Biomedical subjects
Publications and source records attributed to G Flemström.
The gastroduodenal mucosa is a dynamic barrier restricting entry of gastric acid and other potentially hostile luminal contents. Mucosal HCO3(-) is a key element in preventing epithelial damage, and knowledge about HCO3(-) transport processes, including the role of the cystic fibrosis transmembrane conductance regulator channel, and their neurohumoral control are in rapid progress.
Melatonin is released from intestinal enterochromaffin cells and from the pineal gland, but its role in gastrointestinal function is largely unknown. Our aim was to study the involvement of intestinal and central nervous melatonin in the neurohumoral control of the duodenal mucosa-protective bicarbonate secretion. Working in anesthetized rats, we cannulated a 12-mm segment of duodenum with an intact blood supply and titrated the local bicarbonate secretion with pH-stat. Melatonin and receptor ligands were supplied to the duodenum by close intra-arterial infusion. Even at low doses, melatonin and the full agonist 2-iodo-N-butanoyl-5-methoxytryptamine increased duodenal bicarbonate secretion. Responses were inhibited by the predominantly MT2-selective antagonist luzindole but not by prazosin, acting at MT3 receptors. Also, luzindole almost abolished the marked rise in secretion induced by intracerebroventricular infusion of the adrenoceptor agonist phenylephrine. This response was also abolished by sublaryngeal ligation of all nerves around the carotid arteries. However, it was insensitive to truncal vagotomy alone or sympathectomy alone and was unaffected by removal of either the pineal gland or pituitary gland. Thus, melatonin stimulates duodenal bicarbonate secretion via action at enterocyte MT2-receptors and mediates neural stimulation of the secretion.
A number of common diseases in humans, including gastroduodenal ulcer and irritable bowel syndrome, show circadian rhythms in pain and discomfort. The neurohormone melatonin is released from enterochromaffin cells in the intestinal mucosa and from the pineal gland but its role in gastrointestinal function is largely unknown. We have studied the involvement of melatonin in stimulation of the mucosa-protective alkaline secretion by the duodenal mucosa. A 12-mm segment of proximal duodenum with an intact blood supply was cannulated in situ in anesthetized rats and duodenal HCO3- secretion titrated by pH-stat. Duodenal close intra-arterial infusion of melatonin or the full agonist 2-iodo-N-butanoyl- 5-methoxytryptamine significantly increased the secretion and pretreatment with the melatonin (predominantly MT2-receptor specific) antagonist luzindole almost abolished the response. Intracerebroventricular (i.c.v.) infusion of the alpha1-adrenoceptor agonist phenylephrine (12.2 micromol kg(-1) x h(-1)) caused an up to fivefold increased in the alkaline secretion and the melatonin antagonist luzindole or cutting all peri-carotid nerves abolished the duodenal secretory response to i.c.v. phenylephrine. Peripheral melatonin thus stimulates duodenal mucosal HCO3- secretion and endogenous melatonin, very likely released from mucosal enterochromaffin cells, is involved in mediating neural stimulation of the secretion.
After abdominal surgery, luminal HCl fails to induce duodenal contractions in anaesthetized rats. Elevated tissue levels of nitric oxide (NO) and prostaglandins possibly contribute to this observation. The aim of this study was to compare the effects of luminal capsaicin (1.2 mg mL-1), ethanol (15%) and high partial pressure of CO2 (>250 mmHg) with those of HCl (10 mM) in anaesthetized rats. Motility (intraluminal pressure), mucosal permeability [blood-to-lumen clearance of 51Cr-EDTA (51Chromium-labelled ethylenediaminetetraacetate)] and duodenal mucosal bicarbonate secretion (DMBS) were recorded. Three groups of animals were studied: (1) controls, (2) pretreatment with the NO synthase inhibitor N-nitro-L-arginine-methyl-ester (L-NAME) and (3) pretreatment with the cyclo-oxygenase inhibitor indomethacin. Neither capsaicin, ethanol, CO2 nor HCl induced duodenal contractions or affected DMBS in control rats. However, L-NAME induced duodenal contractions that were augmented by capsaicin, ethanol and HCl, but not by CO2. Indomethacin also induced contractions that were reversibly diminished by capsaicin and HCl, but not by ethanol or CO2. Significant increases in mucosal permeability occurred during ethanol perfusion in indomethacin- and L-NAME pretreated rats. In conclusion, NO probably plays a key role in preventing duodenal contractions in response to luminally HCl, capsaicin and ethanol. The HCl-induced effect on motility appears to be independent of CO2 and is not caused by alteration in mucosal integrity.
