Gastrin secretion induced by distention is mediated by gastric cholinergic and vasoactive intestinal peptide neurons in rats

Mechanosensing in the Physiology and Pathology of the Gastrointestinal Tract

Normal gastrointestinal function relies on sensing and transducing mechanical signals into changes in intracellular signaling pathways. Both specialized mechanosensing cells, such as certain enterochromaffin cells and enteric neurons, and non-specialized cells, such as smooth muscle cells, interstitial cells of Cajal, and resident macrophages, participate in physiological and pathological responses to mechanical signals in the gastrointestinal tract. We review the role of mechanosensors in the different cell types of the gastrointestinal tract. Then, we provide several examples of the role of mechanotransduction in normal physiology. These examples highlight the fact that, although these responses to mechanical signals have been known for decades, the mechanosensors involved in these responses to mechanical signals are largely unknown. Finally, we discuss several diseases involving the overstimulation or dysregulation of mechanotransductive pathways. Understanding these pathways and identifying the mechanosensors involved in these diseases may facilitate the identification of new drug targets to effectively treat these diseases.

A semaphorin-plexin-Rasal1 signaling pathway inhibits gastrin expression and protects against peptic ulcers

Peptic ulcer disease is a frequent clinical problem with potentially serious complications such as bleeding or perforation. A decisive factor in the pathogenesis of peptic ulcers is gastric acid, the secretion of which is controlled by the hormone gastrin released from gastric G cells. However, the molecular mechanisms regulating gastrin plasma concentrations are poorly understood. Here, we identified a semaphorin-plexin signaling pathway that operates in gastric G cells to inhibit gastrin expression on a transcriptional level, thereby limiting food-stimulated gastrin release and gastric acid secretion. Using a systematic siRNA screening approach combined with biochemical, cell biology, and in vivo mouse experiments, we found that the RasGAP protein Rasal1 is a central mediator of plexin signal transduction, which suppresses gastrin expression through inactivation of the small GTPase R-Ras. Moreover, we show that Rasal1 is pathophysiologically relevant for the pathogenesis of peptic ulcers induced by nonsteroidal anti-inflammatory drugs (NSAIDs), a main risk factor of peptic ulcers in humans. Last, we show that application of recombinant semaphorin 4D alleviates peptic ulcer disease in mice in vivo, demonstrating that this signaling pathway can be harnessed pharmacologically. This study unravels a mode of G cell regulation that is functionally important in gastric homeostasis and disease.

Gastric Peptides-Gastrin and Somatostatin

Gastric acid secretion (i) facilitates digestion of protein as well as absorption of micronutrients and certain medications, (ii) kills ingested microorganisms, including Helicobacter pylori, and (iii) prevents bacterial overgrowth and enteric infection. The principal regulators of acid secretion are the gastric peptides gastrin and somatostatin. Gastrin, the major hormonal stimulant for acid secretion, is synthesized in pyloric mucosal G cells as a 101-amino acid precursor (preprogastrin) that is processed to yield biologically active amidated gastrin-17 and gastrin-34. The C-terminal active site of gastrin (Trp-Met-Asp-Phe-NH₂ ) binds to gastrin/CCK₂ receptors on parietal and, more importantly, histamine-containing enterochromaffin-like (ECL) cells, located in oxyntic mucosa, to induce acid secretion. Histamine diffuses to the neighboring parietal cells where it binds to histamine H₂ -receptors coupled to hydrochloric acid secretion. Gastrin is also a trophic hormone that maintains the integrity of gastric mucosa, induces proliferation of parietal and ECL cells, and is thought to play a role in carcinogenesis. Somatostatin, present in D cells of the gastric pyloric and oxyntic mucosa, is the main inhibitor of acid secretion, particularly during the interdigestive period. Somatostatin exerts a tonic paracrine restraint on gastrin secretion from G cells, histamine secretion from ECL cells, and acid secretion from parietal cells. Removal of this restraint, for example by activation of cholinergic neurons during ingestion of food, initiates and maximizes acid secretion. Knowledge regarding the structure and function of gastrin, somatostatin, and their respective receptors is providing novel avenues to better diagnose and manage acid-peptic disorders and certain cancers. Published 2020. Compr Physiol 10:197-228, 2020.

Mechanosensitive ion channel Piezo1 is expressed in antral G cells of murine stomach

G cells in the antrum region of the murine stomach produce gastrin, the central hormone for controlling gastric activities. Secretion of gastrin is induced mainly by protein breakdown products but also by distensions of the stomach wall. Although G cells respond to protein fragments via distinct chemosensory receptor types, the mechanism underlying G cell activation upon distention is entirely ambiguous. Mechanosensitive ion channels are considered as potential candidates for such a task. Therefore, we explore the possibility of whether Piezo1, a polymodal sensor for diverse mechanical forces, is expressed in antral G cells. The experimental analyses revealed that the vast majority of G cells indeed expressed Piezo1. Within flask-like G cells at the base of the antral invaginations, the Piezo1 protein was primarily located at the basolateral portion, which is thought to be the release site for the exocytic secretion of gastrin. In the spindle-like G cells, which are oriented parallel to the invaginations, Piezo1 protein was restricted to the cell body where the hormone was also located, whereas the long processes appeared to be devoid of Piezo1 protein. Our results suggest that mechanosensitive channels such as Piezo1, located in close proximity to hormone-release sites, enable G cells to respond directly to antrum distensions with gastrin secretion.

