Bombesin and the brain-gut axis

Bombesin is the first peptide shown to act in the brain to influence gastric function and the most potent peptide to inhibit acid secretion when injected into the cerebrospinal fluid (CSF) in rats and dogs. Bombesin responsive sites include specific hypothalamic nuclei (paraventricular nucleus, preoptic area and anterior hypothalamus), the dorsal vagal complex as well as spinal sites at T9-T10. The antisecretory effect of central bombesin encompasses a variety of endocrine/paracrine (gastrin, histamine) or neuronal stimulants. Bombesin into the CSF induces an integrated gastric response (increase in bicarbonate, and mucus, inhibition of acid, pepsin, vagally mediated contractions) enhancing the resistance of the mucosa to injury through autonomic pathways. The physiological significance of central action of bombesin on gastric function is still to be unraveled.     

Studies on adenyl cyclase in Necturus gastric mucosa

Adenyl cyclase activity of Necturus gastric mucosa was determined by measuring the amount of radioactive adenosine 3′,5′-cyclic monophosphate formed from ³H-labeled adenosine triphosphate. Histamine, pentagastrin, and fluoride added in vitro significantly increased fundic adenyl cyclase activity. The dose response curves show that the affinity of pentagastrin for adenyl cyclase is greater than that of histamine, whereas the peak response to pentagastrin is less than that to histamine. Additive stimulation was not obtained when maximal doses of pentagastrin and histamine were combined. These findings suggest that gastric mucosa contains a single adenyl cyclase unit which is coupled to distinctive selectivity sites for gastrin and histamine. N-Benzyl-3-pyrrolidyl acetate methobromide (AHR-602), the muscarinic compound, caused a significant reduction in adenyl cyclase activity, indicating a different mechanism of stimulation of gastric acid secretion for cholinergic muscarinic compounds.     

Bombesin microinjected into the dorsal vagal complex inhibits vagally stimulated gastric acid secretion in the rat

Medullary sites of action for bombesin-induced inhibition of gastric acid secretion were investigated in urethane-anesthetized rats with gastric fistula. Unilateral microinjection of bombesin or vehicle into the dorsal vagal complex was performed using a glass micropipet and pressure ejection of 100 nl volume; gastric acid output was measured every 10 min by flushing the stomach. Microinjection of vehicle into the dorsal vagal complex did not alter gastric acid secretion (1.9 +/- mumol/10) from preinjection levels (2.9 +/- 0.8 mumol/10 min). Microinjection of the stable thyrotropin-releasing hormone (TRH) analog, RX 77368, at a 77 pmol dose into the dorsal vagal complex stimulated gastric acid secretion for 100 min with a peak response at 40 min (24.1 +/- 3.2 mumol/10 min). Concomitant microinjection of RX 77368 (77 pmol) with bombesin (0.6-6.2 pmol) into the dorsal vagal complex dose dependently inhibited by 35-86% the gastric acid response to the TRH analog. Bombesin (6.2 pmol) microinjected into the dorsal vagal complex inhibited by 17% pentagastrin infusion-induced stimulation of gastric acid secretion (13.2 +/- 0.8 mumol/10 min) whereas intracisternal injection induced a 69% inhibition of the pentagastrin response. These results demonstrate that the dorsal motor complex is a sensitive site of action for bombesin-induced inhibition of vagally stimulated gastric secretion. However, other medullary sites must be involved in mediating the inhibitory effect of intracisternal bombesin on pentagastrin-stimulated gastric acid secretion.     

铃蟾肽(Bombesin)对离体灌流大鼠胃的胃泌素、生长抑素、胰升糖素及胃酸释放的影响

本研究用离体大鼠胃灌流技术来观察铃蟾肽对胃-肠激素及胃酸分泌的影响。2×10~(?)mol/L铃蟾肽以0.3ml/min速度作动脉内输注,可刺激胃酸的分泌,自2.50±0.05×10~(-1)增至5.50±1.50×10~(-1)mEq/min,但与外源性五肽胃泌素无协同作用。铃蟾肽引起两次性的门脉中胃泌索及生长抑素的释放,但抑制胰升糖素释放。这三种激素的基础释放率分别为:胃泌素62±8pg,生长抑素5.9±1.1ng,胰升糖素0.40±0.03ng/min;2×10~(-8)mol/L铃蟾肽以0.3ml/min作动脉内输注,胃泌素及生长抑素的峰值分别为1,000±20pg及12.2±2.0ng/min,胰升糖素的最低值为0.17±0.05ng/min,三种激素的反应均与铃蟾肽的浓度成正比。在胃腔流出液中也可测到上述三种激素,但量要少得多。     

