Mechanism of action of Trolox on duodenal contractility.

Trolox is a hydrophilic analogue of vitamin E. The aim of this work was to study the mechanism of action of Trolox on rabbit duodenal spontaneous motility and contractility. The duodenal contractility studies in vitro were carried out in an organ bath. Trolox (12 mM) reduced the amplitude and frequency of spontaneous contractions and the acetylcholine-induced contractions in the longitudinal and circular smooth muscle of rabbit duodenum. Quinine reverted the Trolox-induced (12 mM) reduction on the amplitude and frequency of spontaneous contractions in the longitudinal and circular muscle. Charibdotoxin and glibenclamide reverted only the amplitude of spontaneous contractions in circular muscle of the duodenum. The decrease of ACh-induced contractions evoked by Trolox 12 mM in the longitudinal and circular smooth muscle of the duodenum was antagonized by quinine in longitudinal and circular muscle and by Bay K8644, 1H-[1,2,4]oxadiazolo [4, 3-α]quinoxalin-1-one (ODQ) and nimesulide in circular muscle. We conclude that in the decrease of duodenal contractility induced by Trolox participate K(+) and Ca(2+) channels, adenylyl cyclase, guanylyl cyclase and cyclooxygenase-2.

Involvement of neuronal nitric oxide synthase (nNOS) in the regulation of migrating motor complex (MMC) in sheep.

The objectives of this study were to evaluate the role of nitric oxide (NO) synthase isoforms (nNOS, eNOS, and iNOS) in the regulation of the migrating motor complex (MMC) in sheep using electromyography and their expression in the gastrointestinal (GI) tract by Western blot (WB) and immunohistochemistry. Intravenous administration of L-NAME or the nNOS inhibitor 7-nitroindazole (7-NI) decreased the MMC interval. Myoelectric activity of intestinal phase II was increased, whereas antral activity was reduced. These effects were blocked by L-arginine. Inhibitors of either iNOS (aminoguanidine and S-methylisothiourea) or eNOS (L-NIO) were ineffective. The NO donor sodium nitroprusside decreased GI myoelectric activity, inhibited the MMC pattern, and prevented the effects induced by L-NAME and 7-NI in the intestine. Intracerebroventricular administration of these agents did not modify GI motility. In the rumen, abomasal antrum, duodenum, and jejunum, WB showed three bands at about 155, 145, and 135kDa corresponding to nNOS, and a 140-kDa band (eNOS); however iNOS was not detected. Positive nNOS immunostaining was observed in neurons of the myenteric and submucous plexus of all GI tissues, while eNOS was found in the endothelial cells, ruminal and intestinal epithelium, as well as in some enteric neurons and in endocrine-like cells of the duodenal Brunner's glands. In contrast, only weak iNOS immunoreactivity was found in ruminal epithelium. Taken together, our results suggest that NO, synthesized at a peripheral level by nNOS, is tonically inhibiting the MMC pattern and intestinal motility in sheep.

Extracellular signal-regulated kinase (ERK) is involved in LPS-induced disturbances in intestinal motility.

BACKGROUND: Lipopolysaccharide (LPS) is a causative agent of sepsis. A relationship has been described between LPS, free radicals, and cyclooxygenase-2 (COX-2). Here, we investigate the role of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases (MAPK) in the effect of LPS on intestinal motility, oxidative stress status, and COX-2 expression. METHODS: Rabbits were injected with (i) saline, (ii) LPS, (iii) U0126, an ERK MAPK inhibitor, or (iv) U0126+LPS. Duodenal contractility was studied in an organ bath with acetylcholine, prostaglandin E(2), and KCl added. Neuromuscular function was assessed by electrical field stimulation (EFS). Neurotransmitter blockers were used to study the EFS-elicited contractile response. The formation of products of oxidative damage to proteins (carbonyls), lipids, [malondialdehyde (MDA), and 4-hydroxyalkenals (4-HDA)] was quantified in plasma and intestine. The protein expression of phospho-ERK (p-ERK), total ERK, and COX-2 in the intestine was measured by western blot, and p-ERK was localized by immunohistochemistry. KEY RESULTS: Acetylcholine, prostaglandin E(2), and KCl-induced contractions decreased with LPS. Electrical field stimulation induced a neurogenic contraction that was reduced by LPS. Lipopolysaccharide increased p-ERK and COX-2 expression and the levels of carbonyls and MDA+4-HDA. U0126 blocked the effect of LPS on acetylcholine, prostaglandin E(2), KCl, and EFS-induced contractions, the levels of carbonyls and MDA+4-HDA and p-ERK and COX-2 expression. Phospho-ERK was detected mostly in the neurons of the myenteric and submucosal ganglia. CONCLUSIONS & INFERENCES: We can suggest that ERK is involved in the mechanism of action of LPS in the intestine.

