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.     

Effect of somatostatin analogs on gastric acid secretion in dogs and rats

The effects of several superactive analogs of somatostatin on gastric acid response to various exogenous and endogenous stimulants were investigated in conscious dogs and rats with gastric fistulae (GF). The inhibition was compared to that induced by somatostatin-14 (S-S-14) at two dose levels. Several octapeptide analogs of somatostatin including D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Trp-NH2 (RC-160) and D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2 (RC-121), which were superactive in tests on suppression of GH levels, were 4-5 times more potent than S-S-14 in inhibiting desglugastrin-stimulated gastric acid secretion in GF dogs. The analog RC-160 also reduced the rise in serum gastrin levels and gastric acid secretion induced by sham feeding (SF) in dogs with gastric and esophageal fistulae (EF), but did not decrease food consumption. Gastric acid secretion induced by histamine (80 micrograms/kg/h) in dogs was not affected by 1-5 micrograms/kg/h of analog RC-121 or by 5 micrograms/kg/h of S-S-14. Analogs RC-160, RC-121, and RC-98-I (D-Trp-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-NH2) and others also powerfully inhibited desglugastrin-induced potent as S-S-14 in dogs but its activity was higher in rats. The results indicate that octapeptide analogs which are superactive in GH-inhibition tests are also more potent than S-S-14 in suppressing gastric acid secretion. These findings may be of clinical value.     

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.     

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.     

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.     

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)     

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.     

Effect of vagotomy and atropine on plasma somatostatin response to a meal in conscious dogs

In 4 conscious dogs with gastric fistulas the somatostatin responses to a meal were measured and compared to the responses seen after i.v. infusion of atropine sulfate (20 and 50 micrograms.kg-1.h-1) or cimetidine (8 mg.kg-1.h-1). The experiments were repeated after truncal vagotomy. The somatostatin responses to bombesin (0.5 micrograms.kg-1.h-1) were also measured before and after vagotomy. Vagotomy decreased basal and postprandial somatostatin levels and reduced the somatostatin responses to feeding during the first 30-min period following the ingestion of the meal but not during subsequent periods. Bombesin-induced somatostatin release was increased after vagotomy. Atropine decreased the somatostatin responses to the meal before and after vagotomy. Cimetidine had no significant effect. These studies suggest that, in conscious dogs, somatostatin released into the circulation is partly under vagal control and that, as for gastrin release, vagal pathways for stimulation and inhibition are present. Our studies also suggest that cholinergic mechanisms are involved in the control of postprandial somatostatin release.     

The effect of atropine on bombesin and gastrin releasing peptide stimulated gastrin, pancreatic polypeptide and neurotensin release in man

The effects of 1-h infusions of bombesin and gastrin releasing peptide (GRP) at 50 pmol/kg per h and neurotensin at 100 pmol/kg per h on gastrin, pancreatic polypeptide (PP) and neurotensin release in man were determined following either saline or atropine infusion (20 micrograms/kg). Bombesin produced a rise in plasma neurotensin from 32 +/- 6 to 61 +/- 19 pmol/l and of PP from 26 +/- 8 to 36 +/- 7 pmol/l. There was a further rise of plasma PP to 50 +/- 13 pmol/l after cessation of the infusion. GRP had no significant effect on plasma neurotensin, but compared to bombesin, produced a significantly greater rise in plasma PP from 34 +/- 6 to 66 +/- 19 pmol/l during infusion. There was no post-infusional increase. At this dose, GRP was as effective as bombesin in releasing gastrin, although unlike bombesin its effect was enhanced by atropine. Neurotensin produced a rise in plasma PP from 17 +/- 4 to 38 +/- 8 pmol/l. Atropine blocked the release of PP during GRP and neurotensin infusion. Atropine had no effect on neurotensin or PP release during bombesin infusion, but did block the rise in plasma PP following bombesin infusion. We conclude that, in contrast to meal-stimulated neurotensin release, bombesin-stimulated neurotensin release is cholinergic independent. Despite structural homology, bombesin and GRP at the dose used are dissimilar in man in their actions and sensitivity to cholinergic blockade.     