Use of AC Impedance Analysis to Study Membrane Changes Related to Acid Secretion in Amphibian Gastric Mucosa

We have applied transepithelial AC impedance techniques to gastric mucosa to reconcile ultrastructural and electrophysiological findings about gastric acid secretion and the mucosal barrier. By fitting impedance data measured at different HCl secretion rates to equivalent circuit models, we extracted capacitances and resistances (as measures of membrane area and ionic conductance, respectively) for the apical and basolateral membranes. The impedance measurements were found to be incompatible with earlier equivalent circuit models that modeled membrane electrical properties as lumped circuits based on one or two cell types. A distributed circuit model was developed that assumed only one dominant electrical pathway (i.e., one cell type), but that incorporated electrical effects arising from long and narrow membrane-lined structures present in the epithelium (e.g., gastric crypts, tubulovesicles, lateral intercellular spaces). This morphologically based model was found to represent the measured data accurately, and to yield values for membrane capacitances consistent with morphometric measurements of membrane areas. The main physiological conclusions from this analysis were as follows: (a) The dominant transepithelial current pathway may reside in the oxyntic cells. (b) The transepithelial conductance increase associated with the onset of acid secretion is entirely due to increased conductance of the apical membrane. This is in turn due entirely to increased area of this membrane, resulting from incorporation of tubulovesicular membrane. (c) When membrane conductances are normalized to actual membrane area by use of membrane capacitances, it turns out that acid secretion is not associated with a change in specific ionic conductance (change in conductance per unit area) at either the apical or basolateral membrane. (d) The puzzlingly low value of transepithelial resistance (≤400 Ω-cm²) arises because there are hundreds or thousands of square centimeters of actual membrane area per square centimeter chamber area. Apical membrane resistance is 25 kΩ-cm² (actual membrane area), implying a tight barrier to back-diffusion of protons.     

Role of Na-K-2Cl cotransporter-1 in gastric secretion of nonacidic fluid and pepsinogen

Na-K-2Cl cotransporter-1 (NKCC) has been detected at exceptionally high levels in the gastric mucosa of several species, prompting speculation that it plays important roles in gastric secretion. To investigate this possibility, we 1) immunolocalized NKCC protein in the mouse gastric mucosa, 2) compared the volume and composition of gastric fluid from NKCC-deficient mice and their normal littermates, and 3) measured acid secretion and electrogenic ion transport by chambered mouse gastric mucosa. NKCC was localized to the basolateral margin of parietal cells, mucous neck cells, and antral base cells. In NKCC-deficient mice, gastric secretions of Na+, K+, Cl-, fluid, and pepsinogen were markedly impaired, whereas secretion of acid was normal. After stimulation with forskolin or 8-bromo-cAMP, chambered corpus mucosa vigorously secreted acid, and this was accompanied by an increase in transmucosal electrical current. Inhibition of NKCC with bumetanide reduced current to resting levels but had no effect on acid output. Although prominent pathways for basolateral Cl- uptake (NKCC) and apical Cl- exit [cystic fibrosis transmembrane conductance regulator (CFTR)] were found in antral base cells, no impairment in gastric secretion was detected in CFTR-deficient mice. Our results establish that NKCC contributes importantly to secretions of Na+, K+, Cl-, fluid, and pepsinogen by the gastric mucosa through a process that is electrogenic in character and independent of acid secretion. The probable source of the NKCC-dependent nonacidic electrogenic fluid secretion is the parietal cell. The observed dependence of pepsinogen secretion on NKCC supports the concept that a nonacidic secretory stream elaborated from parietal cells facilitates flushing of the proenzyme from the gastric gland lumen.     

Desensitization by histamine of H2 receptor-mediated adenylate cyclase activation in the human gastric cancer cell line HGT-1

Short-term treatment of cultured HGT-1 cells with histamine produced a time-dependent (half-life: 20 min) and homologous desensitization of histamine H2 receptor activity mediating cAMP generation in HGT-1 cells and gastric acid secretion in normal gastric mucosa. Histamine treatment resulted in loss of response of the adenylate cyclase to histamine in purified plasma membranes, but had no effect on basal, vasoactive intestinal peptide (VIP)- or NaF-stimulated enzyme activities. We propose that the desensitization of gastric histamine H2 receptor by histamine evidenced in cellular or subcellular preparations from HGT-1 cells could be involved in the physiological regulation and pharmacological control of gastric cell function in man.     