Role of potassium channels in rabbit intestinal motility disorders induced by 2, 2'-azobis (2-amidinopropane) dihydrochloride (AAPH).

Oxidative stress appears to play a role in the pathogenesis of several inflammatory gastrointestinal diseases. Changes in intestinal motility have been reported in different models of intestinal inflammation. The initiating factor of altered motility could be an alteration of gut redox status. The aim of this study was to investigate the effect of oxidative stress evoked by 2, 2'-azobis (2-amidinopropane) dihydrochloride (AAPH) on the intestinal motility of rabbit duodenum and the possible contribution of different K(+) channels in mediating this response. Whole thickness segments of rabbit duodenum were suspended in the direction of the longitudinal or circular smooth muscle fibres in an organ bath to study the effects of AAPH alone, or in the presence of different K(+) channel blockers on the amplitude, frequency and tone of spontaneous contractions. In circular muscle, AAPH 20 mM induced a reduction of the amplitude, the frequency and tone of the spontaneous contractions. In longitudinal muscle, AAPH 10 mM induced a reduction of the amplitude and tone of the spontaneous contractions. The reduction of the amplitude and tone induced by AAPH was reverted by BaCl2 (1 mM) and TEA (5 mM). Charybdotoxin (100 nM) and iberiotoxin (100 nM) only reverted the reduction of the tone induced by AAPH. In conclusion, our results show that the peroxyl radicals released by AAPH reduced the amplitude and the tone of the spontaneous contractions of the longitudinal smooth muscle from rabbit small intestine. Inward rectifier and intermediate and large-conductance Ca(2+)-activated K(+) channels could be involved in these effects.

Inhibition of p38 MAPK improves intestinal disturbances and oxidative stress induced in a rabbit endotoxemia model.

BACKGROUND: Lipopolysaccharide (LPS) decreases intestinal contractility and induces the release of reactive oxygen species, which play an important role in the pathogenesis of sepsis. p38 mitogen-activated protein kinase (MAPK) can be activated by a variety of stimuli such as LPS. The aims of this study were: (i) to investigate the role of p38 MAPK in the effect of LPS on (a) the acetylcholine, prostaglandin E(2) and KCl-induced contractions of rabbit duodenum and (b) the oxidative stress status; (ii) to localize the active form of p38 in the intestine. METHODS: Rabbits were injected with (i) saline, (ii) LPS, (iii) SB203580, a specific p38 MAPK inhibitor or (iv) SB203580 + LPS. Duodenal contractility was studied in an organ bath. SB203580 was also tested in vitro. The protein expression of p-p38 and total p38 was measured by Western blot and p-p38 was localized by immunohistochemistry. The formation of products of oxidative damage to proteins (carbonyls) and lipids (MDA+4-HDA) was quantified in intestine and plasma. KEY RESULTS: ACh, PGE(2) and KCl-induced contractions decreased with LPS. LPS increased phospho-p38 expression and the levels of carbonyls and MDA+4-HDA. SB203580 blocked the effect of LPS on the ACh, PGE(2) and KCl-induced contractions in vivo and in vitro and the levels of carbonyls and MDA+4-HDA. P-p38 was detected in neurons of the myenteric plexus and smooth muscle cells of duodenum. CONCLUSIONS & INFERENCES: Lipopolysaccharide decreases the duodenal contractility in rabbits and increases the production of free radicals. p38 MAPK is a mediator of these effects.

Melatonin and Trolox ameliorate duodenal LPS-induced disturbances and oxidative stress.