The Gastric and Intestinal Microbiome: Role of Proton Pump Inhibitors

PURPOSE OF REVIEW

The discovery of Helicobacter pylori and other organisms colonizing the stomach and the intestines has shed some light on the importance of microbiome in maintaining overall health and developing pathological conditions when alterations in biodiversity are present. The gastric acidity plays a crucial role in filtering out bacteria and preventing development of enteric infections. In this article, we discuss the physiology of gastric acid secretion and bacterial contribution to the composition of gastric and intestinal barriers and review the current literature on the role of proton pump inhibitors (PPIs) in the microbial biodiversity of the gastrointestinal tract.

RECENT FINDINGS

Culture-independent techniques, such as 16S rRNA sequencing, have revolutionized our understanding of the microbial biodiversity in the gastrointestinal tract. Luminal and mucosa-associated microbial populations are not identical. Streptococcus is overrepresented in the biopsies of patients with antral gastritis and may also be responsible for the development of peptic ulcer disease. The use of PPIs favors relative streptococcal abundance irrespective of H. pylori status and may explain the persistence of dyspeptic symptoms in patients on PPI therapy. Increased risk of enteric infections has also been seen in patients taking PPIs. The overuse of PPIs leads to significant shift of the gastrointestinal microbiome towards a less healthy state. With the advent of PPIs, many studies have demonstrated the significant changes in the microbial composition of both gastric and intestinal microbiota. Although they are considered relatively safe over-the-counter medications, PPIs in many cases are over- and even inappropriately used. Future studies assessing the safety of PPIs and their role in the development of microbiome changes should be encouraged.

Transcriptional and Functional Characterization of the G Protein-Coupled Receptor Repertoire of Gastric Somatostatin Cells

In the stomach, somatostatin (SST) acts as a general paracrine negative regulator of exocrine secretion of gastric acid and pepsinogen and endocrine secretion of gastrin, ghrelin, and histamine. Using reporter mice expressing red fluorescent protein (RFP) under control of the SST promotor, we have characterized the G protein-coupled receptors expressed in gastric Sst-RFP-positive cells and probed their effects on SST secretion in primary cell cultures. Surprisingly, besides SST, amylin and PYY were also highly enriched in the SST cells. Several receptors found to regulate SST secretion were highly expressed and/or enriched. 1) The metabolite receptors calcium-sensing receptor and free fatty acid receptor 4 (GPR120) functioned as positive and negative regulators, respectively. 2) Among the neurotransmitter receptors, adrenergic receptors α1a, α2a, α2b, and β1 were all highly expressed, with norepinephrine and isoproterenol acting as positive regulators. The muscarinic receptor M3 acted as a positive regulator, whereas M4 was conceivably a negative regulator. 3) Of the hormone receptors, the GLP-1 and GIP receptors, CCKb (stimulated by both CCK and gastrin) and surprisingly the melanocortin MC1 receptor were all positive regulators. 4) The neuropeptide receptors for calcitonin gene-related peptide, adrenomedullin, and vasoactive intestinal peptide acted as positive regulators, no effect was observed using galanin and nociceptin although transcripts for the corresponding receptors appeared highly expressed. 5) The SST receptors 1 and 2 functioned in an autocrine negative feedback loop. Thus, the article provides a comprehensive map of receptors through which SST secretion is regulated by hormones, neurotransmitters, neuropeptides and metabolites that act directly on the SST cells in the gastric mucosa.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

High k(+)-induced relaxation by nitric oxide in human gastric fundus

This study was designed to elucidate high K(+)-induced relaxation in the human gastric fundus. Circular smooth muscle from the human gastric fundus greater curvature showed stretch-dependent high K(+) (50 mM)-induced contractions. However, longitudinal smooth muscle produced stretch-dependent high K(+)-induced relaxation. We investigated several relaxation mechanisms to understand the reason for the discrepancy. Protein kinase inhibitors such as KT 5823 (1 µM) and KT 5720 (1 µM) which block protein kinases (PKG and PKA) had no effect on high K(+)-induced relaxation. K(+) channel blockers except 4-aminopyridine (4-AP), a voltage-dependent K(+) channel (K(V)) blocker, did not affect high K(+)-induced relaxation. However, N(G)-nitro-L-arginine and 1H-(1,2,4)oxadiazolo (4,3-A)quinoxalin-1-one, an inhibitors of soluble guanylate cyclase (sGC) and 4-AP inhibited relaxation and reversed relaxation to contraction. High K(+)-induced relaxation of the human gastric fundus was observed only in the longitudinal muscles from the greater curvature. These data suggest that the longitudinal muscle of the human gastric fundus greater curvature produced high K(+)-induced relaxation that was activated by the nitric oxide/sGC pathway through a K(V) channel-dependent mechanism.