Dual action of prostaglandin E2 on gastric acid secretion through different EP-receptor subtypes in the rat

We examined the role of prostaglandin E (EP) receptor subtypes in the regulation of gastric acid secretion in the rat. Under urethane anesthesia, the stomach was superfused with saline, and the acid secretion was determined at pH 7.0 by adding 50 mM NaOH. The acid secretion was stimulated by intravenous infusion of histamine or pentagastrin. Various EP agonists were administered intravenously, whereas EP antagonists were given subcutaneously 30 min or intravenously 10 min before EP agonists. PGE(2) suppressed the acid secretion stimulated by either histamine or pentagastrin in a dose-dependent manner. The acid inhibitory effect of PGE(2) was mimicked by sulprostone (EP(1)/EP(3) agonist) but not butaprost (EP(2) agonist) or AE1-329 (EP(4) agonist). The inhibitory effect of sulprostone, which was not affected by ONO-8711 (EP(1) antagonist), was more potent against pentagastrin- (50% inhibition dose: 3.6 mug/kg) than histamine-stimulated acid secretion (50% inhibition dose: 18.0 mug/kg). Pentagastrin increased the luminal release of histamine, and this response was also inhibited by sulprostone. On the other hand, AE1-329 (EP(4) agonist) stimulated the acid secretion in vagotomized animals with a significant increase in luminal histamine. This effect of AE1-329 was totally abolished by cimetidine as well as AE3-208 (EP(4) antagonist). These results suggest that PGE(2) has a dual effect on acid secretion: inhibition mediated by EP(3) receptors and stimulation through EP(4) receptors. The former effect may be brought about by suppression at both parietal and enterochromaffin-like cells, whereas the latter effect may be mediated by histamine released from enterochromaffin-like cells.     

Effect of cortisone acetate on acid secretion induced by histamine, pentagastrin, gastrin and food in dogs]

The effects of long term treatment with cortisone on the gastric secretion induced by histamine, pentagastrin, porcine gastrin and a meal have been investigated in four dogs with both gastric fistula and Heidenhain pouch. Cortisone increased the postcibal acid output and the observed maximal acid response from the pouch to all three exogenous stimuli. The ED 50'S remained unchanged. The same effects although less marked were observed in the innervated stomach. These data indicate that the increased acid secretion observed after long term treatment with cortisone is largely due to an increased secretory capacity of the gastric mucosa. This latter could result from an increase in the number of secretory units or to partial removal of a non competitive inhibitor of gastric secretion.     

Gastric acid secretion in aquaporin-4 knockout mice

The aquaporin-4 (AQP4) water channel has been proposed to play a role in gastric acid secretion. Immunocytochemistry using anti-AQP4 antibodies showed strong AQP4 protein expression at the basolateral membrane of gastric parietal cells in wild-type (+/+) mice. AQP4 involvement in gastric acid secretion was studied using transgenic null (-/-) mice deficient in AQP4 protein. -/- Mice had grossly normal growth and appearance and showed no differences in gastric morphology by light microscopy. Gastric acid secretion was measured in anesthetized mice in which the stomach was luminally perfused (0. 3 ml/min) with 0.9% NaCl containing [(14)C]polyethylene glycol ([(14)C]PEG) as a volume marker. Collected effluent was assayed for titratable acid content and [(14)C]PEG radioactivity. After 45-min baseline perfusion, acid secretion was stimulated by pentagastrin (200 microg. kg(-1). h(-1) iv) for 1 h or histamine (0.23 mg/kg iv) + intraluminal carbachol (20 mg/l). Baseline gastric acid secretion (means +/- SE, n = 25) was 0.06 +/- 0.03 and 0.03 +/- 0.02 microeq/15 min in +/+ and -/- mice, respectively. Pentagastrin-stimulated acid secretion was 0.59 +/- 0.14 and 0.70 +/- 0.15 microeq/15 min in +/+ and -/- mice, respectively. Histamine plus carbachol-stimulated acid secretion was 7.0 +/- 1.9 and 8.0 +/- 1.8 microeq/15 min in +/+ and -/- mice, respectively. In addition, AQP4 deletion did not affect gastric fluid secretion, gastric pH, or fasting serum gastrin concentrations. These results provide direct evidence against a role of AQP4 in gastric acid secretion.     