The effect of somatostatin and 16,16-dimethyl-prostaglandin E2 on bombesin-stimulated canine gastric acid, plasma gastrin and pancreatic polypeptide secretion

We studied the effect of the intravenous infusion of 16,16-dimethylprostaglandin E2 methyl ester (di-M-PGE2) and somatostatin on bombesin-stimulated gastric acid secretion, plasma gastrin and plasma pancreatic polypeptide in four chronic gastric fistula dogs. Bombesin-stimulated gastric acid secretion was significantly inhibited by somatostatin and virtually abolished by di-M-PGE2. Both agents caused significant, but indistinguishable inhibition of gastrin release (P less than 0.05). Bombesin-stimulated pancreatic polypeptide release was also significantly inhibited by both somatostatin and di-M-PGE2; the inhibitory effect of somatostatin was significantly greater than that of di-M-PGE2 (P less than 0.05). This study provides further evidence in support of the complex interrelationships between agents responsible for the modulation of gastrointestinal physiology.     

Modulation of acetylcholine-induced secretion of gastric bombesin-like immunoreactivity by cholinergic and histamine H2-receptors, somatostatin and intragastric pH

Recently we have shown the release of bombesin-like immunoreactivity (BLI) from the isolated perfused rat stomach. In these experiments we have shown that BLI secretion is stimulated by acetylcholine. Gastric inhibitory peptide (GIP) exerts an inhibitory effect which is dependent on the intraluminal pH. The present study was designed to examine further the exact cholinergic mechanisms and to study the interaction between cholinergic and histaminergic mechanisms as well as the effect of the intraluminal pH. Acetylcholine elicited a dose-dependent increase in BLI and gastrin secretion (10(-6) M and 2 X 10(-6)M), whereas somatostatin release was suppressed at luminal pH 7. Blockade of muscarinic cholinergic receptors by atropine (10(-5)M) and nicotinic cholinergic receptors by hexamethonium (10(-5) M) abolished the effect of acetylcholine on all three peptides. Reduction of the intraluminal pH to 2 also abolished acetylcholine-induced stimulation of BLI and gastrin secretion and the inhibition of somatostatin secretion. Changes of intraluminal pH per se had no effect on the secretion of either peptide. Somatostatin (10(-7) M) reduced both BLI and gastrin secretion during stimulation with acetylcholine. The addition of the H2-receptor antagonist cimetidine (10(-5) M) abolished the effect of both doses of acetylcholine on BLI and somatostatin secretion and also the effect of the lower dose of acetylcholine (10(-6) M) on gastrin secretion during luminal pH 7. At luminal pH 2 cimetidine did not alter BLI and somatostatin secretion in response to acetylcholine, however, gastrin release was augmented in the presence of cimetidine. These data demonstrate that the effect of acetylcholine on BLI, gastrin, and somatostatin secretion is mediated by muscarinic and nicotinic cholinergic receptors and also by histamine H2-receptors. Somatostatin inhibits cholinergically induced BLI secretion. The cholinergic effects on BLI, somatostatin and gastrin secretion are abolished during an acidic intragastric pH. In this isolated perfused rat stomach model the inhibitory effect of intraluminal acid on gastrin secretion is, at least in part, mediated by H2-receptors. This suggests that the secretion of bombesin, a potential peptidergic neurotransmitter is modulated by neural, endocrine and local tissue factors and also by alterations of intragastric pH.     

Pancreatic somatostatin-like immunoreactivity suppresses gastric acid secretion in rats.