BACKGROUND AND AIMS: Lipopolysaccharide evokes gastrointestinal motility disturbances and oxidative stress. The aims of the present study were to investigate the effect of melatonin and Trolox in the actions of lipopolysaccharide on duodenal contractility and on lipid peroxidation in rabbit duodenum. METHODS: The in vitro duodenal contractility studies were carried out in organ bath and the levels of malondialdehyde were assayed by spectrophotometry. Duodenal segments were incubated with lipopolysaccharide (0.3 microg mL(-1)). RESULTS: Lipopolysaccharide decreased acetylcholine-induced contractions and increased malondialdehyde and 4-hydroxyalkenals concentrations in homogenates of duodenum. Melatonin reduced the amplitude of spontaneous contractions in duodenal muscle. Acetylcholine-induced contractions were not altered by melatonin in longitudinal and circular muscles. Trolox decreased the amplitude of spontaneous contractions of duodenal muscle. Trolox (1.2 or 4 mM) did not alter acetylcholine-induced contractions in duodenal muscle, but the concentration of 12 mM diminished the frequency of contractions and acetylcholine-induced contractions. Melatonin (0.3 mM) or Trolox (4 mM) diminished malondialdehyde and 4-hydroxyalkenals levels induced by lipopolysaccharide in the duodenum. CONCLUSIONS: Melatonin and Trolox reduce oxidative stress induced by lipopolysaccharide and ameliorate the effect of lipopolysaccharide on duodenal contractility.

A downregulation of nNOS is associated to dysmotility evoked by lipopolysaccharide in rabbit duodenum.

Alterations in gastrointestinal motility have been reported in response to endotoxin. The effects of lipopolysaccharide (LPS) on motility have been attributed to several substances, including prostaglandins and nitric oxide. The aim of this study was to investigate the expression and the contribution of NOS and COX enzymes to the local effect of LPS on ACh-evoked contractions in rabbit duodenum. The ACh evoked contractions were inhibited by LPS in longitudinal and circular muscles of duodenum. L-NNA, aminoguanidine, ODQ, indomethacin, and NS-398 but not NPLA antagonized the inhibitory effect of LPS. Western blot analysis showed protein bands of 155, 130, 70 and 72 kDa for nNOS, iNOS, COX-1 and COX-2 respectively in rabbit duodenum. All of these isoforms were expressed constitutively and only the nNOS was reduced in the presence of LPS. Expression of nNOS, iNOS, COX-1 and COX-2 was detected by inmunohistochemistry in the smooth muscle layers and in the neurons of the myenteric ganglia of rabbit duodenum. In conclusion, LPS locally administered reduces the contractility of rabbit duodenum and a downregulation of nNOS is associated to this effect. The iNOS, COX-1 and COX-2 were expressed constitutively but their expression was not modified by LPS.

Ca2+-activated K+ channels involved in duodenal dismotility induced by ethanol.

The purpose of this study was to investigate the role of K+ channels in duodenal dismotility induced by ethanol in vitro. The amplitude of spontaneous contractions was reduced by ethanol in longitudinal and circular muscle, while frequency did not change. Charybdotoxin antagonized ethanol-induced inhibition of the amplitude of spontaneous contractions. Ethanol decreased ACh-induced contractions and this effect was cancelled out by charybdotoxin. Ca2+-activated K+ channels may be involved in duodenal dismotility induced by ethanol.

The role of tyrosine kinase in prostaglandin E2 and vanadate-evoked contractions in rabbit duodenum in vitro.

Prostaglandin E2 (PGE2) can interact with at least four cell surface receptors (EP1-EP4) in smooth muscle, which evokes a variety of intracellular responses depending on the G protein to which the cell surface receptors are coupled. The activation of G protein-coupled receptors and receptor tyrosine kinases can lead to the phosphorylation of tyrosine residues of various cellular proteins. The aim of this study was to examine the role of tyrosine phosphorylation in PGE2, vanadate and carbachol-evoked contractions. PGE2, vanadate, and carbachol induced contractile motor responses in the longitudinal smooth muscle of rabbit duodenum. PGE2-evoked contractions decreased in the presence of genistein or tyrphostin B44. PGE2-evoked contractions increased in the presence of vanadate. Vanadate-evoked contractions decreased in the presence of genistein. In contrast, tyrphostin 47 increased the vanadate-evoked contractions. Vanadate-evoked contractions were reduced in the presence of Ca2+-free solutions, verapamil, or indomethacin. U-73122 decreased PGE2-evoked contractions. Carbachol-evoked contractions decreased in the presence of genistein, tyrphostin B44 or tyrphostin 47. Our results suggest that PGE2, vanadate or carbachol-evoked contractions are mediated by protein tyrosine phosphorylation. Protein tyrosine phosphorylation might cause an increase in calcium influx through voltage-dependent channels and the release of prostaglandins in the longitudinal smooth muscle of the rabbit duodenum.