Pharmacological profile of a new peptidic gastrin antagonist

Boc-Trp-Met-Asp-NH2 was described as the smallest peptidic fragment which presented gastric antisecretory activity. Some pharmacological aspects of a peptide analogue, Boc-Trp-Leu-Asp-NH2 (Boc-WLD-NH2), were studied on the main biological functions of gastrin. This compound was found to inhibit the binding of gastrin to isolated gastric fundic mucosal cells (IC50 50 microM). On pentagastrin-induced gastric acid secretion in the rat, a dose-dependent inhibition was observed with an ID50 of 55 mumol/kg when pentagastrin (1 microgram/kg per h) was continuously infused and with an ID50 of 7.8 mumol/kg when pentagastrin (1 microgram/kg) was bolus i.v. injected. Similar inhibition was observed on acid secretion induced by pentagastrin in the isolated rat gastric mucosa (IC50 100 microM), whereas the tripeptide had no effect when acid output was triggered by histamine. A dose-dependent inhibition with the tripeptide was shown on pentagastrin induced guinea-pig ileum contractions (IC50 31 microM). The compound had no activity on histamine-stimulated guinea-pig atria (histamine H2-receptor). These results suggest some evidence for a selective antigastrin activity.     

Control of pepsin secretion by regulatory peptides in the rat stomach: comparison with acid secretion.

Previous studies of the control of pepsin secretion by neurohumoral agents showed some discrepancies between in vitro (isolated cells) and in vivo experiments. In the present work, the effects on pepsin secretion of CCK, pentagastrin, secretin, VIP, neurotensin, histamine, and methacholine were reinvestigated in conscious gastric fistula rats, in comparison to acid secretion. ED50's and doses inducing maximal responses were measured to directly compare the potency and efficacy of these substances. Methacholine was the most efficient (maximal response = 4.5 x basal level, ED50 = 1.3 mumol/kg.h) and CCK the most potent (ED50 = 1.9 nmol/kg.h) stimulant, whereas secretin was a potent (ED50 regulators of pepsin secretion in the rat. Pentagastrin and histamine did not stimulate pepsin output, as found by others with isolated chief cells in vitro. Neurotensin and large doses of VIP marginally inhibited pepsin secretion.     

Inhibitory role on gastric secretion of a central NK-3 tachykinin receptor agonist, senktide

When administered intracerebroventricularly, the highly selective NK-3 tachykinin receptor agonist senktide possesses a potent and dose-related inhibitory effect on gastric acid secretion. The central mechanism governing the antisecretory effect of senktide was examined in perfused-stomach rats by studying its influence on gastric acid secretion elicited by the secretagogues histamine, pentagastrin and bethanechol. Given intracerebroventricularly, senktide reduced the acid response to histamine, but not that to pentagastrin or bethanechol. Stimulation of NK-3 receptors in rat brain thus appears to inhibit gastric acid secretion through histaminergic pathways.     

Histamine in stress ulcer prophylaxis in rats.

BACKGROUND: Gastrin and its analogues increase the gastric acid secretion, but also enhance mucosal defense mechanisms. On the other hand, increased formation of histamine leading to an increase in gastric acid secretion is accompanied with gastroprotection and acceleration of gastric ulcer healing. AIM: Of this study was to examine the effect of histamine on stress induced gastric ulcers in rats. METHODS: Male Wistar rats were exposed to water immersion and restrain stress (WRS) for 3.5 h at 23 degrees C. Before WRS rats were pretreated with saline, histamine, ranitidine or omeprazole. RESULTS: WRS produces gastric lesions which were strongly reduced by ranitidine or omeprazole. Also treatment with histamine markedly reduced ulcer area evoked by WRS. Addition of histamine to ranitidine or omeprazole caused an additional reduction in ulcer area. Gastroprotective effect of histamine was accompanied with the increase in gastric blood flow (GBF). Administration of omeprazole or ranitidine alone was without significant effect on GBF. Histamine caused an slight decrease in gastric luminal pH, whereas ranitidine or omeprazole significantly increased gastric luminal pH. Plasma interleukin-1beta was significantly reduced after administration of omeprazole, ranitidine, or histamine, however, the effect of histamine was less pronounced. DNA synthesis was increased after administration of omeprazole, ranitidine or histamine when compared with WRS alone. Administration of histamine in combination with ranitidine or omeprazole caused an additional increase in DNA synthesis. CONCLUSIONS: Histamine exhibits protective effect and increases gastroprotective effect of ranitidine and omeprazole against stress-induced gastric lesions. This effect of histamine seems to be independent on gastric acid secretion but related to the increase in gastric blood flow and the reduction in activation of cytokine cascade.     