Somatostatin-like immunoreactivity (SLI) was extracted from the canine pancreas and purified by ion exchange, affinity chromatography and gel filtration. The 1600 dalton fraction, which is physicochemically similar to synthetic somatostatin was infused into the peripheral circulation of anesthetized rats and its effect upon gastric acid secretion was compared with that of synthetic somatostatin. Both synthetic somatostatin and pancreatic SLI in a dose of 7–8 μg/kg/h suppressed pentagastrin-stimulated gastric acid secretion. It is concluded that the highly purified 1600 dalton fraction of canine pancreatic SLI, like synthetic somatostatin, can exert biological activity upon the stomach of rats.     

Role of cholecystokinin in the control of gastric somatostatin in the rat: in vivo and in vitro studies.

Cholecystokinin (CCK) has been shown to be a powerful stimulus for somatostatin release from isolated canine fundic D-cells in short-term culture. The influence of the CCK analogue caerulein on the secretory activity of the D-cell in the intact stomach in vitro and the effect of elevated plasma levels of endogenous CCK on gastric somatostatin stores in vivo were investigated in the rat. Basal somatostatin secretion from the isolated, vascularly perfused rat stomach preparation was not affected by various doses of caerulein. Slight stimulation of somatostatin-like immunoreactivity (SLI) release by epinephrine was significantly inhibited by caerulein, whereas caerulein did not alter half-maximal stimulation of SLI secretion by isoproterenol. Rats with chronically elevated plasma CCK levels induced by experimental exocrine pancreatic insufficiency did not show any change in tissue concentrations of SLI or in D-cell number, both in the antrum and corpus. These data suggest that CCK--in contrast to dogs--is not an important modulator of gastric somatostatin in the rat.     

Effect of CCK-8 on basal and meal-stimulated somatostatin in the baboon

We measured the ability of CCK-8 alone, a test meal alone, or a combination of the two, to increase peripheral plasma somatostatin levels in the baboon. Baboons received a five-minute intravenous infusion of either CCK-8 (1, 2, or 4 micrograms/kg) or saline prior to a 30-minute meal. CCK-8 administration at all doses resulted in a significant rise of plasma somatostatin-like immunoreactivity (SLI). In addition, ingestion of a meal following a control saline infusion resulted in a significant rise of plasma SLI. However, the meal-related rise in SLI was blunted by prior administration of CCK-8 at all doses, including a dose which did not significantly decrease meal size. CCK-8 administration at all doses also blunted the meal-related rise of plasma insulin and glucose. We conclude that the known ability of CCK-8 to inhibit gastric emptying, as well as to decrease meal size, may account for its suppression of the meal-related SLI release.     

Gastric inhibitory polypeptide stimulated secretion of somatostatinlike immunoreactivity from the stomach: inhibition by acetylcholine or vagal stimulation

The possible involvement of gastric somatostatinlike immunoreactivity (SLI) in the acid inhibitory action of gastric inhibitory polypeptide (GIP) was studied in an isolated perfused rat stomach. GIP, in a dose of 5 or 50 ng/mL, caused a 4- and 12-fold increase in SLI secretion, respectively. At the higher dose level the stimulated secretory rate declined throughout the perfusion suggesting that secretion exceeded the capacity to synthesize SLI under excessive GIP stimulation. Acetylcholine (10 microM) or vagal stimulation (7 V, 10 Hz, 5 ms) completely inhibited GIP-stimulated SLI secretion. It is therefore proposed that the acid inhibitory activity of GIP is probably mediated via release of gastric SLI and this action is under cholinergic control.     

Inhibitory action of galanin on gastric acid secretion in pentobarbital-anesthetized rats

The effects of galanin and two galanin fragments, GAL(9-29) and GAL(15-29), were studied for potential effects on pentagastrin- and bethanechol-stimulated gastric acid secretion in a pentobarbital-anesthetized rat experimental model. At a dose of 10 micrograms/kg/h, galanin potently inhibited pentagastrin-stimulated gastric acid secretion whereas inhibition of bethanechol-stimulated gastric acid secretion was not statistically significant. Simultaneous iv infusion of galanin and atropine did not affect the inhibitory action of the former. In similar experiments, a GAL(15-29) fragment was completely inactive whilst GAL(9-29) retained only about 5% potency. These results indicate that galanin probably induces its inhibitory effects by acting directly on the parietal cells rather than through a cholinergic pathway. They also demonstrate that the rat gastric acid inhibitory activity of galanin depends critically on the integrity of the first fourteen N-terminal amino acids.     

