Effect of indomethacin on bombesin-like immunoreactivity, somatostatin and gastrin secretion from rat stomach

In the present study the effect of indomethacin-induced prostaglandin deficiency was examined on the release of bombesin-like immunoreactivity (BLI), a putative peptidergic neurotransmitter, from the isolated perfused rat stomach. In addition, gastrin and somatostatin (SLI) secretion was determined. Pretreatment of rats with indomethacin (2 mg/kg X h) resulted in a 3-fold increase of basal BLI secretion. In response to acetylcholine (2 X 10(-6) M) BLI rose from 2,000 to 4,000 pg/min, whereas in controls BLI increased from 400 to 1,400 pg/min. While absolute values for BLI secretion were higher in indomethacin-treated stomachs the relative increase above baseline was lower (100 vs. 250%). In control rats the increase in BLI secretion in response to acetylcholine was abolished when the acidity in the gastric lumen was increased from pH 7 to pH 2. After indomethacin, however, the stimulatory effect of acetylcholine during luminal pH 7 and pH 2 was identical. The decrease of SLI by acetylcholine at luminal pH 7 was abolished in indomethacin-treated stomachs in response to 10(-6) M acetylcholine, and 2 X 10(-6) M had even a stimulatory effect on SLI secretion. Indomethacin pretreatment reduced gastrin secretion at luminal pH 7. These data demonstrate that endogenous prostaglandins exert an inhibitory tone on basal and stimulated BLI and stimulated SLI secretion in the rat stomach. It is suggested that endogenous prostaglandins also inhibit the release of a peptidergic neurotransmitter, similar to their effect on the classical neurotransmitters acetylcholine and norepinephrine.     

Vagally induced release of gastrin, somatostatin and bombesin-like immunoreactivity from perfused rat stomach. Effect of stimulation frequency and cholinergic mechanisms

The isolated stomach of rats was vascularly perfused to measure the secretion of gastrin, somatostatin (SLI) and bombesin-like immunoreactivity (BLI). The gastric lumen was perfused with saline pH 7 or pH 2, and electrical vagal stimulation was performed with 1 ms, 10 V and 2, 5 or 10 Hz, respectively. Atropine was added in concentrations of 10⁻⁹ or 10⁻⁷ M to evaluate the role of cholinergic mechanisms. In control experiments, vagal stimulation during luminal pH 2 elicited a significant increase of BLI secretion only at 10 Hz but not at 2 and 5 Hz. Somatostatin release was inhibited independent of the stimulation frequency employed. Gastrin secretion at 2 Hz was twice the secretion rates observed at 5 and 10 Hz, respectively. At luminal pH 7 BLI rose significantly at 5 and 10 Hz. SLI secrtion was decreased by all frequencies. Gastrin secretion at 2 and 5 Hz was twice as high as during stimulation with 10 Hz. Atropine at doses of 10⁻⁹, 10⁻⁸, 10⁻⁷ and 10⁻⁶ M had no effect on basal secretion of BLI, SLI and gastrin. At luminal pH 2, atropine increased dose-dependently the BLI response at 2 and 5 but not at 10 Hz. The decrease of SLI during 2 and 5 Hz but not 10 Hz was abolished by atropine 10⁻⁹ M. SLI was reversed to stimulation during atropine 10⁻⁷ M at all frequencies. The rise of gastrin at 2 Hz was reduced by 50%. At luminal pH 7, atropine had comparable effects with a few differences: the BLI response at 10 Hz was augmented and the gastrin response to 2 and 5 Hz was reduced. In conclusion the present data demonstrate a frequency and pH-dependent stimulation of BLI and gastrin release. The stimulation of BLI is predominantly due to atropine-insensitive mechanisms while muscarinic cholinergic mechanisms exert an inhibitory effect on BLI release during lower stimulation frequencies (2 and 5 Hz) independent of the intragastric pH and also during higher frequencies at neutral pH. Both, atropine sensitive and insensitive mechanisms are activated frequency dependent. The atropine-sensitive cholinergic mechanisms but not the noncholinergic mechanisms involved in regulation of G-cell function are pH and frequency dependent. Somatostatin is regulated largely independent of stimulation frequency and pH by at least two pathways involving cholinergic mechanisms of different sensitivity to atropine. These data suggest a highly differentiated regulation of BLI, gastrin and SLI secretion and the interaction between these systems awaits further elucidation.     

