Cell biology of gastric acid secretion

The genetic regulation of the gastric transcriptome is associated with metabolic and obesity-related traits and diseases

Tissue-specific gene expression and gene regulation lead to a better understanding of tissue-specific physiology and pathophysiology. We analyzed the transcriptome and genetic regulatory profiles of two distinct gastric sites, corpus and antrum, to identify tissue-specific gene expression and its regulation. Gastric corpus and antrum mucosa biopsies were collected during routine gastroscopies from up to 431 healthy individuals. We obtained genotype and transcriptome data and performed transcriptome profiling and expression quantitative trait locus (eQTL) studies. We further used data from genome-wide association studies (GWAS) of various diseases and traits to partition their heritability and to perform transcriptome-wide association studies (TWAS). The transcriptome data from corpus and antral mucosa highlights the heterogeneity of gene expression in the stomach. We identified enriched pathways revealing distinct and common physiological processes in gastric corpus and antrum. Furthermore, we found an enrichment of the single nucleotide polymorphism (SNP)-based heritability of metabolic, obesity-related, and cardiovascular traits and diseases by considering corpus- and antrum-specifically expressed genes. Particularly, we could prioritize gastric-specific candidate genes for multiple metabolic traits, like NQO1 which is involved in glucose metabolism, MUC1 which contributes to purine and protein metabolism or RAB27B being a regulator of weight and body composition. Our findings show that gastric corpus and antrum vary in their transcriptome and genetic regulatory profiles indicating physiological differences which are mostly related to digestion and epithelial protection. Moreover, our findings demonstrate that the genetic regulation of the gastric transcriptome is linked to biological mechanisms associated with metabolic, obesity-related, and cardiovascular traits and diseases. NEW & NOTEWORTHY We analyzed the transcriptomes and genetic regulatory profiles of gastric corpus and for the first time also of antrum mucosa in 431 healthy individuals. Through tissue-specific gene expression and eQTL analyses, we uncovered unique and common physiological processes across both primary gastric sites. Notably, our findings reveal that stomach-specific eQTLs are enriched in loci associated with metabolic traits and diseases, highlighting the pivotal role of gene expression regulation in gastric physiology and potential pathophysiology.

Remodeling of the gastric environment in Helicobacter pylori-induced atrophic gastritis

Helicobacter pylori colonization of the human stomach is a strong risk factor for gastric cancer. To investigate H. pylori-induced gastric molecular alterations, we used a Mongolian gerbil model of gastric carcinogenesis. Histologic evaluation revealed varying levels of atrophic gastritis (a premalignant condition characterized by parietal and chief cell loss) in H. pylori-infected animals, and transcriptional profiling revealed a loss of markers for these cell types. We then assessed the spatial distribution and relative abundance of proteins in the gastric tissues using imaging mass spectrometry and liquid chromatography with tandem mass spectrometry. We detected striking differences in the protein content of corpus and antrum tissues. Four hundred ninety-two proteins were preferentially localized to the corpus in uninfected animals. The abundance of 91 of these proteins was reduced in H. pylori-infected corpus tissues exhibiting atrophic gastritis compared with infected corpus tissues exhibiting non-atrophic gastritis or uninfected corpus tissues; these included numerous proteins with metabolic functions. Fifty proteins localized to the corpus in uninfected animals were diffusely delocalized throughout the stomach in infected tissues with atrophic gastritis; these included numerous proteins with roles in protein processing. The corresponding alterations were not detected in animals infected with a H. pylori ∆cagT mutant (lacking Cag type IV secretion system activity). These results indicate that H. pylori can cause loss of proteins normally localized to the gastric corpus as well as diffuse delocalization of corpus-specific proteins, resulting in marked changes in the normal gastric molecular partitioning into distinct corpus and antrum regions.IMPORTANCEA normal stomach is organized into distinct regions known as the corpus and antrum, which have different functions, cell types, and gland architectures. Previous studies have primarily used histologic methods to differentiate these regions and detect H. pylori-induced alterations leading to stomach cancer. In this study, we investigated H. pylori-induced gastric molecular alterations in a Mongolian gerbil model of carcinogenesis. We report the detection of numerous proteins that are preferentially localized to the gastric corpus but not the antrum in a normal stomach. We show that stomachs with H. pylori-induced atrophic gastritis (a precancerous condition characterized by the loss of specialized cell types) exhibit marked changes in the abundance and localization of proteins normally localized to the gastric corpus. These results provide new insights into H. pylori-induced gastric molecular alterations that are associated with the development of stomach cancer.