K+ channels involved in contractility of rabbit small intestine.

Most excitable cells, including gastrointestinal smooth muscle cells, express several types of K+ channels. The aim of this study was to examine the types of K' channels involved in the contractility of longitudinal smooth muscle of rabbit small intestine in vitro. Spontaneous contractions and KCl-stimulated contractions were reduced by atropine, phentolamine, propranolol, suramin, tetrodotoxin and indomethacin. The amplitude and tone of spontaneous contractions were increased by apamin, charybdotoxin, iberiotoxin, E4031, tetraetylammonium (TEA) and BaCl2. The frequency of contractions was reduced in the presence of apamin and TEA and increased by charybdotoxin. It was found that 4-aminopyridine increased the tone of spontaneous contractions and reduced the amplitude and frequency of contractions. Glibenclamide did not modify the amplitude, frequency or tone of contractions. KCl-stimulated contractions were increased by E4031, were not modified by apamin, glibenclamide, NS1619 or diazoxide, and were reduced by charybdotoxin, TEA, 4-aminopyridine or BaCl2. These results suggest that both Ca2+-activated K+ channels of small and high conductance, and HERG K+ channels and inward rectifier K+ channels participate in spontaneous contractions of small intestine. On the other hand, voltage-dependent K+ channels, HERG K+ channels, inward rectifier K+ channels and high conductance Ca2+-activated K+ channels are involved in KCl-stimulated contractions.

The role of NO in the contractility of rabbit small intestine in vitro: effect of K+ channels.

Nitric oxide (NO) is an inhibitory neurotransmitter of intestinal smooth muscle cells. The aim of this study was to determine the role of NO in the contractility of rabbit small intestine smooth muscle in vitro. The amplitude, frequency and tone of spontaneous contractions in longitudinal and circular smooth muscle of duodenum, jejunum and ileum were determined and the sodium nitroprusside (SNP), acetylcholine (ACh) and KCl responses were quantified. L-NAME, L-NNA, L-arginine and D-arginine did not affect the amplitude, frequency and tone of spontaneous contractions. ODQ (10(-6) M) increased the tone of spontaneous contractions of the types of tissues examined, and the amplitude in ileum, without modifying the frequency. SNP (10(-4) M) evoked relaxations that were not influenced by atropine (10(-6) M) plus guanethidine (10(-6) M), apamin (10(-8) M) or glybenclamide (10(-6) M), but were increased by TTX (10(-6) M) and verapamil (10(-7) M). SNP-induced relaxations were reduced by charybdotoxin (10(-8) M) and ODQ (10(-6) M). ODQ (10(-5) M) reduced ACh-induced contractions, but it did not influence KCl-evoked contractions. Those results suggest that NO modulates the spontaneous contractions of small intestine in rabbits. This effect is mediated by cGMP and Ca2+-dependent K+ channels of large conductance.

The role of Ca2+ in the contractility of rabbit small intestine in vitro.

This study evaluated the role of Ca2+ in spontaneous and ACh- and KCl-induced contractions in longitudinal and circular smooth muscle from rabbit small intestine in vitro. In the first experiment, the amplitude, frequency and tone of spontaneous contractions in longitudinal and circular smooth muscle of small intestine were determined and, in the second experiment, the ACh- and KCl-induced responses of longitudinal and circular smooth muscle were measured. Atropine and guanethidine reduced the amplitude and tone of contractions in longitudinal and circular muscle, but reduced the frequency of contractions in circular muscle, only. TTX attenuated the amplitude of contractions and decreased the tone of contractions in longitudinal muscle, but increased the tone in circular muscle. Ca2+-free solutions, verapamil, nifedipine and caffeine diminished the three parameters of spontaneous contractions. Thapsigargin and cyclopiazonic acid increased the amplitude and tone of contractions in ileum longitudinal muscle, only, and cyclopiazonic acid increased the amplitude of contractions in circular muscle. Ca2+-free solutions, verapamil, nifedipine, thapsigargin, cyclopiazonic acid, and caffeine diminished ACh- and KCl-induced contractions. Those results suggest that extracellular Ca2+ plays a role in spontaneous contractions, and extracellular and intracellular Ca2+ participate in the ACh- and KCl-induced contractions of rabbit small intestine.

Role of prostaglandins in lipopolysaccharide effects on K+-induced contractions in rabbit small intestine.