Release of bombesin-like immunoreactivity from the isolated perfused rat stomach

In the present study the release of bombesin-like immunoreactivity (BLI), somatostatin and gastrin was determined form the isolated perfused rat stomach. Gastric inhibitory polypeptide (GIP, 2 X 10(-9) M) had no effect on BLI while stimulating somatostatin and gastrin release. In these experiments the luminal pH of the stomach was kept at pH 7. Reduction of the luminal pH to 2 resulted in an inhibition of BLI secretion by GIP while gastrin release was abolished and somatostatin remained unaffected compared to luminal pH 7. Acetylcholine (10(-6) and 2 X 10(-6) M) elicited a dose-dependent stimulation of BLI secretion while gastrin was stimulated and somatostatin secretion suppressed independent of the administered dose. The present data demonstrate that release of bombesin-like immunoreactivity can be modulated by intestinal hormones and neurotransmitters and is integrated into the complex system of gastrointestinal neuroendocrine regulation.     

Effect of vasoactive intestinal peptide, peptide histidine isoleucine and growth hormone-releasing factor-40 on bombesin-like immunoreactivity, somatostatin and gastrin release from the perfused rat stomach

Bombesin-like immunoreactivity (BLI) has been demonstrated in neurons of the gastrointestinal tract and gastric BLI secretion can be demonstrated in response to the classical neurotransmitter acetylcholine. Since structurally related peptides VIP, PHI and GRF have to be considered as peptidergic neurotransmitters it was of interest to determine their effect on gastric BLI secretion. Additionally, somatostatin (SLI) and gastrin secretion was examined. The isolated stomach of overnight fasted rats was perfused with Krebs-Ringer buffer via the celiac artery and the effluent was collected via the portal vein. The gastric lumen was perfused with isotonic saline at pH7 or pH2. All four peptides were tested at a dose of 10(-11) M and 10(-8) M at both pH levels and in addition the effect of VIP and PHI was examined at 10(-14) M and 10(-12) M during luminal pH2. At luminal pH7 VIP and PHI stimulated SLI release at 10(-8) M but had no effect on BLI or gastrin secretion. rGRF and hpGRF were both ineffective on SLI and gastrin release while rGRF inhibited and hpGRF stimulated BLI secretion. This effect was not dose related. At luminal pH2 all four peptides stimulated BLI secretion. Stimulation by PHI was already observed at a dose of 10(-14) M while VIP elicited a stimulatory effect at 10(-12) M. PHI at the two lowest concentrations of 10(-14) and 10(-12) M elicited a stimulation of SLI and gastrin release while the same doses of VIP and the higher doses of all four peptides had no effect on SLI and gastrin secretion at an acidic intraluminal pH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Somatostatin and gastrin release into the gastric lumen in rats

Somatostatin and gastrin release into the gastric lumen was investigated in anaesthetized, vagally intact rats. The stomach was perfused at a flow rate of 0.5 mL.min-1. During perfusion with 0.1 M HCl or buffers of varying pH the somatostatin ans gastrin concentrations in the perfusate were less than 10 pg.mL -1 and approximately 30 pg.mL-1, respectively. Peptone caused a gastrin concentrations in the perfusate were less than 10 pg.mL-1 and approximately 30 pg.mL-1, respectively. Peptone caused a slight pH-independent increase in somatostatin release; gastrin release was unchanged despite an increase in serum gastrin from a basal of 15 +/- 4 to 155 +/- 34 pg.mL-1 during peptone stimulation. intravenous infusion of carbachol (1 microgram.kg-1.min-1) strongly stimulated luminal somatostatin and gastrin release (from 5 +/- 1 to 192 +/- 52 pg.mL-1 and from 27 +/- 5 to 198 +/- 41 pg.mL-1, respectively) during perfusion with 0.1 M HCl. Phosphate buffer perfusion at pH 7.5 abolished the cholinergic-mediated somatostatin release but the gastrin response was unaffected. It is suggested that changes of luminal hormone concentrations in the rat stomach do not reflect the secretory activity of the endocrine cells in the gastric mucosa.     

Serotoninergic control of somatostatin and gastrin release from the isolated rat stomach

The influence of exogenous serotonin on the secretion of gastric somatostatin and gastrin was investigated under in vitro conditions using an isolated, vascularly perfused rat stomach preparation. Serotonin stimulated gastrin release, maximal effects were observed at 10(-6) M which increased gastrin levels by 78%; on the contrary, somatostatin secretion was inhibited (maximal inhibition of 56% at 10(-6) M). Changes in hormone secretion in response to serotonin were reversed by combined blockade of 5-HT1 and 5-HT2 receptors by methysergide and blockade of 5-HT2 receptors by ketanserin (10(-5) and 10(-6) M, respectively), and of cholinoreceptors by atropine (10(-5) M). It is concluded that in rats in vitro serotonin inhibits release of gastric somatostatin and stimulates gastrin secretion via specific serotonin receptors but muscarinic cholinergic receptors are also involved.     