Nitric Oxide-mediated Relaxation by High K in Human Gastric Longitudinal Smooth Muscle

This study was designed to elucidate high-K(+)induced response of circular and longitudinal smooth muscle from human gastric corpus using isometric contraction. Contraction from circular and longitudinal muscle stripes of gastric corpus greater curvature and lesser curvature were compared. Circular smooth muscle from corpus greater curvature showed high K(+) (50 mM)-induced tonic contraction. On the contrary, however, longitudinal smooth muscle strips showed high K(+) (50 mM)-induced sustained relaxation. To find out the reason for the discrepancy we tested several relaxation mechanisms. Protein kinase blockers like KT5720, PKA inhibitor, and KT5823, PKG inhibitor, did not affect high K(+)-induced relaxation. K(+) channel blockers like tetraethylammonium (TEA), apamin (APA), glibenclamide (Glib) and barium (Ba(2+)) also had no effect. However, N(G)-nitro-L-arginine (L-NNA) and 1H-(1,2,4) oxadiazolo (4,3-A) quinoxalin-1-one (ODQ), an inhibitor of soluble guanylate cyclase (sGC) and 4-AP (4-aminopyridine), voltage-dependent K(+) channel (K(V)) blocker, inhibited high K(+)-induced relaxation, hence reversing to tonic contraction. High K(+)-induced relaxation was observed in gastric corpus of human stomach, but only in the longitudinal muscles from greater curvature not lesser curvature. L-NNA, ODQ and K(V) channel blocker sensitive high K(+)-induced relaxation in longitudinal muscle of higher portion of corpus was also observed. These results suggest that longitudinal smooth muscle from greater curvature of gastric corpus produced high K(+)-induced relaxation which was activated by NO/sGC pathway and by K(V) channel dependent mechanism.

Control of gastric acid secretion in health and disease

Recent milestones in the understanding of gastric acid secretion and treatment of acid-peptic disorders include the (1) discovery of histamine H(2)-receptors and development of histamine H(2)-receptor antagonists, (2) identification of H(+)K(+)-ATPase as the parietal cell proton pump and development of proton pump inhibitors, and (3) identification of Helicobacter pylori as the major cause of duodenal ulcer and development of effective eradication regimens. This review emphasizes the importance and relevance of gastric acid secretion and its regulation in health and disease. We review the physiology and pathophysiology of acid secretion as well as evidence regarding its inhibition in the management of acid-related clinical conditions.

Binding of isolectin IB4 to neurons of the mouse enteric nervous system

The plant lectin, IB4, binds to primary afferent neurons of dorsal root and trigeminal ganglia, where it is selective for nociceptive neurons. In the enteric nervous system of the guinea-pig IB4 labels intrinsic primary afferent neurons, which are believed to have roles as nociceptors. Here we investigate whether IB4 binding is also a marker of intrinsic primary afferent neurons in the mouse. Neurons that bound IB4 were common in the enteric plexuses of the small intestine and colon. Labeled neurons were rare in the stomach, and absent from the esophagus and gallbladder. Binding was to the cell surface, initial parts of axons and to clumps in the cytoplasm. Similar binding occurred on small and medium sized neurons of dorsal root, nodose and trigeminal ganglia. In the enteric nervous system, IB4 revealed large round or oval (type II) neurons, type I neurons with prominent laminar dendrites and small neurons of myenteric ganglia. The type II neurons were immunoreactive for calretinin, and some type I neurons were immunoreactive for nitric oxide synthase. Most neurons in the submucosal ganglia bound IB4, and some of these were vasoactive intestinal peptide immunoreactive. Thus IB4 binds to specific subgroups of enteric neurons in the mouse. These include intrinsic primary afferent neurons, but other neurons, including secretomotor neurons, are labeled. The results suggest that IB4 is not a specific label for enteric nociceptive neurons.

Peripheral PACAP inhibits gastric acid secretion through somatostatin release in mice

1. Studies in rats suggest that PACAP modulates gastric acid secretion through the release of both histamine and somatostatin. 2. We characterized the effects of exogenous PACAP on gastric acid secretion in urethane-anesthetized mice implanted with a gastric cannula and in conscious 2-h pylorus ligated mice, and determined the involvement of somatostatin and somatostatin receptor type 2 (SSTR2) by using somatostatin immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. 3. Urethane-anesthetized wild-type mice had low basal acid secretion (0.10+/-0.01 micromol (10 min)(-1)) compared with SSTR2 knockout mice (0.93+/-0.07 micromol (10 min)(-1)). Somatostatin antibody and PRL-2903 increased basal secretion in wild-type mice but not in SSTR2 knockout animals. 4. In wild-type urethane-anesthetized mice, PACAP-38 (3-270 microg kg(-1) h(-1)) did not affect the low basal acid secretion, but inhibited the acid response to pentagastrin, histamine, and bethanechol. 5. In wild-type urethane-anesthetized mice pretreated with somatostatin antibody or PRL-2903 and in SSTR2 knockout mice, peripheral infusion of PACAP-38 or somatostatin-14 did not inhibit the increased basal gastric acid secretion. 6. In conscious wild-type mice, but not in SSTR2 knockout mice, PACAP-38 inhibited gastric acid secretion induced by 2-h pylorus ligation. The antisecretory effect of PACAP-38 was prevented by immunoneutralization of somatostatin. 7. These results indicate that, in mice, peripheral PACAP inhibits gastric acid secretion through the release of somatostatin and the activation of SSTR2 receptors. There is no evidence for stimulatory effects of PACAP on acid secretion in mice.

