An update on acid secretion

Expression of Adenosine A2B Receptor and Adenosine Deaminase in Rabbit Gastric Mucosa ECL Cells

Adenosine is readily available to the glandular epithelium of the stomach. Formed continuously in intracellular and extracellular locations, it is notably produced from ATP released in enteric cotransmission. Adenosine analogs modulate chloride secretion in gastric glands and activate acid secretion in isolated parietal cells through A_2B adenosine receptor (A2BR) binding. A functional link between surface A2BR and adenosine deaminase (ADA) was found in parietal cells, but whether this connection is a general feature of gastric mucosa cells is unknown. Here we examine whether A2BR is expressed at the membrane of histamine-producing enterochromaffin-like (ECL) cells, the major endocrine cell type in the oxyntic mucosa, and if so, whether it has a vicinity relationship with ADA. We used a highly homogeneous population of rabbit ECL cells (size 7.5–10 µm) after purification by elutriation centrifugation. The surface expression of A2BR and ADA proteins was assessed by flow cytometry and confocal microscopy. Our findings demonstrate that A2BR and ADA are partially coexpressed at the gastric ECL cell surface and that A2BR is functional, with regard to binding of adenosine analogs and adenylate cyclase activation. The physiological relevance of A2BR and ADA association in regulating histamine release is yet to be explained.

Stimulation of gastric acid secretion by rabbit parietal cell A2B adenosine receptor activation

Adenosine modulates different functional activities in many cells of the gastrointestinal tract; some of them are believed to be mediated by interaction with its four G protein-coupled receptors. The renewed interest in the adenosine A2B receptor (A2BR) subtype can be traced by studies in which the introduction of new genetic and chemical tools has widened the pharmacological and structural knowledge of this receptor as well as its potential therapeutic use in cancer and inflammation- or hypoxia-related pathologies. In the acid-secreting parietal cells of the gastric mucosa, the use of various radioligands for adenosine receptors suggested the presence of the A2 adenosine receptor subtype(s) on the cell surface. Recently, we confirmed A2BR expression in native, nontransformed parietal cells at rest by using flow cytometry and confocal microscopy. In this study, we show that A2BR is functional in primary rabbit gastric parietal cells, as indicated by the fact that agonist binding to A2BR increased adenylate cyclase activity and acid production. In addition, both acid production and radioligand binding of adenosine analogs to isolated cell membranes were potently blocked by selective A2BR antagonists, whereas ligands for A1, A2A, and A3 adenosine receptors failed to abolish activation. We conclude that rabbit gastric parietal cells possess functional A2BR proteins that are coupled to Gs and stimulate HCl production upon activation. Whether adenosine- and A2BR-mediated functional responses play a role in human gastric pathophysiology is yet to be elucidated.

Hypergastrinemia

Gastrin is an important hormone of the digestive system, which assists gastric acid secretion. It may be pathologically elevated in conditions such as Zollinger-Ellison syndrome, or due to common medications such as proton pump inhibitors. In this review we provide an overview of the pathophysiology and medical causes of hypergastrinemia, diagnostic testing and clinical consequences of chronic hypergastrinemia.

The Golgi apparatus in the endomembrane-rich gastric parietal cells exist as functional stable mini-stacks dispersed throughout the cytoplasm

Background information. Acid-secreting gastric parietal cells are polarized epithelial cells that harbour highly abundant and specialized, H⁺,K⁺ ATPase-containing, tubulovesicular membranes in the apical cytoplasm. The Golgi apparatus has been implicated in the biogenesis of the tubulovesicular membranes; however, an unanswered question is how a typical Golgi organization could regulate normal membrane transport within the membrane-dense cytoplasm of parietal cells.

Results. Here, we demonstrate that the Golgi apparatus of parietal cells is not the typical juxta-nuclear ribbon of stacks, but rather individual Golgi units are scattered throughout the cytoplasm. The Golgi membrane structures labelled with markers of both cis- and trans-Golgi membrane, indicating the presence of intact Golgi stacks. The parietal cell Golgi stacks were closely aligned with the microtubule network and were shown to participate in both anterograde and retrograde transport pathways. Dispersed Golgi stacks were also observed in parietal cells from H⁺,K⁺ ATPase-deficient mice that lack tubulovesicular membranes.