Effects of naloxone on basal and vagus nerve-induced secretions of GRP, gastrin, and somatostatin from the isolated perfused rat stomach

The effects of naloxone, an opiate antagonist, on basal and vagus nerve-induced secretions of GRP, gastrin, and somatostatin were examined using the isolated perfused rat stomach prepared with vagal innervation. Naloxone (10(-6) M) significantly inhibited basal somatostatin secretion in the presence and absence of atropine and of hexamethonium, whereas basal GRP and gastrin secretion was not affected by naloxone. Electrical stimulation (10 Hz, lms duration, 10V) of the distal end of the subdiaphragmatic vagal trunks elicited a significant increase in both GRP and gastrin but a decrease in somatostatin. Naloxone (10(-6) M) failed to affect these responses in the presence or absence of atropine. On the other hand, when hexamethonium was infused, naloxone significantly inhibited both the GRP and gastrin responses to electrical vagal stimulation. Somatostatin secretion was unchanged by vagal stimulation during the infusion of hexamethonium with or without naloxone. These findings suggest that basal somatostatin secretion is under the control of an opiate neuron and that opioid peptides might be involved in vagal regulation of GRP and gastrin secretion.     

Neural, hormonal, and paracrine regulation of gastrin and acid secretion.

Physiological stimuli from inside and outside the stomach coverage on gastric effector neurons that are the primary regulators of acid secretion. The effector neurons comprise cholinergic neurons and two types of non-cholinergic neurons: bombesin/GRP and VIP neurons. The neurons act directly on target cells or indirectly by regulating release of the hormone, gastrin, the stimulatory paracrine amine, histamine, and the inhibitory paracrine peptide, somatostatin. In the antrum, cholinergic and bombesin/GRP neurons activated by intraluminal proteins stimulate gastrin secretion directly and, in the case of cholinergic neurons, indirectly by eliminating the inhibitory influence of somatostatin (disinhibition). In turn, gastrin acts on adjacent somatostatin cells to restore the secretion of somatostatin. The dual paracrine circuit activated by antral neurons determines the magnitude of gastrin secretion. Low-level distention of the antrum activates, preferentially, VIP neurons that stimulate somatostatin secretion and thus inhibit gastrin secretion. Higher levels of distention activate predominantly cholinergic neurons that suppress antral somatostatin secretion and thus stimulate gastrin secretion. In the fundus, cholinergic neurons activated by distention or proteins stimulate acid secretion directly and indirectly by eliminating the inhibitory influence of somatostatin. The same stimuli activate bombesin/GRP and VIP neurons that stimulate somatostatin secretion and thus attenuate acid secretion. In addition, gastrin and fundic somatostatin influence acid secretion directly and indirectly by regulating histamine release. Acid in the lumen stimulates somatostatin secretion, which attenuates acid secretion in the fundus and gastrin secretion in the antrum.     

Vagal modulation of arginine- and glucagon-induced pancreatic somatostatin secretion

In order to elucidate the role of the vagus nerve in the regulation of pancreatic somatostatin secretion, the effect of electrical stimulation of the vagus on the isolated perfused rat pancreas was studied. Somatostatin release induced by 19 mM arginine in the presence of 11 mM glucose or 10(-6)M glucagon in the presence of 5.5 mM glucose was suppressed by vagal stimulation. This suppressive effect on somatostatin was eliminated in the presence of 10(-5)M atropine plus glucagon, while somatostatin release was significantly enhanced in the presence of atropine plus arginine. We conclude that pancreatic somatostatin secretion may be regulated not only by a cholinergic inhibitory neuron but also by a stimulatory non-cholinergic neuron.     