Gastric atrial natriuretic peptide regulates endocrine secretion in antrum and fundus of human and rat stomach

Atrial natriuretic peptide (ANP) is present in gastric mucosa and preferentially binds to two subtypes of natriuretic peptide receptors (NPR), NPR-A and NPR-C. The present study examines the role of endogenous ANP in regulating endocrine secretion in rat and human stomachs. NPR-A protein expression and transcripts were identified in rat antral and fundic mucosa by Western blot and RT-PCR. In superfused rat and human antral and fundic segments, ANP (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion. In antrum, this was accompanied by a decrease in gastrin, and in fundus, this was accompanied by a decrease in histamine secretion. Changes in gastrin and histamine secretion reflected changes in somatostatin secretion and were abolished by somatostatin antibody. The NPR-A receptor antagonist anantin 1) inhibited basal somatostatin secretion and 2) abolished the somatostatin, gastrin, and histamine responses to ANP. We conclude that endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion from both antrum and fundus of rat and human stomach. Stimulation of somatostatin secretion is coupled to inhibition of gastrin secretion in the antrum and inhibition of histamine secretion in the fundus.

Reciprocal paracrine pathways link atrial natriuretic peptide and somatostatin secretion in the antrum of the stomach

Atrial natriuretic peptide (ANP) as well as its receptor, NPR-A, have been identified in gastric antral mucosa, suggesting that ANP may act in a paracrine fashion to regulate gastric secretion. In the present study, we have superfused antral mucosal segments obtained from rat stomach to examine the paracrine pathways linking ANP and somatostatin secretion in this region.ANP (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion (EC(50), 0.3 nM). The somatostatin response to ANP was unaffected by the axonal blocker tetrodotoxin but abolished by addition of the selective NPR-A antagonist, anantin. Anantin alone inhibited somatostatin secretion by 18+/-3% (P<0.005), implying that endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion. Somatostatin (1 pM to 1 microM) caused a concentration-dependent decrease in ANP secretion (EC(50), 0.7 nM) that was abolished by addition of the somatostatin subtype 2 receptor (sst2) antagonist, PRL2903. Neutralization of ambient somatostatin with somatostatin antibody (final dilution 1:200) increased basal ANP secretion by 70+/-8% (P<001), implying that endogenous somatostatin inhibits ANP secretion. We conclude that antral ANP and somatostatin secretion are linked by paracrine feedback pathways: endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion, and endogenous somatostatin, acting via the sst2 receptor, inhibits ANP secretion.

Mechanisms for regulation of gastrin and somatostatin release from isolated rat stomach during gastric distention

AIM: To investigate the intragastric mechanisms for regulation of gastric neuroendocrine functions during gastric distention in isolated vascularly perfused rat stomach.

METHODS: Isolated vascularly perfused rat stomach was prepared, then the gastric lumen was distended with either 5, 10 or 15 mL pH7 isotonic saline during a period of 20 min. During the distention, the axonal blocker tetrodotoxin (TTX), the cholinergic antagonist atropine, or the putative somatostatin-antagonist cyclo [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)] were applied by vascular perfusion. The releases of gastrin and somatostatin were then examined by radioimmunoassay.

RESULTS: The graded gastric distention caused a significant volume-dependent decrease in gastrin secretion [-183 ± 75 (5 mL), -385 ± 86 (10 mL) and -440 ± 85 (15 mL) pg/20 min] and a significant increase of somatostatin secretion [260 ± 102 (5 mL), 608 ± 148 (10 mL) and 943 ± 316 (15 mL) pg/20 min]. In response to 10 mL distention, the infusion of either axonal blocker TTX (10^-6 M) or cholinergic blocker atropine (10^-7 M) had a similar affect. They both attenuated the decrease of gastrin release by approximately 50%, and attenuated the increase of somatostatin release by approximately 40%. The infusion of somatostatin-antagonist cyclo [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)] (10^-6 M) attenuated the decrease of gastrin release by about 60%. Furthermore, combined infusion of the somatostatin-antagonist and atropine completely abolished distention-induced inhibition of gastrin release.

CONCLUSION: The present data suggest that distention of isolated rat stomach stimulates somatostatin release via cholinergic and non-cholinergic TTX-insensitive pathways. Both somatostatin and intrinsic cholinergic pathways are responsible for distention-induced inhibition of gastrin release.

PACAP 1-38 as neurotransmitter in the porcine antrum

The concentration of PACAP 1-38 in porcine antrum amounted to 15.4+/-7.9 and 20.3+/-8 pmol/g tissue in the mucosal and muscular layers. PACAP immunoreactive (IR) fibres innervated the muscular (co-localised with VIP) and submucosal/mucosal layers (some co-storing VIP and CGRP) including myenteric and submucosal plexus and blood vessels. Only myenteric nerve cell bodies contained PACAP-IR (co-storing VIP). In isolated perfused antrum, vagus nerve stimulation (8 Hz) and capsaicin (10(-5) M) increased PACAP 1-38 release. PACAP 1-38 (10(-9) M) increased substance P (SP), gastrin releasing peptide (GRP) and VIP release. PACAP 1-38 (10(-8) M) inhibited gastrin secretion and stimulated somatostatin secretion and motility dose-dependently. PACAP-induced motility was strongly inhibited by the antagonist PACAP 6-38 but also by atropine and substance P-antagonists (CP99994/SR48968) but PACAP 6-38 had no effect on vagus-induced secretion or motility.