AIM: The mediators of the pathophysiologcal symptoms of septic shock are not completely understood. The aim of this work was to investigate the effect of lipopolysaccharide (LPS) on the K+-induced response of longitudinal segments of rabbit small intestine in vitro and the possible role of prostaglandins. METHODS AND RESULTS: Rabbits were treated with intravenously injected LPS. After 90 min animals were killed and intestinal segments were mounted in an organ bath. Lipopolysaccharide (0.2 microg kg-1) inhibited K+-induced contractions (60 mm) by 68% in duodenum, 58% in jejunum and 52% in ileum. Indomethacin antagonized LPS actions when injected 15 min before LPS. PGE2 reduced K+-induced contractions, imitating LPS effects. In contrast, contractions induced by K+ increased when intestinal segments were incubated in vitro with LPS for 90 min. The LPS (0.3 microg mL-1) increased K+-induced contractions (60 mm) by 46% in duodenum, 63% in jejunum and 85% in ileum. The LPS effect was antagonized by indomethacin at 10-6 m in duodenum and jejunum and at 10-8 m in ileum. PGE2 evoked dose-dependent contractions when added to the bath in duodenum, jejunum and ileum. CONCLUSION: These results suggest that effect of LPS on K+-induced contractions in the rabbit small bowel may be mediated by prostaglandin E2.

Central tumour necrosis factor-alpha mediates the early gastrointestinal motor disturbances induced by lipopolysaccharide in sheep.

Cytokines are involved in fever and other symptoms of the acute phase response induced by endotoxins. The aim of this work was to study the involvement of central tumour necrosis factor-alpha (TNF-alpha) in the changes induced by lipopolysaccharide (LPS) on gastrointestinal (GI) motility in sheep. Body temperature and myoelectric activity of the antrum, duodenum and jejunum was recorded continuously. Intravenous (i.v.) administration of LPS (0.1 micro g kg-1)-induced hyperthermia, decreased gastrointestinal myoelectric activity and increased the frequency of the migrating motor complex (MMC). These effects started 40-50 min after LPS and lasted for 6-7 h. TNF-alpha (50 and 100 ng kg-1) mimicked these effects when injected intracerebroventricularly (i.c.v.) but not i.v. Pretreatment with soluble recombinant TNF receptor (TNFR:Fc, 10 micro g kg-1, i.c.v.) abolished the TNF-induced actions and reduced those evoked by LPS. Furthermore, the effects induced by either LPS or TNF were suppressed by prior i.c.v. injection of indomethacin (100 micro g kg-1). In contrast, the i.v. injections of TNFR:Fc or indomethacin were ineffective. Our data suggest that LPS disturbs GI motility in sheep through a central pathway that involves TNF-alpha and prostaglandins sequentially.

Effect of lipopolysaccharide on rabbit small intestine muscle contractility in vitro: role of prostaglandins.

The purpose of this study was to investigate the effect of lipopolysaccharide (LPS) on spontaneous contractions and acetylcholine (ACh) induced contractions of rabbit intestinal segments in vitro, with two different protocols: intestinal segments isolated from LPS-treated rabbits and intestinal segments incubated with LPS. The frequency of spontaneous movements decreased significantly in LPS-treated rabbits at 2 microg kg-1 in the duodenum and 20 microg kg-1 in the duodenum, jejunum and ileum. LPS (0.2 microg kg-1) reduced significantly the ACh contractions (10-6 mol L-1) in the duodenum (61%), jejunum (48%) and ileum (21%). Indomethacin (1, 5 and 10 mg kg-1) administered 15 min before LPS (0.2 microg kg-1) antagonized the LPS effects on the ACh-induced contractions. Prostaglandin (PG)E2 (8 microg kg-1) inhibited significantly the frequency of spontaneous contractions in the ileum and reduced the ACh-induced contractions in the three segments, mimicking the LPS effects. The amplitude and frequency of contractions in rabbit intestinal segments previously incubated with LPS (0.03, 0.3, 3 and 30 microg mL-1) were not modified with respect to the control. The ACh-induced contractions (10-4 mol L-1) were significantly reduced after 90 min of incubation with LPS. The inhibition of LPS (0.3 microg mL-1) was 43% in the duodenum, 35% in the jejunum and 17% in the ileum. Indomethacin added before LPS blocked the effect of LPS on the ACh-induced contractions in the duodenum, jejunum and ileum. These results show that LPS decreases intestinal contractility in rabbits and suggest that PGs are implicated in these actions.