Somatostatin,prostaglandin E2 and atropine inhibition of the gastric actions of bombesin in the dog

Bombesin, acetylcholine, prostaglandins and somatostatin are all thought to be involved in the regulation of gastrin release and gastric secretion. We have studied the effects of low doses of atropine, 16-16(Me)₂-prostaglandin E₂ (PGE₂) and somatostatin-14 on bombesin-stimulated gastrin release and gastric acid and pepsin secretion in conscious fistula dogs. For reference, synthetic gastrin G-17 was studied with and without somatostatin. Bombesin, in a dose-related manner, increased serum gastrin, which in turn stimulated gastric acid and pepsin secretion in a serum gastrin, concentration-dependent manner. Somatostatin inhibited gastrin release by bombesin as well as the secretory stimulation by G-17; the combination of sequential effects resulted in a marked inhibition of bombesin-stimulated gastric acid and pepsin secretion. PGE₂ also strongly inhibited gastrin release and acid and pepsin secretion. Atropine had no significant effect on gastrin release, but greatly inhibited gastric secretion. Thus somatostatin and PGE₂ inhibited at two sites, gastrin release and gastrin effects, while atropine affected only the latter.     

Control of rat gastric somatostatin release by gamma-aminobutyric acid (GABA)

The influence of gamma-aminobutyric acid (GABA) on gastric somatostatin and gastrin release was studied using an isolated perfused rat stomach preparation. GABA dose-dependently inhibited somatostatin release (maximal inhibition of 44% at 10(-5)M GABA), whereas gastrin secretion was not affected. The GABA agonist muscimol led to a decrease in somatostatin release of similar magnitude. The GABA-induced changes were partially reversed by 10(-5)M atropine. Gastrin secretion was not influenced by either protocol. It is concluded that GABA as a putative neurotransmitter in the enteric nervous system is inhibitory to rat gastric somatostatin release in vitro via cholinergic pathways.     

Effect of galanin on gastrin and somatostatin release from the rat stomach

Galanin has been shown to be present in the gastrointestinal tract, pancreas and CNS. In the rat stomach, immunohistochemical studies have revealed the presence of galanin in the intrinsic nervous system suggesting a function as putative neurotransmitter or neuromodulator which could affect neighbouring exo- or endocrine cells. Therefore this study was performed to determine the effect of galanin on the secretion of gastrin and somatostatin-like immunoreactivity (SLI) from the isolated perfused rat stomach. The stomach was perfused via the celiac artery and the venous effluent was collected from the portal vein. The luminal content was kept at pH 2 or 7 Galanin at a concentration of 10(-10), 10(-9) and 10(-8) M inhibited basal gastrin release by 60-70% (60-100 pg/min; p less than 0.05) at luminal pH 7. At luminal pH 2 higher concentrations of galanin (10(-9) and 10(-8) M) decreased basal gastrin secretion by 60-70% (60-100 pg/min; p less than 0.05). This inhibitory effect was also present during infusion of neuromedin-C, a mammalian bombesin-like peptide that stimulates gastrin release. SLI secretion remained unchanged during galanin administration. The inhibitory action of galanin on gastrin secretion was also present during the infusion of tetrodotoxin suggesting that this effect is not mediated via neural pathways. The present data demonstrate that galanin is an inhibitor of basal and stimulated gastrin secretion and has to be considered as an inhibitory neurotransmitter which could participate in the regulation of gastric G-cell function.     

Somatostatin, misoprostol and galanin inhibit gastrin- and PACAP-stimulated secretion of histamine and pancreastatin from ECL cells by blocking specific Ca2+ channels

The oxyntic mucosa is rich in ECL cells. They secrete histamine and chromogranin A-derived peptides, such as pancreastatin, in response to gastrin and pituitary adenylate cyclase-activating peptide (PACAP). Secretion is initiated by Ca2+ entry. While gastrin stimulates secretion by opening L-type and N-type Ca2+ channels, PACAP stimulates secretion by activating L-type and receptor-operated Ca2+ channels. Somatostatin, galanin and prostaglandin E2 (PGE2) inhibit gastrin- and PACAP-stimulated secretion from the ECL cells. In the present study, somatostatin and the PGE2 congener misoprostol inhibited gastrin- and PACAP-stimulated secretion 100%, while galanin inhibited at most 60-65%. Bay K 8644, a specific activator of L-type Ca2+ channels, stimulated ECL-cell secretion, an effect that was inhibited equally effectively by somatostatin, misoprostol and galanin (75-80% inhibition). Pretreatment with pertussis toxin, that inactivates inhibitory G-proteins, prevented all three agents from inhibiting stimulated secretion (regardless of the stimulus). Pretreatment with nifedipine (10 microM), an L-type Ca2+ channel blocker, reduced PACAP-evoked pancreastatin secretion by 50-60%, gastrin-evoked secretion by approximately 80% and abolished the response to Bay K 8644. The nifedipine-resistant response to PACAP was abolished by somatostatin and misoprostol but not by galanin. Gastrin and PACAP raised the intracellular Ca2+ concentration in a biphasic manner, believed to reflect mobilization of internal Ca2+ followed by Ca2+ entry. Somatostatin and misoprostol blocked Ca2+ entry (and histamine and pancreastatin secretion) but not mobilization of internal Ca2+. The present observations on isolated ECL cells suggest that Ca2+ entry rather than mobilization of internal Ca2+ triggers exocytosis, that gastrin and PACAP activate different (but over-lapping) Ca2+ channels, that somatostatin, misoprostol and galanin interact with inhibitory G-proteins to block Ca2+ entry via L-type Ca2+ channels, and that somatostatin and misoprostol (but not galanin) in addition block N-type and/or receptor-operated Ca2+ channels.     