Amelioration of alcohol‑induced gastric mucosa damage by oral administration of food‑polydeoxyribonucleotides

Gastritis refers to inflammation caused by injury to the gastric epithelium, which is usually due to excessive alcohol consumption and prolonged use of nonsteroidal anti-inflammatory drugs. Millions of individuals worldwide suffer from this disease. However, the lack of safe and promising treatments makes it urgent to explore and develop leads from natural resources. Therefore, food as medicine may be the best approach for the treatment of these disorders. The present study described the protective effects of food-polydeoxyribonucleotides (f-PDRNs) in a rat model of gastric mucosal injury induced by HCl-EtOH. Administration of f-PDRN was performed with low-PRF002 (26 mg/kg/day), medium-PRF002 (52 mg/kg/day) and high-PRF002 (78 mg/kg/day) on the day of autopsy. The site of damage to the mucous membrane was also analysed. In addition, an increase in gastric juice pH, total acidity of gastric juice and decrease in gastric juice secretion were confirmed, and gastric juice secretion-related factors corresponding to the administration of f-PDRN were analysed. Administration of f-PDRN reduced the mRNA expression of histamine H2 receptor, muscarinic acetylcholine receptor M3, cholecystokinin 2 receptor and H⁺/K⁺ ATPase related to gastric acid secretion and downregulation of histamine, myeloperoxidase and cyclic adenosine monophosphate. In addition, it was histologically confirmed that the loss of epithelial cells and the distortion of the mucosa were recovered in the group in which f-PDRN was administered compared to the model group with gastric mucosa damage. In summary, the present study suggested that f-PDRN has therapeutic potential and may have beneficial effects if taken regularly as a food supplement.

Gastroprotective Effects of Cudrania tricuspidata Leaf Extracts by Suppressing Gastric cAMP and Increasing Gastric Mucins

Cudrania tricuspidata has been used as an East Asian folk remedy to treat various symptoms. Recently, scientific evidence of the efficacy of C. tricuspidata has emerged. The objective of this study was to elucidate protective role of C. tricuspidata in the gastric mucosa using pylorus-ligated Sprague-Dawley rats and primary parietal cells. C. tricuspidata ethanol extracts attenuated gastric mucosal damage, secretion, and juice acidity in pylorus-ligated rats; however, it did not affect expression of gastric acid-related genes [muscarinic acetylcholine receptor M3 receptor (M3R), histamine H2-receptors (H2R), and cholecystokinin-2/gastrin receptors (CCK2R)] or serum gastrin concentrations. Furthermore, extracts greatly reduced levels of gastric cyclic adenosine monophosphate (cAMP) and significantly increased mRNA levels of gastric-type mucins (MUC5AC and MUC6). To identify the mode of action of C. tricuspidata extract in regulating gastric acid secretion, intracellular cAMP and mRNA for H2R, M3R, and CCK2R were measured in primary parietal cells. mRNA levels of H2R, M3R, and CCK2R did not significantly differ following treatment with C. tricuspidata extract, whereas cAMP induced by the H2R-specific agonist was significantly decreased. C. tricuspidata may therefore reduce gastric acid secretion by inhibiting H2R activity rather than regulating mRNA expression. These finding suggest that ethanol extracts of C. tricuspidata inhibit H2R-related gastric acid secretion and increase gastric mucus to help prevent gastric mucosal damage. Therefore, C. tricuspidata extract has potential to be used in foods and medicines to prevent diseases related to gastric mucosal damage, such as gastritis and functional dyspepsia.

Modest Regulation of Digestive Performance Is Maintained through Early Ontogeny for the American Alligator, Alligator mississippiensis