BACKGROUND: The effects of gamma-aminobutyric (GABA(A))-receptor ligands and peripheral-type benzodiazepine (BZ) ligands on duodenal mucosal bicarbonate secretion (DBS) were studied in thiobarbiturate-anesthetized rats. METHODS: A segment of proximal duodenum was perfused, and bicarbonate secretion was continuously titrated by pH-stat. In some experiments the vagus nerves were dissected free and cut at the neck level. RESULTS: Luminal GABA (10(-8) to 10(-6) M) increased (P < 0.01) DBS dose-dependently, and the GABA(A) antagonist (+)-bicuculline (0.1-2.5 mg/kg intravenously) caused a similar increase (P < 0.01) in DBS. Vagotomy abolished the latter response, suggesting a centrally elicited nervous action of bicuculline mediated by the vagal nerves. This was supported by the absence of an effect of bicuculline administered intra-arterially close to the duodenum. The GABA(A) agonist muscimol had no effect on DBS when administered intravenously (0.01-0.25 mg/kg) or into a lateral brain ventricle (2 microg/kg/h) but counteracted any stimulation by subsequent intravenous bicuculline. The 'peripheral-type' BZ agonist 4'-chlorodiazepam increased DBS when infused close intra-arterially but had no effect when administered intravenously. CONCLUSIONS: Inhibition of GABA(A) receptors related to cholinergic excitatory pathways in the central nervous system stimulates duodenal mucosal bicarbonate secretion and may increase the local mucosal defense. The stimulation by luminal GABA may reflect modulation of the local mucosal control of duodenal bicarbonate secretion.
The aim of this study was to investigate the permeability of the adherent mucus gel layer in rat duodenum in vivo to macromolecules applied in the lumen. Rats were anesthetized with thiobarbiturate, and the duodenum was perfused with isotonic NaCl solution containing large-molecular-size secretagogues. Effects on mucosal HCO(-)(3) secretion and blood-to-lumen (51)chromium-labeled EDTA clearance were used as indexes that compounds had migrated across the mucus layer. Exposure to a low concentration of papain (10 U/100 ml) for 30 min removed the mucus layer without damage to the epithelium and induced or markedly enhanced HCO(-)(3) secretory responses to cholera toxin (molecular mass of 85 kDa) or glucagon (3.5 kDa). Water extracts from a VacA cytotoxin (89 kDa) producing Helicobacter pylori strain, but not from a toxin-negative isogenic mutant, caused a small increase in HCO(-)(3) secretion but only after the mucus layer had been removed by papain. The duodenal surface mucus gel thus significantly restricts migration of macromolecules to the duodenal surface. Release of bacterial toxins at the cell-mucus interface may enhance or be a prerequisite for their effects on the gastrointestinal mucosa.
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Neurokinin A (NKA) induces duodenal motility and increases mucosal permeability and bicarbonate secretion in the in situ perfused duodenum in anesthetized rats. In the present study, the NKA-induced increase in mucosal permeability was potentiated by luminal perfusion with lidocaine and diminished by vasoactive intestinal peptide (VIP) but unaltered by elevated intraluminal pressure. Elevation of intraluminal pressure, however, potentiated the stimulatory effect of NKA on bicarbonate secretion. In contrast, the tachykinin decreased the rate of alkalinization in rats subjected to elevated intraluminal pressure and treated with indomethacin. Similarly, NKA partially inhibited the VIP-stimulated bicarbonate secretion. Luminal lidocaine did not affect the secretory response to NKA. The motility induced by NKA was unaffected by VIP or lidocaine but decreased by elevated intraluminal pressure. It is concluded that the NKA-induced increase in duodenal mucosal bicarbonate secretion is independent of neurons and possibly mediated by prostanoids. The increase in mucosal permeability in response to NKA may be suppressed by mucosal nerves, perhaps utilizing VIP as one of the transmitters.