Conclusions. These results indicate that the unusual organization of individual Golgi stacks dispersed throughout the cytoplasm of these terminally differentiated cells is likely to be a developmentally regulated event.

Protein intake, calcium balance and health consequences

High-protein (HP) diets exert a hypercalciuric effect at constant levels of calcium intake, even though the effect may depend on the nature of the dietary protein. Lower urinary pH is also consistently observed for subjects consuming HP diets. The combination of these two effects was suspected to be associated with a dietary environment favorable for demineralization of the skeleton. However, increased calcium excretion due to HP diet does not seem to be linked to impaired calcium balance. In contrast, some data indicate that HP intakes induce an increase of intestinal calcium absorption. Moreover, no clinical data support the hypothesis of a detrimental effect of HP diet on bone health, except in a context of inadequate calcium supply. In addition, HP intake promotes bone growth and retards bone loss and low-protein diet is associated with higher risk of hip fractures. The increase of acid and calcium excretion due to HP diet is also accused of constituting a favorable environment for kidney stones and renal diseases. However, in healthy subjects, no damaging effect of HP diets on kidney has been found in either observational or interventional studies and it seems that HP diets might be deleterious only in patients with preexisting metabolic renal dysfunction. Thus, HP diet does not seem to lead to calcium bone loss, and the role of protein seems to be complex and probably dependent on other dietary factors and the presence of other nutrients in the diet.

Zinc salts provide a novel, prolonged and rapid inhibition of gastric acid secretion

OBJECTIVES

The overproduction of acid and the associated illnesses linked to hypersecretion have a lifetime prevalence of 25-35% in the United States. Although a variety of pharmaceutical agents have been used to reduce the production of acid, alarming new evidence questions the long-term efficacy and safety of the agents. These issues coupled with the delayed onset of action and the return of symptoms in over 60% of the patients is less than satisfactory. The purpose of this study was to determine whether administration of a zinc salt could lead to a rapid and sustained increase in gastric pH in both animals and in humans and provide a new rapid acid suppression therapy.

METHODS

Intracellular pH was measured with 2',7'-bis-(2-carboxyethyl)-5-and-6-carboxy-fluorescein in both human and rat gastric glands following an acid load±a secretagogue. In a separate series of studies, whole stomach acid secretion was monitored in rats. A final study used healthy human volunteers while monitoring with a gastric pH measurement received placebo, zinc salt, or a zinc salt and proton pump inhibitor (PPI).

RESULTS

We demonstrate that exposure to ZnCl(2) immediately abolished secretagogue-induced acid secretion in isolated human and rat gastric glands, and in intact rat stomachs. Chronic low-dose zinc exposure effectively inhibited acid secretion in whole stomachs and isolated glands. In a randomized cross-over study in 12 volunteers, exposure to a single dose of ZnCl(2) raised intragastric pH for over 3  h, including a fast onset of effect.

CONCLUSIONS

Our findings demonstrate that zinc offers a novel rapid and prolonged therapy to inhibit gastric acid secretion in human and rat models.

Elevated serum gastrin levels in Jervell and Lange-Nielsen syndrome: a marker of severe KCNQ1 dysfunction?

BACKGROUND

The potassium channel I(Ks), which is encoded by the KCNQ1 gene, is expressed in organ systems including the inner ear, kidneys, lungs, intestine, and stomach in addition to the heart. Increasing evidence indicates that I(Ks) in the stomach plays an essential role in enabling gastric acid production. It is not known whether gastric acid production is disordered in patients with long QT type 1. Serum gastrin levels become elevated in subjects with disordered gastric acid production.

OBJECTIVE

The purpose of this study was to evaluate serum gastrin levels, as a surrogate for impaired gastric acid secretion, in patients with KCNQ1 mutations, and to see if gastrin levels correlate with severity of cardiac disease.

METHODS

Fasting serum gastrin levels were measured using a standardized immunometric technique in an index case and 12 subjects with known KCNQ1 mutations.