Evidence for the involvement of 5-HT4 receptors in the 5-hydroxytryptamine-induced pattern of migrating myoelectric complex in sheep.

1. The effects induced by 5-hydroxytryptamine (5-HT) on gastrointestinal myoelectric activity in conscious sheep were recorded through electrodes chronically implanted and analysed by computer. The 5-HT receptors and the cholinergic neuronal pathways involved in these actions were investigated. 2. The intravenous (i.v.) administration of 5-HT (2, 4 and 8 micrograms kg-1 min-1, 5 min) induced an antral inhibition concomitant with a duodenal activity front that migrated to the jejunum, followed by a period of intestinal inactivity. This myoelectric pattern closely resembled that observed in the phases III and I of the migrating myoelectric complex (MMC) in sheep. The 0.5 microgram kg-1 min-1 dose evoked the same pattern in only two out of the six animals used. Likewise, the 1 microgram kg-1 min-1 dose similarly affected four of the six animals. In addition, a transient stimulation was observed in the antrum and jejunum when the two highest doses were used. 3. The 5-HT1 antagonist, methiothepin (0.1 mg kg-1), the 5-HT2 antagonists, ritanserin (0.1 mg kg-1) and ketanserin (0.3 mg kg-1), the 5-HT3 antagonists, granisetron (0.2 mg kg-1) and ondansetron (0.5 mg kg-1), as well as the 5-HT4 antagonist, GR113808 (0.2 mg kg-1), did not modify the spontaneous gastrointestinal myoelectric activity. However, the cholinoceptor antagonists, atropine (0.2 mg kg-1) and hexamethonium (2 mg kg-1), inhibited gastrointestinal activity. 4. When these antagonists were injected i.v. 10 min before 5-HT (2 or 4 micrograms kg-1 min-1, 5 min), only GR113808, atropine and hexamethonium were able to modify the 5-HT-induced actions, all of them being completely blocked by the three antagonists. 5. Our data show that 5-HT initiates a MMC-like pattern in the gastrointestinal area in sheep through 5-HT4 receptors. Furthermore, these actions are mediated by cholinergic neural pathways involving muscarinic and nicotinic receptors. However, our results do not indicate a role for either 5-HT1, 5-HT2 or 5-HT3 receptors in the 5-HT-induced effects.

The effect of Mn2+, Zn2+, Ba2+ and Ca2+ on spontaneous motility in sheep duodenum in vitro.

1. The effects of several ions, Mn2+, Zn2+, Ba2+ and Ca2+, on spontaneous motility were investigated in longitudinal smooth muscle strips from sheep duodenum, in vitro. 2. Mn2+ (0.5-1.5 mM) and Zn2+ (0.5-5 mM) inhibited both the amplitude and frequency of motility in Krebs solution and in Ca(2+)-free medium. 3. Ba2+ (0.5-5 mM) evoked three types of contractile responses: (i) an increase in the frequency and a reduction of the amplitude of spontaneous contractions; (ii) a slight increase in muscle tone of the phasic contractions; and (iii) a rapid initial phasic contraction followed by slowly fading contraction. Ca2+ induced two kinds of responses in spontaneous motility: (i) a fast phasic contraction, followed by an increase in the amplitude and frequency of phasic contractions with no changes in its tone; and (ii) an increase in the amplitude of contractions. 4. The Ba(2+)-induced contractions were inhibited by EDTA, verapamil and diltiazem, but were not modified by sodium nitroprusside. The Ca(2+)-induced contractions were reduced by verapamil and diltiazem. 5. Our results show that Mn2+ and Zn2+ behave as inhibitors of sheep duodenum motility. In contrast, Ba2+ and Ca2+ stimulate motility. It is suggested that Ba2+ can penetrate the cells through voltage-dependent Ca2+ channels and behave as a partial substitute for Ca2+.

Effects of 5-hydroxytryptamine agonists on myoelectric activity of the forestomach and antroduodenal area in sheep.