Ontogeny of gastric acidity in the ovine fetus

Hypoacidity and hypergastrinaemia have been reported in the newborn human. However, little is known about in utero gastric acid secretion, and the relationship to fetal plasma gastrin levels. The longitudinal pattern of development of basal and stimulated gastric acid secretion in the non-anaesthetized fetal sheep has been studied during the last 45 days of gestation. Fetuses had cannulae inserted into the jugular vein, carotid artery and stomach. Gastric juice and blood was sampled daily from 101 days gestation until birth (145 days). Intermittent basal acid secretion began between 120 and 133 days of gestation. These fluctuations in gastric juice pH continued until birth. Overall there was a decline in gastric pH from 7.5 +/- 0.2 (SEM), for fetuses 101-105 days to 4.3 +/- 0.5 by 131-135 days. Mean fetal plasma gastrin was higher than maternal levels after 111-115 days but no correlation between fetal plasma gastrin levels and gastric pH could be demonstrated. Pentagastrin and histamine infusion did not stimulate acid secretion in fetuses younger than 115 days. After this age the fetuses became responsive to both pentagastrin and histamine. In contrast, cholinergic stimulation, using bethanechol, did not stimulate acid production until 10 to 15 days later, suggesting a hierarchy in the development of the control of acid secretion in the fetus. The lack of response to endogenous gastrin and the hierarchy in the control of acid secretion suggest either a lack of receptors on the parietal cell or the presence of an inhibitor of acid secretion. These studies are relevant to human physiology since the present findings show that the sheep and human have a similar gastrin/acid profile at birth.     

Inhibition of gastric acid secretion in dogs by neurotensin

Neurotensin is a tridacapeptide which has been isolated from bovine hypothalamus. The action of synthetic neurotensin was studied on gastric acid secretion in dogs provided with gastric pouches. Intravenously infused neurotensin, 50 ng × kg^?1 × min^?1, was found to produce a considerable inhibition of pentagastrin stimulated gastric acid secretion. On the other hand, there was no sign of inhibition of histamine induced gastric acid secretion. The experiments show that neurotensin, isolated from the central nervous system is a potent gastric secretory inhibitor and that it has a selective action in inhibiting gastric acid responses to pentagastrin but not to histamine.     

Effect of metiamide on acid secretion from isolated kitten fundic mucosa.

Isolated kitten fundic mucosa demonstrates low rates of spontaneous acid secretion in vitro when bathed in Krebs-Henseleit solution, and responds consistently to histamine, pentagastrin, and acetylcholine placed in the bath. High rates of spontaneous secretion or secretion in response to histamine and pentagastrin were significantly inhibited by addition of metiamide (5 X 10(-3) M); but mucosa stimulated by constant addition of acetylcholine chloride (10(-4) M) was not inhibited by metiamide at the same concentration.     

N-acetyl-GRP(20-26)-O-CH3 reverses intracisternal bombesin-induced inhibition of gastric acid secretion in rats

Intracisternal injection of 19 pmoles of bombesin in light-ether-anesthetized rats, five minutes after intracisternal vehicle, produced a 75% and 63% inhibition in gastric acid output and concentration, respectively, in 2-hour pylorus-ligated rats. Pretreatment of rats with the characterized peripheral bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 (1 nmole) reversed the inhibitory effect of bombesin on gastric acid output and concentration. In contrast, the related bombesin antagonist N-acetyl-GRP-O-CH2-CH3 (1 nmole) was ineffective in this system. In urethane-anesthetized, acute gastric fistula rats infused with pentagastrin, intracisternal N-acetyl-GRP(20-26)-O-CH3 protected against the inhibition in gastric acid output produced by intracisternal bombesin (19 pmoles). Thus the recently characterized peripheral bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 also appears to be effective in antagonizing central bombesin-induced inhibition in gastric acid secretion in two models. This represents a first report of a synthetic bombesin antagonist effective in reversing central bombesin-induced effects on gastric function.     

Effect of bombesin-stimulated gastrin on gastric acid secretion in man

The effect of bombesin on gastrin release and gastric acid secretion was investigated in 10 healthy volunteers. Bombesin (0.6 μg · Kg^?1 · hr^?1) produced a significantly higher $\text{(}\text{p}\text{<}\text{0.001)}$ increase in plasma gastrin levels (86.7 11.1 pmo/1 than after a protein meal (39.6 ± 5.6 pmol1/1). The gastric acid secretory response to bombesin (12.1 ± 2.9 mEq · hr^?1) was however significantly lower $\text{(}\text{p}\text{<}\text{0.005)}$ than the maximal response produced by pentagostrin (20.9 ± 3.5 mEq · hr^?1) at the dose of 6 μg · Kg^?1. Atropine did not modify gastrin release induced by bombesin but significantly reduced gastric acid secretion $\text{(}\text{p}\text{<}\text{0.01)}$. From the data presented it may be hypothesized that less biologically active forms of gastrin and/or other peptides inhibiting the gastrin effect upon gastric acid secretion may be released by bombesin.