Somatostatin release induced by gastrin-releasing peptide in man. Effect of proximal gastric vagotomy and cholinergic blockade

The influence of intragastric pH on the basal release of somatostatin has been studied in healthy controls and in duodenal ulcer patients. In addition the somatostatin response to gastrin-releasing peptide infusion has been evaluated both regarding the effect of intragastric pH and the influence of vagal innervation and muscarinic blockade. No difference was found in basal blood levels, when changing the intraluminal pH, although a slightly higher basal somatostatin concentration was noticed in patients with duodenal ulcer disease. Neither proximal gastric vagotomy nor cholinergic blockade had any effect on basal somatostatin concentrations. GRP infused in stepwise increasing doses from 20 pmol/kg/h to 400 pmol/kg/h induced a small but significant response. This effect of GRP was most evident, when the stomach was perfused with 0.1 M HCl. The small, somatostatin response to GRP infusion was not influenced by vagal denervation of the parietal cell area, neither by cholinergic blockade. Despite the previously observed effects of vagotomy and cholinergic blockade on gastrin release induced by GRP, a corresponding inverse effect on somatostatin is not apparent.     

Somatostatin inhibition of gastrin gene expression: involvement of pertussis toxin-sensitive and -insensitive pathways.

These studies were performed to determine the intracellular pathways involved in regulating gastrin gene expression. The inclusion of 10(-4) M forskolin or 10(-4) M dibutyryl cyclic AMP (DBcAMP) in incubation medium containing dog antral mucosa resulted in 249% and 323% increases, respectively, in gastrin mRNA levels. The stimulatory effects of forskolin and DBcAMP were both inhibited significantly by 10(-6) M somatostatin. Preincubation of antral mucosa with pertussis toxin nearly abolished the inhibitory effects of somatostatin on gastrin mRNA stimulated by forskolin, but had no effect following DBcAMP. To examine whether calcium-dependent pathways might be involved in regulating gastrin gene expression, antral mucosa was incubated with increasing concentrations of calcium or the ionophore ionomycin. Both agents produced only modest increases in gastrin mRNA, which were abolished by the addition of somatostatin to the incubation medium. These studies indicate that somatostatin appears to inhibit gastrin gene expression through mechanisms involving both pertussis toxin-sensitive and -insensitive pathways.     

Somatostatin potentiates cholinergic neurotransmission in ferret trachea

We studied the effect of somatostatin on contractile responses to electrical field stimulation (EFS) in isolated ferret tracheal segments. Somatostatin (up to 10(-5) M) did not change resting tension, but it potentiated the contractile response to EFS dose dependently, with a maximum effect at 10(-6) M. Thus, at a concentration of 10(-6) M, somatostatin significantly decreased the mean log of EFS frequency producing 50% of maximum contraction from a control value of 0.52 +/- 0.07 to 0.24 +/- 0.06 (SE) Hz (P less than 0.01). The potentiating effect of somatostatin (10(-6) M) was not inhibited by hexamethonium, indomethacin, BW755C, pyrilamine, methysergide, or D,Pro2,D,Trp7,9-SP, but it was inhibited by atropine or by the somatostatin antagonist cyclo[7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]. In contrast to EFS-induced contraction, contractions produced by acetylcholine (10(-9) to 10(-3) M) were not affected by somatostatin at a concentration of 10(-6) M. These results suggest that somatostatin potentiates contractions produced by EFS via presynaptic cholinergic mechanisms and probably through a specific somatostatin receptor.     