The American alligator, Alligator mississippiensis, is an opportunistic carnivore that experiences an ontogenetic shift in food and feeding habits with an increase in body size. Alligators frequently feed on invertebrates and small fish as neonates and transition to feeding less frequently on larger vertebrates as they grow. We hypothesized that alligators experience an ontogenetic shift in the regulation of intestinal performance-modest regulation with frequent feeding early in life and wider regulation with less frequent feeding as they increase in body size. We tested this hypothesis by comparing postprandial responses in metabolic rate, organ masses, intestinal histology, digestive hydrolase activities, and intestinal nutrient uptake rates among neonate, juvenile, and subadult alligators. With feeding, alligators of all three age classes experienced a rapid increase in metabolic rate that peaked within 2 d and thereafter declined more slowly to prefeeding rates. Specific dynamic action increased with body mass and was equivalent to 32% of meal energy. For each age class, the majority of organs did not change in wet and dry mass with feeding. For subadult alligators, luminal gut pH varied regionally due to the acidic stomach, which continued to remain acidic with fasting. With feeding, epithelial enterocytes are remodeled from a pseudostratified to a stratified architecture and become infiltrated with lipid droplets. Feeding did not generate any significant change in the thickness of intestinal tissues, though it did induce an increase in enterocyte width and volume for subadults. For each age class, feeding generally did not result in significant changes in pancreatic trypsin, intestinal aminopeptidase, and intestinal nutrient uptake activities and capacities. Mass-specific nutrient uptake rates varied among age classes due to the higher rates exhibited by neonates. Among age classes, intestinal uptake capacities scaled allometrically (mass exponents <1) with body mass. Across these three age classes, the modest regulation of digestive performance with feeding and fasting for alligators appears to be ontogenetically conserved.

Helicobacter pylori associated factors in the development of gastric cancer with special reference to the early-onset subtype

Nowadays, gastric cancer is one of the most common neoplasms and the fourth cause of cancer-related death on the world. Regarding the age at the diagnosis it is divided into early-onset gastric carcinoma (45 years or younger) and conventional gastric cancer (older than 45). Gastric carcinomas are rarely observed in young population and rely mostly on genetic factors, therefore provide the unique model to study genetic and environmental alternations. The latest research on early-onset gastric cancer are trying to explain molecular and genetic basis, because young patients are less exposed to environmental factors predisposing to cancer. In the general population, Helicobacter pylori, has been particularly associated with intestinal subtype of gastric cancers. The significant association of Helicobacter pylori infection in young patients with gastric cancers suggests that the bacterium has an etiologic role in both diffuse and intestinal subtypes of early-onset gastric cancers. In this paper we would like to ascertain the possible role of Helicobacter pylori infection in the development of gastric carcinoma in young patients. The review summarizes recent literature on early-onset gastric cancers with special reference to Helicobacter pylori infection.

Recent advances in the molecular and functional characterization of acid/base and electrolyte transporters in the basolateral membranes of gastric and duodenal epithelial cells

All segments of the gastrointestinal tract are comprised of an elaborately folded epithelium that expresses a variety of cell types and performs multiple secretory and absorptive functions. While the apical membrane expresses the electrolyte transporters that secrete or absorb electrolytes and water, basolateral transporters regulate the secretory or absorptive rates. During gastric acid formation, Cl⁻/HCO₃⁻ and Na(+) /H(+) exchange and other transporters secure Cl⁻ re-supply as well as pH and volume regulation. Gastric surface cells utilize ion transporters to secrete HCO₃⁻, maintain pH(i) during a luminal acid load and repair damaged surface areas during the process of epithelial restitution. Na(+)/H(+) exchange and Na(+)/HCO₃⁻ cotransport serve basolateral acid/base import for gastroduodenal HCO₃⁻ secretion. The gastric and duodenal epithelium also absorbs salt and water. Recent molecular information on novel ion transporters expressed in the gastric and duodenal epithelium has exploded; however, a function has not been found yet for all transporters. The purpose of this review is to summarize current knowledge on the molecular identity and cellular function of basolateral ion transporters in the gastric and duodenal epithelium.

Digestive challenges for vertebrate animals: microbial diversity, cardiorespiratory coupling, and dietary specialization

The digestive system is the interface between the supply of food for an animal and the demand for energy and nutrients to maintain the body, to grow, and to reproduce. Digestive systems are not morphologically static but rather dynamically respond to changes in the physical and chemical characteristics of the diet and the level of food intake. In this article, we discuss three themes that affect the ability of an animal to alter digestive function in relation to novel substrates and changing food supply: (1) the fermentative digestion in herbivores, (2) the integration of cardiopulmonary and digestive functions, and (3) the evolution of dietary specialization. Herbivores consume, digest, and detoxify complex diets by using a wide variety of enzymes expressed by bacteria, predominantly in the phyla Firmicutes and Bacteroidetes. Carnivores, such as snakes that feed intermittently, sometimes process very large meals that require compensatory adjustments in blood flow, acid secretion, and regulation of acid-base homeostasis. Snakes and birds that specialize in simple diets of prey or nectar retain their ability to digest a wider selection of prey. The digestive system continues to be of interest to comparative physiologists because of its plasticity, both phenotypic and evolutionary, and because of its widespread integration with other physiological systems, including thermoregulation, circulation, ventilation, homeostasis, immunity, and reproduction.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Effect of nitric oxide on histamine-induced cytological transformations in parietal cells in isolated human gastric glands