CONCLUSION

PACAP 1-38 may be involved in antral motility and secretion by interacting with cholinergic, SP-ergic, GRP-ergic and/or VIP-ergic neurones, and may also be involved in afferent reflex pathways.

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.

Innervation of the gastric mucosa

A plethora of neuronal messengers ("classical" transmitters, gaseous messengers, amino acid transmitters, and neuropeptides) are capable of mediating or modulating gastric functions. Accordingly, the stomach is richly innervated. Gastric nerves are either intrinsic to the gastric wall, i.e., they have their cell bodies in the intramural ganglia and thus belong to the enteric nervous system, or they reach the stomach from outside, originating in the brainstem, in sympathetic ganglia, or in sensory ganglia. Topographically, the nerve fibers in the stomach reach all layers from the most superficial portions of the gastric glands to the outer smooth muscle layer. This wide distribution implies that virtually all different cell types may be reached by neuronal messengers. Within the gastric mucosa endocrine and paracrine cells (e.g., gastrin cells, ECL cells, somatostatin cells), exocrine cells (parietal cells, chief cells, mucous cells), smooth muscle cells, and stromal cells are regulated by neuronal messengers. The sensory innervation, responding to capsaicin, plays an important role in mucosal protection, and in ulcer healing. Presumably also other nerves are involved and a plasticity in the neuropeptide expression has been demonstrated at the margin of gastric ulcers. Taken together, available data indicate a complex interplay between hormones, paracrine messengers and neuronal messengers, growth factors and cytokines in the regulation of gastric mucosal activities such as secretion, local blood flow, growth, and restitution after damage.

Neural mediation of vasoactive intestinal polypeptide inhibitory effect on jejunal alanine absorption

It was recently shown that vasoactive intestinal polypeptide (VIP) inhibits rat jejunal alanine absorption, an effect that was significantly reduced by vagotomy. This study assesses the role of capsaicin-sensitive primary afferents (CSPA) and the myenteric plexus in the inhibition of rat jejunal alanine absorption by VIP. Continuous intravenous infusion of VIP (11.2 ng . kg-1 . min-1) reduced alanine absorption by 60% in sham control rats and by 20% in rats neonatally treated with capsaicin (P < 0.01). In in vitro experiments, VIP decreased alanine uptake by jejunal strips isolated from sham control rats in a dose-dependent manner. In the presence of 40 nM VIP, alanine uptake by full-thickness jejunal strips was reduced by 54% in sham control rats and by 25% in rats neonatally treated with capsaicin (P < 0.001). On the other hand, VIP reduced alanine uptake by mucosal scrapings by 25% in sham rats compared with 9% reduction in neonatally treated rats. Chemical ablation of the extrinsic innervation and jejunal myenteric plexuses by pretreatment with benzalkonium chloride significantly (P < 0.001) reduced basal alanine absorption and the inhibitory effect of VIP. Moreover, incubation of intestinal strips with tetrodotoxin and atropine reduced significantly (P < 0.05) the inhibitory effect of VIP on alanine absorption. These data suggest that VIP exerts its inhibitory effect on alanine absorption through the CSPA fibers and the myenteric plexus. The neuronal circuitry of this inhibitory process may involve cholinergic muscarinic mechanisms.

Relationship between fundic endocrine cells and gastric acid secretion in hypersecretory duodenal ulcer diseases

BACKGROUND

Acid hypersecretion is associated with duodenal ulcer disease in the following conditions: Zollinger-Ellison syndrome (ZES) and antral gastrin cell hyperfunction (AGCH) due to hypergastrinaemia, or hypersecretory duodenal ulcer (HDU) without hypergastrinaemia.

AIM

To evaluate whether quantitative changes in fundic ECL and D cells may be involved in acid hypersecretion.

PATIENTS AND METHODS

Seven ZES, six AGCH and six HDU Helicobacter pylori-positive patients were compared. Basal (BAO) and pentagastrin-stimulated gastric acid secretions (PAO), and morphometry of fundic ECL and D cells were performed. The six AGCH and six HDU patients were investigated again using the same tests 1 year after H. pylori eradication.

RESULTS

Median PAO values were no different in all the hypersecretory conditions studied. The median volume density of ECL cells in ZES was significantly higher than in controls (2.75, range 1.74-5.8 vs. 0.73, 0.52-1.11: P < 0.05), whereas it was in the control range in AGCH and HDU patients (0.77, range 0.20-1.39 and 0.99, range 0.42-1.51; respectively). The count of fundic D cells was significantly lower in AGCH patients than in all other investigated groups (median 0.16, range 0.1-0.52; P < 0.05). Cure of infection in AGCH and HDU patients did not modify the ECL cell volume density, whereas a significant increase in the count of fundic D cells was observed in AGCH patients. Thus, the ECL/D cell index was significantly affected in AGCH patients (P < 0.05), being higher during H. pylori infection (median 6, range 0.7-9.25) than after the cure (median 2.12, range 1.10-3.5). BAO and PAO were not affected by H. pylori eradication in either group.

CONCLUSIONS

The study provides evidence, for the first time, that quantitative alterations in the fundic endocrine cells are not involved in acid hypersecretion of patients with hypersecretory states, and that eradication of H. pylori does not restore normal acid secretion values.