The effects of substance P, histamine and histamine antagonists on somatostatin and gastrin release from the isolated perfused rat stomach

Secretion of somatostatin-like immunoreactivity (SLI) from the isolated perfused rat stomach has been shown to be inhibited by substance P. The present study was initiated to examine the possibility that this action of substance P was mediated via release of histamine. Substance P (1 microM) reduced basal secretion of SLI in agreement with earlier studies. Neither pyrilamine nor cimetidine influenced this action. Basal immunoreactive gastrin (IRG) secretion was unaffected by substance P. Addition of pyrilamine during substance P perfusion increased IRG secretion whereas addition of cimetidine resulted in a delayed decrease on removal of both compounds. Histamine (1 and 10 microM) increased SLI secretion and reduced IRG secretion. Pyrilamine increased and cimetidine decreased IRG secretion but neither drug influenced SLI secretion. Pyrilamine had no effect on histamine-stimulated SLI secretion but inhibition of IRG secretion by histamine was converted to stimulation. Cimetidine potentiated histamine stimulation of SLI secretion and inhibition of IRG secretion. In conclusion: (1) substance P inhibition of SLI secretion is unlikely to be mediated via release of histamine. (2) The gastrin cell appears to have both H1- and H2-receptors which mediate opposite actions but H1-receptor-mediated inhibition is predominant. (3) Histamine weakly stimulates SLI secretion but there may be both inhibitory and stimulatory pathways acting via H2- and H1-receptors, respectively.     

The effect of somatostatin on baseline and stimulated acid secretion and vascular histamine release from the totally isolated vascularly perfused rat stomach

The effect of somatostatin-(1-14) (S1-14) on the gastrin- and histamine-induced acid secretion and gastrin-evoked vascular histamine release was studied in isolated vascularly perfused rat stomachs being continuously perfused by a gassed buffer containing 10% ovine erythrocytes and 50 microM isobutyl methylxanthine (IMX). Concentrations of gastrin (520 pM) and histamine, (0.5 microM) were chosen to give acid secretion in the same range (61.5 +/- 7.0 and 49.4 +/- 9.4 mumol/60 min). S1-14 induced a concentration-dependent decrease in acid secretion stimulated by both gastrin and histamine. Even at the lowest concentration examined (0.1 nM) somatostatin gave a significant inhibition of both gastrin- and histamine-stimulated acid secretion. The inhibitory effect was, however, most marked for gastrin-stimulated acid secretion (P less than 0.05 at 1 nM concentration of S1-14). Gastrin gave an immediate and marked vascular histamine release which was inhibited by somatostatin in the higher concentrations (1.0 and 5.0 nM). Somatostatin at the lowest concentration tested (0.1 nM) did not inhibit the gastrin-induced vascular histamine release although it did inhibit acid secretion. Furthermore, baseline histamine release was not affected by somatostatin. This study suggests that somatostatin inhibits acid secretion both via a direct effect of the parietal cell and by inhibiting gastrin-induced histamine release. Baseline histamine release is regulated by a mechanism not sensitive to somatostatin.     

Intestinal serotonin acts as paracrine substance to mediate pancreatic secretion stimulated by luminal factors

We recently demonstrated that luminal factors such as osmolality, disaccharides, and mechanical stimulation evoke pancreatic secretion by activating 5-hydroxytryptamine subtype 3 (serotonin-3, 5-HT3) receptors on mucosal vagal afferent fibers in the intestine. We hypothesized that 5-HT released by luminal stimuli acts as a paracrine substance, activating the mucosal vagal afferent fibers to stimulate pancreatic secretion. In the in vivo rat model, luminal perfusion of maltose or hypertonic NaCl increased 5-HT level threefold in intestinal effluent perfusates. Similar levels were observed after intraluminal 10(-5) M 5-HT perfusion. These treatments did not affect 5-HT blood levels. In a separate study, intraduodenal, but not intraileal, 5-HT application induced a dose-dependent increase in pancreatic protein secretion, which was not blocked by the CCK-A antagonist CR-1409. Acute vagotomy, methscopolamine, or perivagal or intestinal mucosal application of capsaicin abolished 5-HT-induced pancreatic secretion. In conscious rats, luminal 10(-5) M 5-HT administration produced a 90% increase in pancreatic protein output, which was markedly inhibited by the 5-HT3 antagonist ondansetron. In conclusion, luminal stimuli induce 5-HT release, which in turn activates 5-HT3 receptors on mucosal vagal afferent terminals. In this manner, 5-HT acts as a paracrine substance to stimulate pancreatic secretion via a vagal cholinergic pathway.