastric juice and gastric mucosa. The gastric mucosal hypertrophy regressed and secretion of acid in response to pentagastrin returned (PAO pg:9.0 mmol/h). These findings suggest that the carcinoid tumour was producing a substance which stimulated increased local synthesis of prostaglandin E2 in the gastric mucosa, with concomitant gastric mucosal hypertrophy and inhibition of gastric acid secretion.

Involvement of prostaglandins in histamine-induced fluid and electrolyte secretion by rat colon

Histamine increased the transmural potential difference across rat colon in-vivo and induced a net secretion of fluid. Both effects were inhibited by indomethacin. Histamine increased the potential difference and short-circuit current, and reduced tissue resistance in colonic sheets in-vitro. This response was reduced in the absence of chloride in the bathing medium or in the presence of serosal frusemide, suggesting that histamine stimulated electrogenic chloride secretion by the colon. The rise in short-circuit current induced by histamine was calcium-dependent since it was reduced in the absence of serosal calcium or in the presence of serosal verapamil. Indomethacin, a cyclo-oxygenase inhibitor, and mepacrine, a phospholipase inhibitor, both caused a dose-dependent inhibition of the electrical response of colonic sheets to histamine, without affecting the rise in short-circuit current induced by prostaglandin E2. The stimulation of chloride secretion induced by histamine in rat colon therefore appears to be mediated by an increased production of prostaglandins.

Intestinal zinc transport: influence of streptozotocin-induced diabetes, insulin and arachidonic acid

The influence of arachidonic acid (AA) on the zinc flux rates of jejunal segments, isolated from streptozotocin-induced diabetic rats injected with saline or with insulin, was investigated using an Ussing chamber technique. Although the zinc flux rates from mucosa-to-serosa (Jms) of normal rats were inhibited by addition of 5 microM AA to the jejunal segment bathing medium (46.4 +/- 5.0 vs 32.6 +/- 4.3 nmol/hr/cm2), AA had no effect on the Jms of diabetic rats either with or without insulin treatment. Induction of diabetes also significantly reduced Jms (46.4 +/- 5.0 vs 22.1 +/- 4.9 nmol/hr/cm2), but 3 day insulin treatment (NPH 8 U/Kg/day subcutaneously) did not reverse this effect (29.2 +/- 5.1 nmol/hr/cm2). Addition of AA to the serosal side did not significantly alter the zinc flux rate from serosa-to-mucosa (Jsm) in either control, diabetic or diabetic rats treated with insulin. The net zinc absorption rate (Jnet) of jejunal segments was decreased in diabetic rats compared to controls (13.2 +/- 3.0 vs -0.7 +/- 2.1 nmol/hr/cm2), but normalization of blood glucose with 3 day insulin treatment did not increase Jnet. Addition of AA was associated with a tendency to increase zinc uptake capacity. This change reached statistical significance in insulin treated diabetic rats. Short-circuit current (Isc) for diabetic rats was increased compared to controls but addition of AA to the mucosal side bathing medium decreased Isc in all groups. The results indicate that the zinc flux rate in the small intestine of streptozotocin-induced diabetic rats is decreased, that zinc uptake capacity of the small intestine does not directly reflect the zinc flux rate across the small intestine, and that AA or one of its metabolites may play a significant role in the control of the zinc flux across the intestinal epithelium.

Indirect effects of bradykinin on ion transport in rat colon descendens: mediated by prostaglandins and enteric neurons

The effect of bradykinin on two preparations of rat colon descendens was examined. In a mucosa-submucosa preparation consisting of the submucosal plexus, the mucosal plexus and the epithelium bradykinin (10(-10)-5 X 10(-9) mol.l-1) caused an increase in Isc, Gt and Pd which was to more than 70% diminished by TTX. However, in a mucosa preparation consisting of only the mucosal plexus and the epithelium bradykinin caused an increase in Isc, Gt and Pd, which was not affected by TTX. Ten times higher concentrations of bradykinin were needed in the mucosa preparation to reach the same effects as in the mucosa-submucosa preparation. All effects of bradykinin were markedly reduced in the presence of indomethacin indicating that they were mediated by prostaglandins in both preparations. The bradykinin effect in the mucosa-submucosa preparation but not in the mucosa preparation was reduced about 50% by atropine. The results suggest that bradykinin activates prostaglandin synthesis. Prostaglandins subsequently stimulate neurons in the submucosal plexus which induce a secretory response on the epithelium partially mediated by a muscarinic receptor. In a high concentration bradykinin due to the induction of prostaglandin synthesis can also activate directly the mucosal epithelium.

Receptor regulation of ion transport in the intestinal epithelium

The active transport of ions by the intestinal epithelium is regulated by a number of enteric neurotransmitters, hormones and other substances. Our knowledge of the receptors mediating the actions of these substances is generally fragmentary. This review summarizes current knowledge on the location and functional characteristics of transmitter receptors regulating transport function in the small intestine, highlighting recent research on cholinergic and bradykinin receptors.