Vagal stimuli increase duodenal mucosal HCO-3 secretion and may provide anticipatory protection against acid injury, but duodenal enterocyte (duodenocyte) responses and cholinoceptor selectivity have not been defined. We therefore developed a stable primary culture model of duodenocytes from rats and humans. Brief digestion of scraped rat duodenal mucosa or human biopsies with collagenase/dispase yielded cells that attached to the extracellular matrix Matrigel within a few hours of plating. Columnar cells with villus enterocyte morphology that exhibited spontaneous active movement were evident between 1 and 3 days of culture. Rat duodenocytes loaded with fura 2 responded to carbachol with a transient increase in intracellular calcium concentration ([Ca2+]i), with an apparent EC50 of approximately 3 microM. In a first type of signaling pattern, [Ca2+]i returned to basal or near basal values within 3-5 min. In a second type, observed in cells with enlarged vacuoles characteristic of crypt cell morphology, the initial transient increase was followed by rhythmic oscillations. Human duodenocytes responded with a more sustained increase in [Ca2+]i, and oscillations were not observed. Rat as well as human duodenocytes also responded to CCK-octapeptide but not to vasoactive intestinal polypeptide. Equimolar concentrations (100 nM) of the subtype-independent muscarinic antagonist atropine and the M3 antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide prevented the response to 10 microM carbachol, whereas the M1 antagonist pirenzepine and the M2 antagonists methoctramine and AF-DX 116BS had no effect at similar concentrations. Responses in rat and human duodenocytes were similar. A new agonist-sensitive primary culture model for rat and human duodenocytes has thus been established and the presence of enterocyte CCK and muscarinic M3 receptors demonstrated.
The aim of this study was to examine the integrative response to neurokinin A (NKA) on duodenal mucosal permeability, bicarbonate secretion, fluid flux, and motility in an in situ perfusion model in anesthetized rats. Intravenous infusion of NKA (100, 200, and 400 pmol.kg-1.min-1) induced duodenal motility. Furthermore, duodenal mucosal bicarbonate secretion, fluid output, and mucosal permeability increased in response to NKA. Pretreatment with the nicotinic antagonist hexamethonium did not change the response in any of the parameters investigated, whereas the NK2-receptor antagonist MEN 10,627 effectively inhibited all responses to NKA. Indomethacin induced duodenal motility and stimulated bicarbonate secretion. In indomethacin-treated rats, NKA further increased motility but decreased indomethacin-stimulated bicarbonate secretion by 70%. The NKA-induced increase in mucosal permeability was unaltered by indomethacin. It is concluded that NKA not only induces motility but also increases mucosal permeability and fluid output. Furthermore, the neuropeptide may have both stimulative and inhibitory effects on bicarbonate secretion. All responses to NKA are dependent on NK-2 receptor activation but are not mediated through nicotinic receptors.
We have studied the effects of intracerebral administration of selective alpha-adrenergic agonists on duodenal bicarbonate secretion. Duodenum free of Brunner's glands was cannulated in situ in anesthetized rats, and bicarbonate secretion into the luminal reperfusate was continuously titrated by pH stat. Infusion of the alpha 1-selective adrenoceptor agonist, phenylephrine (1,000-2,500 micrograms.kg-1.h-1), into a lateral brain ventricle increased (P < 0.01) duodenal bicarbonate secretion. Pretreatment with prazosin, an alpha 1-antagonist, significantly (P < 0.01) reduced the stimulatory effect when infused into the lateral ventricle (30 micrograms.kg-1.h-1), but not when administered intravenously (1,000 micrograms.kg-1.h-1). Hexamethonium (10 mg.kg-1.h-1 iv) abolished stimulation, whereas cervical vagotomy, epidural blockade, and naloxone were each without effect. Vasopressin, vasopressin antagonists, ts, and oxytocin did not affect basal secretion. Intracerebro-ventricular administration of the alpha 2-adrenoceptor agonist, clonidine (1,000 micrograms.kg-1.h-1), in contrast to alpha 1-receptor activation, decreased (P < 0.01) the secretion. Thus central nervous adrenoceptors influence duodenal mucosal bicarbonate te secretion, and alpha 1-adrenoceptor stimulation may provide protection against luminal acid. This potent stimulation was not mediated by the vagal nerves, spinal cord pathways, or the release of beta-endorphin but involves nicotinic, possibly enteric nervous transmission.
This study examines the integrative response of several duodenal functions to nitric oxide synthase inhibition. Effects of the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) were studied in anesthetized rats, using in situ duodenal perfusion. L-NAME increased bicarbonate secretion, permeability, and fluid secretion and induced motility. Injection of L-arginine abolished L-NAME-induced motility and lowered the secretion of bicarbonate and fluid. Pretreatment with the nicotinic receptor antagonist hexamethonium prevented the rise in bicarbonate secretion and motility in response to L-NAME but did not affect the increase in mucosal permeability. Atropine diminished the L-NAME-induced increases in permeability, motility, and fluid secretion. The adrenolytic drug guanethidine did not alter the responses to the inhibitor. These results suggest that nitric oxide inhibits duodenal motility and bicarbonate secretion by suppressing a stimulatory, nicotinic receptor-dependent, neural mechanism. The L-NAME-induced contractions involve both a cholinergic, atropine-sensitive pathway and nonadrenergic, noncholinergic neural transmission. Muscarinic receptors also mediate part of the L-NAME-induced increases in mucosal permeability and fluid secretion.