RESULTS

An adult female with Jervell and Lange-Nielsen syndrome (JLNS; with KCNQ1 nonsense mutations p.Arg518X and p.Arg190AlafsX95 ) presented with multiple gastric carcinoid tumors and grossly elevated serum gastrin levels (943-1,570 pmol/L; normal 6-55 pmol/L) and absent acid secretion. Gastrin levels in two girls with JLNS, unrelated to the index case (missense mutations p.Leu266Pro and Gly269Ser), also were high (305 and 229 pmol/L). Gastrin levels were normal in 10 KCNQ1 heterozygous single mutation carriers, even in those with severe long QT syndrome, including three heterozygous family members of the JLNS subjects.

CONCLUSION

JLNS may be associated with elevated gastrin levels, impaired acid secretion, and risk of gastric carcinoid tumors. Among KCNQ1 single mutation carriers, gastrin levels were normal and did not appear to be linked to the severity of clinical expression of long QT syndrome.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

Gastroesophageal reflux disease (GERD): is there more to the story?

Gastroesophageal reflux disease (GERD) affects both men and women worldwide, with the most common symptom of GERD being frequent heartburn. If left untreated, more serious diseases including esophagitis and/or esophageal cancer may result. GERD has been commonly held to be the result of gastric acid refluxing into the esophagus. Recent work, however, has shown that there are acid-producing cells in the upper aerodigestive tract. In addition, acid-producing bacteria located within the upper gastrointestinal tract and oral cavity may also be a contributing factor in the onset of GERD. Proton pump inhibitors (PPIs) are commonly prescribed for treating GERD; these drugs are designed to stop the production of gastric acid by shutting down the H(+)/K(+)-ATPase enzyme located in parietal cells. PPI treatment is systemic and therefore significantly different than traditional antacids. Although a popular treatment choice, PPIs exhibit substantial interpatient variability and commonly fail to provide a complete cure to the disease. Recent studies have shown that H(+)/K(+)-ATPases are expressed in tissues outside the stomach, and the effects of PPIs in these nongastric tissues have not been fully explored. Likewise, acid-producing bacteria containing proton pumps are present in both the oral cavity and esophagus, and PPI use may also adversely affect these bacteria. The use of PPI therapy is further complicated by the two philosophical approaches to treating this disease: to treat only symptoms or to treat continuously. The latter approach frequently results in unwanted side effects which may be due to the PPIs acting on nongastric tissues or the microbes which colonize the upper aerodigestive tract.

No potassium, no acid: K+ channels and gastric acid secretion

The gastric H+-K+-ATPase pumps H+ into the lumen and takes up K+ in parallel. In the acid-producing parietal cells, luminal KCNE2/KCNQ1 K+ channels play a pivotal role in replenishing K+ in the luminal fluid. Inactivation of KCNE2/KCNQ1 channels abrogates gastric acid secretion and dramatically modifies the architecture of gastric mucosa.

Physiological and clinical significance of enterochromaffin-like cell activation in the regulation of gastric acid secretion

Gastric acid plays an important role in digesting food (especially protein), iron absorption, and destroying swallowed micro-organisms. H+ is secreted by the oxyntic parietal cells and its secretion is regulated by endocrine, neurocrine and paracrine mechanisms. Gastrin released from the antral G cell is the principal physiological stimulus of gastric acid secretion. Activation of the enterochromaffin-like (ECL) cell is accepted as the main source of histamine participating in the regulation of acid secretion and is functionally and trophically controlled by gastrin, which is mediated by gastrin/CCK-2 receptors expressed on the ECL cell. However, long-term hypergastrinemia will induce ECL cell hyperplasia and probably carcinoids. Clinically, potent inhibitors of acid secretion have been prescribed widely to patients with acid-related disorders. Long-term potent acid inhibition evokes a marked increase in plasma gastrin levels, leading to enlargement of oxyntic mucosa with ECL cell hyperplasia. Accordingly, the induction of ECL cell hyperplasia and carcinoids remains a topic of considerable concern, especially in long-term use. In addition, the activation of ECL cells also induces another clinical concern, i.e., rebound acid hypersecretion after acid inhibition. Recent experimental and clinical findings indicate that the activation of ECL cells plays a critical role both physiologically and clinically in the regulation of gastric acid secretion.