Effect of gamma-aminobutyric acid on bombesin-evoked release of somatostatin and gastrin from isolated rat stomach

The effect of gamma-aminobutyric acid (GABA) on basal and bombesin (BBS)-stimulated release of somatostatin (SLI) and gastrin from isolated perfused rat stomach was examined. In the control study, BBS at a dose of 10 nM significantly stimulated release of SLI and gastrin. Infusion of GABA (1-1000 nM) caused a depression of SLI release induced by BBS (10 nM) in a dose-dependent fashion. However, at doses used in this study GABA had no effect on either basal level of SLI and gastrin or BBS-elicited gastrin release. These results indicate that GABA can specifically modulate BBS-induced SLI release from rat stomach.     

Autonomic nervous control of gastric somatostatin secretion from the perfused rat stomach

The effect of parasympathetic and sympathetic nerve stimulation on the secretion of gastric somatostatin and gastrin has been studied in an isolated perfused rat stomach preparation. Stimulation of the vagus nerve inhibited somatostatin secretion and increased gastrin release. Splanchnic nerve stimulation increased somatostatin release during simultaneous atropine perfusion, but not in its absence, whereas gastrin secretion was unchanged. The secretory activity of the gastric D-cell was therefore reciprocally influenced by the sympathetic and parasympathetic nerves but sympathetic stimulation was only effective during muscarinic blockade.     

Gastrin secretion by ovine antral mucosa in vitro

The effect on gastrin and somatostatin release in sheep of stimulatory and inhibitory peptides and pharmacological agents was investigated using an in vitro preparation of ovine antral mucosa. Carbachol stimulated gastrin release in a dose-dependent manner but had no effect on somatostatin release. As atropine blocked the effect of carbachol, cholinergic agonists appear to stimulate gastrin secretion directly through muscarinic receptors on the G-cell and not by inhibition of somatostatin secretion. Both vasoactive-intestinal peptide (VIP) and gastric-inhibitory peptide (GIP) increased somatostatin release but did not inhibit basal gastrin secretion, although VIP was effective in reducing the gastrin response to Gastrin-releasing peptide (GRP). Porcine and human GRP were stimulatory to gastrin secretion in high doses but bombesin was without effect. The relative insensitivity to GRP (not of ovine origin) previously reported from intact sheep may be caused either by a high basal release of somatostatin or by the ovine GRP receptor or peptide differing from those of other mammalian species.     

Effect of histamine and cimetidine on amino acid meal-stimulated gastrin release at a controlled intragastric pH in healthy human beings

Results of several experiments have suggested that histamine-2 receptors play an inhibitory role in regulating gastrin release. We evaluated this prospectively in healthy human beings by infusing intravenously either histamine (0.33 μg/kg/min) or cimetidine (3.33 mg/min) during a continuous 3-h intragastric infusion of a 3% mixed amino acid meal, a potent stimulus of gastrin release. In order to be certain that effects of histamine or cimetidine on gastrin release were independent of their known effects on gastric acid secretion, intragastric pH was maintained at 5.0 by in vivo intragastric titration with sodium bicarbonate or hydrochloric acid. Although histamine and cimetidine had significant effects on gastric acid secretion, neither significantly affected the rises in serum gastrin concentrations during intragastric amino acid infusion. For example, mean gastrin rises above basal concentrations were 39 ± 9 pg/ml on the control day, 39 ± 9 pg/ml on the histamine day and 44 ± 11 pg/ml on the cimetidine day (P > 0.05). Thus, blockade or stimulation of H₂-receptors at the doses tested had no effect on gastrin release in response to an amino acid meal in humans when intragastric pH was maintained at 5.0.