Duodenal mucosal secretion of bicarbonate is one main mechanism in the protection of this epithelium against luminal acid. The duodenal secretagogue vasoactive intestinal polypeptide, at doses not affecting mucosal blood flow, protects against acid-induced morphological changes. Some sigma receptor ligands, which increase the duodenal alkaline secretion, prevent duodenal but not gastric mucosal ulceration. Dopamine D-1 receptor agonists and the peripherally acting catechol-O-methyl-transferase (COMT) inhibitor nitecapone stimulate the bicarbonate secretion in the rat and a similar increase in secretion has been observed in human volunteers. COMT inhibitors decrease tissue degradation of catecholamines, including dopamine. The D-2 agonist bromocriptine, in contrast, decreases the secretion. These results, indicating that the bicarbonate secretion is stimulated via peripheral dopamine D-1 receptors, are supported by the finding that dopamine D-1 (but not D-2) agonists increase the production of cyclic AMP in isolated duodenal enterocytes. The increase in mucosal alkaline secretion may contribute to the previously observed ulceroprotective actions of dopaminergic compounds.
BACKGROUND/AIMS: The gastric mucosa is covered by a continuous layer of bicarbonate-containing mucus gel; the question arises how acid, formed in the gastric glands, moves into the lumen. METHODS: The pH in the gastric mucus gel and gel thickness were measured in anesthetized rats with pH-sensitive microelectrodes (tip diameter, 1-5 microns). RESULTS: During pentagastrin (40 micrograms.kg-1.h-1) stimulation of acid secretion, the pH was higher in the gel than in the lumen (pH 2) up to a distance of 115 +/- 18 microns from the epithelial surface and maximal (pH 7.2 +/- 0.1) at the surface. A similar pH gradient was recorded at luminal pH 3. After omeprazole (10 mumol/kg) inhibition of endogenous acid secretion and with exogenous acid in the lumen, the pH profile was broader: 204 +/- 26 microns at luminal pH 2 and 231 +/- 63 microns at luminal pH 3. In contrast, the pH at the epithelial surface was lower (pH 6.8-6.9). The gel thickness (200-300 microns) was similar in all groups. CONCLUSIONS: The significantly higher surface pH in acid-secreting stomachs probably reflects better availability of interstitial mucosal bicarbonate. Bulk transport of secreted acid in channels created by the gland luminal hydrostatic pressure may additionally act to limit acidification of the mucus gel.
Bicarbonate secretion by duodenum distal to the Brunner's glands area was titrated in situ in rats anesthetized with thiobarbiturate. The unselective muscarinic antagonist atropine (0.4 mg/kg) inhibited secretion stimulated by bethanechol (15 micrograms.kg-1.h-1) but not that stimulated by carbachol (15 micrograms.kg-1.h-1). The nicotinic antagonist hexamethonium (10 mg.kg-1.h-1), however, abolished the latter response. The muscarinic M1-selective antagonists pirenzepine and telenzepine (0.025, 0.25 and 2.5 mg/kg) did not decrease but caused a dose-dependent rise in duodenal mucosal HCO3- secretion, an effect abolished by cervical vagotomy or infusion of the alpha-adrenoceptor antagonist phentolamine (0.1 mg.kg-1.h-1). Phentolamine alone caused a sustained increase in secretion. McN-A-343 (0.025, 0.25, and 2.5 mg/kg), an M1-selective agonist and ganglionic stimulator, increased the HCO3- secretion; this effect was not prevented by vagotomy but was attenuated by pirenzepine. Intracerebroventricular infusion of pirenzepine and telenzepine did not cause any changes in secretion. These findings suggest that peripheral muscarinic M1 and nicotinic receptors mediate cholinergic stimulation of duodenal mucosal HCO3- secretion. Pirenzepine and telenzepine may act stimulatory by antagonizing muscarinic M1-transmission in peripheral sympathetic ganglia, thus decreasing postsynaptic adrenergic inhibition.