Increased sensitivity of gastrin cells to gastric distension following antral denervation in the rat

1. Secretion of the antral hormone gastrin is increased by protein in the gastric lumen and by nervous reflexes. We have examined the relative importance of luminal and neuronal mechanisms, by lesioning the antral innervation using benzalkonium chloride. 2. Benzalkonium chloride was applied to the serosa of the antrum in anaesthetized rats. In some animals, a stainless-steel cannula was also implanted in the corpus. Animals were allowed 10 days to recover. Plasma gastrin was measured by radioimmunoassay and mRNAs encoding gastrin, somatostatin and histidine decarboxylase were measured by Northern blot. 3. Antral denervation was associated with gastric retention after fasting, and elevated plasma gastrin (28.4 +/- 7 pM compared with 7.6 +/- 1.0 pM in controls). When fasted control or denervated rats were refed, plasma gastrin increased 3-fold in both cases. A gastrin-releasing peptide antagonist inhibited the post-prandial rise in plasma gastrin in control rats, but had no effect in antrally denervated rats. 4. In fasted, antrally denervated rats with a gastric fistula, basal gastric acid secretion was depressed 3-fold, and plasma gastrin concentrations were similar to controls. 5. Distension of the stomach with peptone via a barostat attached to the gastric cannula (5 cm H2O, 30 min), produced 3-fold increases in plasma gastrin in both control and denervated rats. However, distension with a non-nutrient solution at pH 6.0 had no effect in controls, but increased gastrin to a similar extent to peptone in denervated rats; distension with 50 mM HCl had no effect in either control or denervated rats. 6. Somatostatin and gastrin mRNA abundances in the antrum were depressed by about 35% by antral denervation, but somatostatin mRNA in the corpus was unchanged; GAPDH mRNA abundance was unaffected by antral denervation. 7. The data suggest that luminal nutrient releases gastrin in the rat, in vivo, via activation of antral neurons secreting gastrin-releasing peptide, and that the antral innervation normally inhibits G-cell responses to non-nutrient distension of the stomach. After antral denervation, gastric distension with a non-nutrient solution is an adequate stimulus for gastrin release.

Influences of fat and carbohydrate on postprandial sleepiness, mood, and hormones

Paired studies were conducted in 18 healthy volunteers (9 men, 9 women) to investigate whether differences in mood and daytime sleepiness induced by high-fat-low-carbohydrate (CHO) and low-fat-high-CHO morning meals were associated with specific hormonal responses. Plasma insulin concentrations were significantly higher after low-fat-high-CHO meals, and cholecystokinin (CCK) concentrations were significantly higher after high-fat-low-CHO meals. Subjects tended to feel more sleepy and less awake 2-3 h after the high-fat-low-CHO meal, and ratings of fatigue were significantly greater 3 h after the high-fat-low-CHO meal than after the low-fat-high-CHO meal. The results of the present study are consistent with the hypothesis that there is an association between the lassitude experienced after a meal and the release of CCK.

Role of vagal fibers and bombesin/gastrin-releasing peptide-neurons in distention-induced gastrin release in rats

In the rat the exact role of vagal fibers and the interaction between the extrinsic and intrinsic neural system in distention-induced gastrin release are still a matter of debate. Accordingly, the aim of the present study was to examine the contribution of afferent and efferent vagal fibers as well as intrinsic neurons on gastrin response to gastric distention. In anesthetized rats graded gastric distention by 5, 10 and 15 ml saline for 20 min caused a significant volume-dependent increase of plasma gastrin levels by 12+/-6 pg/ml (5 ml saline, n = 8, P =0.05), 26+/-7 pg/ml (10 ml saline, n = 10, P < 0.05) and 37+/-7 pg/ml (15 ml saline, n = 8, P < 0.01 ), respectively. To examine the role of the extrinsic vagal innervation, gastrin response to distention was studied in anesthetized rats after bilateral truncal vagotomy (n = 9) or selective afferent vagotomy following pretreatment with capsaicin (n = 6). Stimulation of gastrin release by 10 ml distention in sham-operated control rats was reversed to an inhibition after truncal vagotomy (26+/-7 vs. -11+/-4 pg/ml; P<0.05) and capsaicin-treatment (37+/-18 vs. -34+/-11 pg/ml; P<0.05). A contribution of cholinergic mechanisms to this vagovagal-mediated stimulation of distention-induced gastrin release was excluded, since atropine (100 microg/kg/h; n = 8) further augmented distention-stimulated gastrin release. Since bombesin/gastrin-releasing peptide (GRP)-neurons contribute to vagally stimulated gastrin secretion, we have examined gastrin response to distention in the presence of the specific bombesin-receptor antagonist D-Phe6-BN(6-13)OMe (400 microg/kg/h: n = 10). This bombesin-antagonist completely reduced distention-stimulated gastrin release in vivo. In contrast, distention of the isolated, extrinsically denervated stomach significantly decreased gastrin release by 13+/-5 pg/min (5 ml saline, n = 8, P < 0.05), 28+/-8 pg/min (10 ml saline, n = 11, P < 0.05) and 35+/-10 pg/min (15 ml saline, n = 8, P < 0.01), respectively, without changing the activity of bombesin/GRP-neurons. Distention-induced decrease of gastrin release was attenuated to 50 percent by atropine (10(-7) M: n = 10) or tetrodotoxin (TTX) (10(-6) M; n = 10), respectively. These data demonstrate, that in anesthetized rats distention-stimulated gastrin secretion depends on the activation of a vagovagal reflex and intrinsic bombesin/GRP-neurons. In contrast distention of the isolated rat stomach inhibits gastrin release in part via intrinsic cholinergic pathways and other as yet unknown mechanisms.