Previous studies have shown that nitric oxide (NO) inhibits histamine-induced gastric acid secretion in isolated human gastric glands. NO synthase has been found to be present in the human oxyntic mucosa and has been suggested to serve as a paracrine regulator of gastric acid secretion. Histamine stimulation of parietal cells induces cytoskeletal rearrangements, recruitment of H+/K+-ATPase-rich tubulovesicles to the apical membrane and expansion of intracellular canaliculi. The aim of the present study was thus to investigate (i) the effect of an NO donor on histamine-induced cytological transformations and (ii) the influence of increased [Ca2+]i on NO-induced morphological changes in human parietal cells. Human gastric glands were isolated and subjected to the NO donor SNAP prior to histamine administration. [Ca2+]i was increased by photolysis of the caged Ca2+ compound NP-EGTA. The distribution of F-actin, ezrin, and H+/K+-ATPase was assessed by confocal microscopy. Ultrastructural analysis was performed using transmission electron microscopy. SNAP did not influence the histamine-induced translocation of F-actin, ezrin, and H+/K+-ATPase but prevented an increase in the canalicular size. Elevation of [Ca2+]i in resting cells was found to mimic histamine-induced intraparietal cell transformations; however, NO-induced parietal cell morphology was unaffected by a rise in [Ca2+]i. These results indicate that NO inhibits secretion of fluid into the canalicular lumen without affecting membrane recruitment and that this effect is Ca2+-insensitive.

Intragastric acidity after switching from 5-day treatment with intravenous pantoprazole 40 mg/d to 5-day treatment with oral esomeprazole 40 mg/d or pantoprazole 40 mg/d: an open-label crossover study in healthy adult volunteers

BACKGROUND

After a course of IV proton pump inhibitor therapy, patients might require continued oral antisecretory therapy. A direct comparison of therapeutic alternatives could assist physicians in decisions regarding optimal acid-suppressive therapy. Oral esomeprazole might control intragastric acidity more effectively compared with other acid-suppressive agents after IV therapy.

OBJECTIVE

The aim of this study was to compare intragastric acid control on day 5 of administration of esomeprazole magnesium versus pantoprazole 40 mg PO QD after switching from 5 days of treatment with pantoprazole 40 mg IV in healthy volunteers.

METHODS

This randomized, open-label, comparative, 2-way crossover study was conducted at the Oklahoma Foundation for Digestive Research, Oklahoma City, Oklahoma, between October and December 2004. Healthy, Helicobacter pylori-negative adults were randomly assigned to 1 of 2 dosing sequences: pantoprazole IV followed by esomeprazole PO or pantoprazole IV followed by pantoprazole PO. All study medications were administered for 5 days at a dose of 40 mg QD. IV pantoprazole was administered over 2 minutes; all medications were administered 30 minutes before breakfast. There was a 10- to 21-day washout period between each 10-day dosing period. All doses were administered at the study site. Before oral study drug administration on days 1 and 5, 24-hour pH monitoring was performed using a pH catheter positioned 10 cm distal to the lower esophageal sphincter in the stomach. The primary end point was percentage of time with pH >4 (%t pH >4) during the 24-hour pH-monitoring period. Tolerability was assessed using spontaneous reporting, laboratory analysis, and vital-sign measurement.

RESULTS

Of 42 subjects randomized to treatment sequences, 4 were withdrawn during the study because of invalid pH data; 38 subjects (24 men, 14 women; mean [SD] age, 25.2 [8.1] years) had assessable data. Day-5 %t pH >4 was 68.5% with esomeprazole and 53.3% with pantoprazole (P < 0.001). Day-1 %t pH >4 was 62.5% with esomeprazole and 51.0% with pantoprazole (P < 0.001). The most common adverse events were rhinitis (2 subjects each with pantoprazole IV and PO; 1 subject with esomeprazole) and headache (2 subjects with esomeprazole; 1 subject with pantoprazole IV).