BACKGROUND: Catechol-O-methyltransferase (COMT) inhibition prevents tissue degradation of catecholamines including dopamine. This study was undertaken to investigate the effect of intraluminal nitecapone, a peripherally acting COMT inhibitor, on duodenal mucosal bicarbonate secretion in humans and to compare the effect with that of the prostaglandin E1 analogue misoprostol. METHODS: The duodenal bulb was isolated by means of a three-balloon six-channel tube as previously described. Basal bicarbonate secretion and secretion after intraluminal administration of 30 and 150 mg nitecapone were determined in 11 healthy subjects. In 7 of these subjects, effects of intraluminal administration of 30 and 150 micrograms of misoprostol were studied in a second experiment. RESULTS: Even the lower dose of misoprostol increased duodenal bicarbonate secretion from 121 +/- 12 to 221 +/- 36 and the lower dose of nitecapone from 149 +/- 18 to 277 +/- 48 microEq.cm-1 x h-1, respectively (P < 0.05). With 150 micrograms of misoprostol or 150 mg of nitecapone there was a further increase in secretion to 296 +/- 33 (P < 0.01) and 421 +/- 36 (P < 0.001) microEq.cm-1 x h-1, respectively. The rise in bicarbonate secretion in response to nitecapone was associated with some increase in the release of prostaglandin E2 to the luminal perfusate. CONCLUSIONS: It seems likely that peripheral COMT inhibition increases duodenal mucosal bicarbonate secretion and protection by inhibition of mucosal degradation of dopamine, an increase similar in magnitude to that obtained by a prostaglandin E1 analogue.
BACKGROUND: The catechol-O-methyl-transferase (COMT) inhibitor nitecapone, which prevents mucosal degradation of dopamine, and some dopamine receptor agonists ameliorate gastroduodenal mucosal damage. Therefore, their effects on mucosal bicarbonate secretion were studied. METHODS: Duodenum just distal to the Brunner's glands area was cannulated in situ in anesthetized rats. Bicarbonate secretion into the luminal perfusate and transmucosal electrical potential difference (PD) were recorded. RESULTS: Intravenous dopamine (50 micrograms.kg-1 x h-1) increased bicarbonate secretion twofold, and a higher rate of infusion (250 micrograms.kg-1 x h-1) resulted in a further increase. Neither dose affected the PD. The dopamine D1 agonist SKF-38393 (10-50 micrograms/kg) and the COMT inhibitor nitecapone (50-500 micrograms/kg) caused dose-dependent increases in secretion, similar to that observed with dopamine. Domperidone, a peripherally acting dopamine antagonist, inhibited the stimulatory effects of SKF-38393 and nitecapone. Hexamethonium or the alpha-adrenoceptor antagonist phentolamine, in contrast, did not affect the response to nitecapone. Intracerebroventricular administration of nitecapone was without effect. CONCLUSIONS: A probable electroneutral component of duodenal mucosal bicarbonate secretion is stimulated via peripheral dopamine D1 receptors. This may contribute to the previously observed ulceroprotective actions of dopaminergic compounds.
Secretion of bicarbonate increases the pH at the duodenal mucosal surface, a process which contributes to the protection against acid/pepsin injury. Previously, we have shown that dopaminergic compounds stimulate the duodenal bicarbonate secretion in situ, in the anaesthetized rat, through an action on peripheral dopamine D1 receptors. In order to study the possible involvement of cyclic adenosine-3',5'-monophosphate (cAMP) as an intracellular mediator in enterocytes isolated from rat duodenum, cells were collected by a combination of enzyme treatment and calcium chelation. Two major cell fractions, one mainly from villi and the other mainly of crypt origin, were studied. In the villus cell fraction, the activity of alkaline phosphatase was 1.6 +/- 0.2 mumol mg protein-1 min-1 and that of sucrase 98.8 +/- 16.4 nmol mg protein-1 min-1. In the crypt fraction, activities were 0.7 +/- 0.1 and 28 +/- 10.5, respectively. Effects of dopamine, two selective dopamine receptor agonists and vasoactive intestinal peptide (VIP) on intracellular accumulation of cAMP were examined by radio-immunoassay (RIA). In the crypt cell fraction, VIP (10(-7) M) caused an increase in cAMP which was maximal after 5 min (78 +/- 28% above control, P < 0.01). In the villus cell fraction, maximal responses to VIP (60 +/- 24% above control, P < 0.05), did not occur until after 60 min of incubation. In contrast, there were no significant differences between villi and crypt enterocytes in respect to effects of dopamine, the dopamine D1-receptor agonist SKF-38393 and the D2-receptor agonist quinpirole.(ABSTRACT TRUNCATED AT 250 WORDS)