To increase knowledge of the role of 5-hydroxytryptamine (5-HT) receptors in the regulation of reticuloruminal, omasal and antroduodenal myoelectric activity in sheep, the effects of 5-HT agonists on forestomach and antroduodenal myoelectric activity have been investigated in conscious sheep. 5-Carboxamidotryptamine, methysergide, alpha-methyl-5-HT, 2-methyl-5-HT, cisapride, zacopride or metoclopramide were infused intravenously for 5 min and myoelectric recordings were obtained from electrodes chronically implanted in the reticulum, rumen (dorsal sac), omasal body, abomasal antrum and duodenal bulb. The integrated activity of the reticular and ruminal spike bursts was modified only by the highest doses of alpha-methyl-5-HT, 2-methyl-5-HT, metoclopramide and cisapride. A phase III-like activity pattern was recorded in the antroduodenal area with all 5-HT-ergic agents and a dose-dependent inhibition of myoelectric activity was recorded in both reticulorumen and omasum at the same time as the antroduodenal effects. In the forestomach, methysergide alone induced inhibition of ruminal secondary contractions; 5-HT, alpha-methyl-5-HT, cisapride and metoclopramide, moreover, evoked an initial dose-dependent increase in antral activity. These results suggest that 5-HT1, 5-HT2, 5-HT3 and 5-HT4 receptors are involved in the regulation of the migrating myoelectric complex in sheep and in the genesis of forestomach hypomotility that is occasionally recorded concomitantly with the spontaneous duodenal phase III in sheep. 5-HT4 receptors also have a prokinetic action in the antral area.

5-Hydroxytryptamine induces forestomach hypomotility in sheep through 5-HT4 receptors.

The effects evoked by 5-hydroxytryptamine (5-HT; serotonin) on forestomach myoelectric activity were investigated in conscious sheep. Myoelectric signals were recorded with electrodes chronically implanted in the reticulum, rumen (dorsal sac) and omasal body, and were analysed by a computer-based method. The 5-HT receptors and the neuronal pathways involved in these actions were studied. The intravenous (i.v.) infusion of 5-HT (8 micrograms kg-1 min-1 for 5 min) evoked an inhibition of activity of the whole forestomach. Methiothepin, injected i.v. at 0.1 mg kg-1, inhibited rumen secondary contractions and omasum activity. However, forestomach activity remained unchanged after the administration of 0.2 mg kg-1 of ketanserin, ondansetron, tropisetron, GR-113808, phentolamine, propranolol, domperidone and naloxone. Atropine (0.2 mg kg-1), hexamethonium (2 mg kg-1) or haloperidol (0.1 mg kg-1) abolished rumen secondary cycles and inhibited omasum activity. In addition, atropine also suppressed primary cycles. GR-113808 blocked all 5-HT-induced effects. Furthermore, atropine or hexamethonium prevented the 5-HT-evoked inhibition of reticulorumen primary cycles. In contrast, the remaining antagonists did not alter the 5-HT-evoked forestomach hypomotility. In conclusion, 5-HT induces inhibition of forestomach myoelectric activity through 5-HT4 receptors, these actions being mediated by cholinergic neural pathways involving muscarinic and nicotinic receptors. However, adrenergic, dopaminergic or opiate pathways are not implicated.

Effect of motilin, somatostatin and bombesin on gastroduodenal myoelectric activity in sheep.

The effects of motilin, erythromycin, somatostatin and bombesin on antroduodenal myoelectric activity were investigated in conscious sheep. Myoelectric recordings were obtained from electrodes chronically implanted on the antrum and duodenal bulb. Peptides or erythromycin were infused intravenously (i.v.) during 5 min. Antagonists were injected i.v. as a bolus. Neither motilin (2.5-80 ng/kg/min) nor erythromycin (2-16 micrograms/kg/min) modified the antroduodenal myoelectric activity, although a single bolus of these compounds (250 ng/kg and 50 micrograms/kg respectively) increased the antral activity. Somatostatin at 5 ng/kg/min induced a decrease in the myoelectric activity of antrum and duodenum. However, doses of 10 to 40 ng/kg/min evoked a duodenal phase III-like activity with a subsequent quiescence period and a concomitant inhibition of the antral activity. These effects were reproduced by bombesin (2.5 to 40 ng/kg/min). Furthermore, an initial increase in the myoelectric activity and in the frequency of slow waves were recorded in the antrum when the highest doses were used. On the other hand, atropine (0.2 mg/kg) or hexamethonium (2 mg/kg) caused a long-lasting inhibition of antroduodenal myoelectric activity. These cholinergic antagonists abolished the effects induced by somatostatin (20 ng/kg/min) but not those evoked by bombesin but not motilin are putative modulators of the migrating myoelectric complex (MMC) in sheep. Moreover, a cholinergic neural pathway is involved in the somatostatin but not in the bombesin-induced effects.