CONCLUSIONS

The results of this study in healthy adult volunteers suggest that switching from pantoprazole 40 mg IV to esomeprazole 40 mg PO QD more effectively suppresses intragastric acid compared with switching from pantoprazole 40 mg IV to pantoprazole 40 mg PO QD. All 3 treatments were well tolerated.

A combined experimental and quantum chemical study on the putative protonophoric activity of thiocyanate

Inhibition of gastric acid secretion by thiocyanate is explained by a protonophoric mechanism assuming that thiocyanate induces a H(+) back flux from the acidic gastric lumen into the parietal cells of gastric mucosa. Protonophoric activity of thiocyanate was examined by swelling measurements using rat liver mitochondria and theoretically by quantum chemical methods. Mitochondria suspended in K-thiocyanate medium plus nigericin (an H/K-exchanger) swelled when the medium pH was acidic, indicating that SCN(-) initiates a transfer of H(+) across the inner membrane. To rationalize the protonophoric activity of thiocyanate, we considered the dehydration of SCN(-) to be critical for transmembranal H(+) transfer. For modeling this process, various hydrate clusters of SCN(-) and Cl(-) were generated and optimized by density functional theory (DFT) at the B3-LYP/6-311++G(d,p) level. The cluster hydration energy was lower for SCN(-) than for Cl(-). The total Gibbs free energies of hydration of the ions were estimated by a hybrid supermolecule-continuum approach based on DFT. The calculated hydration energies also led to the conclusion that SCN(-) is less efficiently solvated than Cl(-). Due to the easier removal of the hydration shell of SCN(-) relative to Cl(-), SCN(-) is favored in going across the membrane, giving rise to the protonophoric activity.

Direct measurement of gastric H+/K+-ATPase activities in patients with or without Helicobacter pylori-associated chronic gastritis

AIM: The role of Helicobacter pylori (H pylori ) infection in gastric acid secretion of patients with chronic gastritis remains controversial. This study was designed to elucidate the effect of H pylori on H⁺/K⁺-ATPase activities in gastric biopsy specimens.

METHODS: Eighty-two patients with chronic gastritis who had undergone upper endoscopy were included in this study. H pylori infection was confirmed by rapid urease test and histology. Gastric H⁺/K⁺-ATPase activities and serum gastrin concentrations were measured by an enzymatic method and radioimmunoassay, respectively. For those patients who received triple therapy for eradicating H pylori, changes in the activity of gastric H⁺/K⁺-ATPase and serum gastrin levels were also measured.

RESULTS: The mean gastric H⁺/K⁺-ATPase activity in H pylori-positive group (42 patients) was slightly higher than that in H pylori-negative group (29 patients) (169.65±52.9 and 161.38±43.85 nmol P_i/(mg·h), respectively, P = 0.301). After eradication of H pylori, the gastric H⁺/K⁺-ATPase activities slightly decreased compared to prior therapy (165.03±59.50 and 158.42±38.93 nmol P_i/(mg·h), respectively, P = 0.805). The mean basal gastrin concentration was slightly higher in H pylori-positive patients than in H pylori-negative patients (87.92±39.65 pg/mL vs 75.04± 42.57 pg/mL, P = 0.228). The gastrin levels fell significantly after the eradication of H pylori. (Before treatment 87.00±30.78 pg/mL, after treatment 64.73±18.96 pg/mL, P = 0.015).

CONCLUSION: Gastric H⁺/K⁺-ATPase activities are not associated with H pylori status in patients with chronic gastritis.

The role of gastric acid in preventing foodborne disease and how bacteria overcome acid conditions

The secretion of hydrochloric acid by the stomach plays an important role in protecting the body against pathogens ingested with food or water. A gastric fluid pH of 1 to 2 is deleterious to many microbial pathogens; however, the neutralization of gastric acid by antacids or the inhibition of acid secretion by various drugs may increase the risk of food- or waterborne illnesses. Peptic ulcer disease is often treated by decreasing or eliminating gastric acid secretion, and such treatment blocks the protective antibacterial action of gastric fluid. The majority of peptic ulcer disease cases originate from Helicobacter pylori infections. Treatment of H. pylori-induced peptic ulcers with antibiotics reduces the need for drugs that inhibit gastric acid secretion and thereby diminishes the risk of food- and waterborne illness for peptic ulcer disease patients. Many bacterial pathogens, such as Escherichia coli, Salmonella Typhimurium, and H. pylori, can circumvent the acid conditions of the stomach by developing adaptive mechanisms that allow these bacteria to survive in acid environments. As a consequence, these bacteria can survive acidic stomach conditions and pass into the intestinal tract, where they can induce gastroenteritis.