The G cell

The study of gastrin continues to serve as an excellent model for gastrointestinal regulatory processes. This review highlights some recent advances in the field by outlining gastrin biosynthesis, summarizing current understanding of gastrin receptors, describing the regulation of gastrin release, and discussing the clinical implications of gastrin in the pathogenesis of peptic ulcer disease. Emphasis is on three emerging areas of gastrin research: the novel finding that one of gastrin's posttranslational processing intermediates has biological activity distinct from that of the mature peptide; elucidation of gastrin's signal transduction mechanisms that mediate the trophic effects of the peptide; and the role of gastrin in peptic ulcer disease pathogenesis secondary to Helicobacter pylori infection.

Intrinsic innervation of the stomach of the fetal pig: an immunohistochemical study of VIP-immunoreactive nerve fibres and cell bodies

Using an immunohistochemical technique, the presence and distribution of vasoactive intestinal polypeptide (VIP) was investigated in cryostat sections, both tangential and transverse, of the fetal pig's stomach. In all fetuses and in all gastric segments investigated, VIP-like immunoreactive (IR) nerve-cell bodies were seen in all intramural ganglia, and VIP-IR nerve fibres were found in all layers of the gastric wall except the tunica serosa. Consequently, VIP-IR nerve fibres were found to form a periglandular network, to accompany arterioles, to interconnect the intramural ganglia, to encircle both VIP-IR-negative and -positive neurons, and were found in all muscle layers. Despite the fact that VIP-IR seems to be restricted to the intramural nervous elements, some non-specific-reacting VIP-IR glandular cells were noticed in the basal parts of the fundic, antral and pyloric gastric glands. The distribution pattern of VIP in the fetal pig resembles that of the adult pig. This suggests a possible functional role for VIP during fetal life and/or puts forward the suggestion that the stomach of a fetal pig from the second half of the gestation period is prepared, from then on, for postnatal function. High similarities with regard to the general distribution pattern of VIP in the stomach have also been noted between the fetal pig and humans, proving once more that the fetal pig can serve as a good animal model in several research areas. Finally, the morphological data provided here may, combined with the physiological significance of VIP, contribute to a better insight into the physiopathology of economically important gastro-intestinal disorders in the pig, such as gastric ulceration.

Hormonal response to enteral feeding and the possiblerole of peptide YY in pathogenesis of enteral feeding-related diarrhoea

Diarrhoea is a common complication of enteral feeding. Previous studies have demonstrated a secretion of water and electrolytes in the ascending colon during intragastric but not intraduodenal enteral feeding. The cause of this secretion is likely to be neurohumoral in origin. This study was designed to examine the hormonal responses to enteral feeding. In vivo segmental colonic perfusion studies were undertaken. Before and at hourly intervals during these studies serum was taken for estimations of neurotensin (NT), pancreatic glucagon (PG), peptide YY (PYY) and vasoactive intestinal polypeptide (VIP). During fasting there was a median ascending colonic absorption of water in all groups. During feeding there was a net secretion in the ascending colon in both gastric groups and in the high load duodenal group, but not in the low load duodenal group. During these studies the PYY levels remained unchanged from fasting in the low and high load gastric groups. In the low and high load duodenal groups the PYY levels increased. The NT levels increased only in the high load duodenal group. There were no other changes in NT or in PG or VIP levels either between fasting and feeding, or between the gastric and duodenal groups. PYY is known to stimulate intestinal absorption. The absence of a rise during intragastric feeding may be important in the underlying mechanisms of enteral feeding-induced colonic secretion and hence enteral feeding-related diarrhoea.

Pyloric antral inhibition of gastrin release

Inconsistencies and omissions in current explanations for the well known depressions of gastric acid secretion and blood gastrin levels following acidification by antral stimulants are discussed. Evidence is presented which favors a reciprocal sensitivity relationship between the fundic mucosa and the antral G cells, such that blood gastrin levels rise when the secreting fundic mucosa is compromised and acid secretion in response to exogenous gastrin is increased when G cells are depressed or reduced. The functional connections between the two phenomena are considered to be nervous.

Vagal control of fasting somatostatin levels

The relationship between fasting intestinal motility, plasma concentration of somatostatin and vagal integrity was examined in four conscious dogs. Small intestinal motility was recorded using subserosally implanted bipolar electrodes. The cervical vagosympathetic trunks, previously isolated in skin loops, were blocked by cooling. In the fasted state, peaks in somatostatin concentration were observed during phase III of the migrating myoelectric complex (MMC). During vagal blockade, small intestinal MMCs persisted but with phase II being absent or decreased in duration in the duodenum and upper jejunum. Somatostatin levels significantly decreased to below the basal levels observed prior to blockade. No cycling of somatostatin levels was evident during the period of vagal blockade. Upon termination of vagal cooling, normal motility returned and somatostatin levels returned to their pre-blockade levels.

IN CONCLUSION

(1) plasma somatostatin levels cycle with phase III of the MMC in the upper small intestine; (2) the cycling of fasting somatostatin concentrations is primarily dependent upon intact vagal pathways; and (3) basal plasma somatostatin levels are in part vagally dependent.