Gastric function and its contribution to the postprandial metabolic response of the Burmese python Python molurus

The large intact prey ingested by Burmese pythons require considerable processing by the stomach before passage into the small intestine. To investigate the function and cost of gastric digestion and its contribution to postprandial metabolic response for the Burmese python, I examined the rate of gastric digestion, the postprandial profile of gastric pH and the effects of decreasing gastric workload on the metabolic cost of digestion, referred to as specific dynamic action (SDA). Ingested meal mass (equivalent to 25% of snake body mass) was reduced by 18% within 1 day postfeeding, by which time intragastric pH had decreased from 7.5 to 2. Gastric pH was maintained at 1.5 for the next 5-7 days, after which it returned to 7.5. The SDA generated by digesting an intact rat meal was reduced by 9.1%, 26.0%, 56.5% and 66.8%, respectively, when pythons were fed steak, ground rat, liquid diet or ground rat directly infused into the small intestine. The production of HCl and enzymes and other gastric functions represent an estimated 55% of the python's SDA generated from the digestion of an intact rodent meal. Additional contributors to SDA include protein synthesis (estimated 26%), gastrointestinal upregulation (estimated 5%) and the activities of the pancreas, gallbladder, liver, kidneys and intestines during digestion (estimated 14%). Operating on a 'pay before pumping' principle, pythons must expend endogenous energy in order to initiate acid production and other digestive processes before ingested nutrients can be absorbed and channeled into metabolic pathways.

Fast freeze-fixation/freeze-substitution reveals the secretory membranes of the gastric parietal cell as a network of helically coiled tubule. A new model for parietal cell transformation

The parietal cell of the gastric mucosa undergoes rapid morphological transformation when it is stimulated to produce hydrochloric acid. In chemically fixed cells, this process is seen as a reduction in number of cytoplasmic ‘tubulovesicles’ as the apical surface of the cell progressively invaginates to increase the secretory surface area. It is widely believed that the tubulovesicles represent stored secretory membrane in the cytoplasm of the unstimulated cell, which is incorporated into the apical membrane upon stimulation, because they share H+,K+-ATPase activity with the apical membrane. However, fusion of tubulovesicles with the apical membrane concomitant with parietal cell activation has never been convincingly demonstrated. We have used fast freeze-fixation and freeze-substitution to study stages of morphological transformation in these cells. Tubulovesicles were not seen in the cytoplasm of any of our cryoprepared cells. Instead, the cytoplasm of the unstimulated cell contained numerous and densely packed helical coils of tubule, each having an axial core of cytoplasm. The helical coils were linked together by connecting tubules, lengths of relatively straight tubule. Lengths of straight connecting tubule also extended from coils lying adjacent to the apical and canalicular surfaces and ended at the apical and canaliculus membranes. Immunogold labelling with alpha- and beta-subunit-specific antibodies showed that the gastric H+,K+-ATPase was localized to the membranes of this tubular system, which therefore represented the configuration of the secretory membrane in the cytoplasm of the unstimulated parietal cell. Stimulation of the cells with histamine and isobutylmethylxanthine lead to modification of the tubular membrane system, correlated with progressive invagination of the apical membrane. The volume of the tubule lumen increased and, as this occurred, the tight spiral twist of the helical coils was lost, indicating that tubule distension was accounted for by partial unwinding. This exposed the cores of cytoplasm in the axes of the coils as rod-shaped elements of a three-dimensional reticulum, resembling a series of microvilli in random thin sections. Conversely, treatment with the H2 antagonist cimetidine caused severe contraction of the tubular membrane system and intracellular canaliculi. Our results indicate that tubulovesicles are an artifact of chemical fixation; consequently, they cannot have a role in parietal cell transformation. From our findings we propose an alternative model for morphological transformation in the parietal cell. This model predicts cytoskeleton-mediated control over expansion and contraction of the tubular membrane network revealed by cryopreparation. The model is compatible with the localization of cytoskeletal components in these cells.