Vagal control of nitric oxide and vasoactive intestinal polypeptide release in the regulation of gastric relaxation in rat

1. Gastric motility and neurotransmitter release in response to vagal stimulation were studied using a vascularly isolated perfused rat stomach. Gastric motor responses were recorded by a strain gauge force transducer implanted on the proximal stomach. 2. Electrical stimulation of vagal trunk (0.5-20 Hz) produced a triphasic response which was composed of a rapid transient relaxation (first phase) followed by a phasic contraction (second phase) and a delayed prolonged relaxation (third phase). Maximum responses of the first, second and third phase were observed at 2.5, 5 and 10 Hz, respectively. Intra-arterial infusion of tetrodotoxin (0.1 microM) or hexamethonium (100 microM) completely abolished the triphasic response. 3. The nitric oxide (NO) biosynthesis inhibitor NG-nitro-L-arginine (L-NNA; 100 microM) significantly antagonized the rapid relaxation but had no effect on the delayed relaxation, while vasoactive intestinal polypeptide (VIP) antagonist (1 microM) significantly reduced the delayed relaxation without affecting the rapid relaxation. 4. In response to vagal stimulation, NO production ([3H]citrulline formation in gastric tissue preloaded with [3H]arginine) was maximum at 2.5 Hz, whereas VIP release into the venous effluent was largest at 10 Hz. Hexamethonium abolished vagal-stimulated NO production and VIP release. L-NNA had no effect on VIP release in response to vagal stimulation. 5. The nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperizinium (DMPP; 100 microM) also caused a triphasic response similar to that observed with vagal stimulation and produced a significant increase in VIP and NO formation. DMPP-evoked VIP release was not affected by L-NNA. Similarly, DMPP-evoked NO production was not antagonized by VIP antagonist. 6. These results suggest that vagus nerve stimulation evokes NO and VIP release via nicotinic synapses which cause different modes of relaxation of the stomach. There is no interaction between NO and VIP release in response to vagal stimulation.

Purification and characterization of a vasoactive intestinal polypeptide-degrading endoprotease from porcine antral mucosal membranes

A neutral endoprotease was isolated from porcine antral mucosa and purified to homogeneity as examined by SDS-polyacrylamide gel electrophoresis (PAGE). Throughout the purification, t-butyloxycarbonyl-Arg-Val-Arg-Arg-4- methylcoumaryl-7-amide (MCA) was used as a substrate, which was found to be hydrolyzed specifically by the enzyme at the Arg-Arg bond. Unexpectedly, however, the enzyme was also found to hydrolyze vasoactive intestinal polypeptide (VIP) fairly specifically and more efficiently when various neuropeptides and related peptides were examined as substrates. It could degrade VIP by cleaving three peptide bonds not containing an arginine residue(s) with Km = 7.7 x 10(-6) M and kcat/Km = 7.4 x 10(6) M-1 s-1 (at pH 7.6 in the presence of 0.1% Lubrol PX), whereas only secretin, substance P, and a few others were hydrolyzed at much slower rates among the various peptides examined. Both activities toward the MCA substrate and VIP behaved in parallel throughout the purification procedures and showed essentially the same pH optimum and susceptibility toward various inhibitors and detergents. Therefore, both activities are thought to be due to the same enzyme. This endoprotease required 0.001% or a higher concentration of a detergent such as Lubrol PX or Triton X-100 for its maximal activity. Its optimum pH was about 7.5 and the molecular weight was estimated to be approximately 37,000 by SDS-PAGE. This enzyme was strongly inhibited by serine protease inhibitors such as diisopropyl-fluorophosphate and phenylmethanesulfonyl fluoride. It was also inhibited by p-chloromercuribenzoic acid, but not by some other cysteine protease inhibitors. Therefore, the enzyme appears to be most likely a kind of serine protease although its possibility as a cysteine protease cannot be completely excluded. Analysis of its cleavage specificity toward various oligopeptides indicated the possibility that the protease might recognize a specific amino acid sequence(s) and/or conformation in the vicinity of the cleavage site of the target peptide. Various characteristics of the endoprotease suggest that it is a novel membrane-bound neuropeptide-degrading endoprotease fairly specific for VIP.

Regulation of gastrin release in the dog by alpha 2-adrenoceptors

1. The purpose of the present study was to analyse the effects of the alpha 2-adrenoceptor agonist medetomidine and the antagonist yohimbine on gastrin release in conscious dogs. 2. Gastrin secretion was investigated under both basal conditions and stimulation by 2-deoxy-D-glucose, food or bombesin. 3. Basal gastrin under fasting conditions was significantly reduced by medetomidine and increased by yohimbine. 4. 2-deoxy-D-glucose-induced gastrin increase was fully inhibited by medetomidine; this effect was antagonized by yohimbine. 5. Medetomidine significantly inhibited food-induced increase in plasma gastrin; under these conditions yohimbine was without effect per se, but significantly antagonized the inhibitory action of medetomidine. 6. Gastrin release induced by bombesin was not affected by medetomidine or yohimbine. 7. These results suggest that alpha 2-adrenoceptors play an inhibitory role under conditions in which gastrin release is mainly mediated through cholinergic and non-cholinergic nervous pathways; in contrast, they do not indicate the presence of alpha 2-adrenoceptors on G cells of the dog stomach.