ECL cell histamine mobilization and parietal cell stimulation in the rat stomach studied by microdialysis and electron microscopy

Evaluation of Gastric pH and Gastrin Concentrations in Horses Subjected to General Inhalation Anesthesia in Dorsal Recumbency

The prevalence of gastric disorders in high-performance horses, especially gastric ulceration, ranges from 50 to 90%. These pathological conditions have negative impacts on athletic performance and health. This study was designed to evaluate changes in gastric pH during a 24 h period and to compare gastrin concentrations at different time points in horses undergoing general inhalation anesthesia and dorsal recumbency. Twenty-two mixed-breed mares weighing 400 ± 50 kg and aged 8 ± 2 years were used. Of these, eight were fasted for 8 h and submitted to 90 min of general inhalation anesthesia in dorsal recumbency. Gastric juice samples were collected prior to anesthesia (T0), and then at 15 min intervals during anesthesia (T15-T90). After recovery from anesthesia (45 ± 1 min), samples were collected every hour for 24 h (T1 to T24) for gastric juice pH measurement. During this period, mares had free access to Bermuda grass hay and water and were fed a commercial concentrate twice (T4 and T16). In a second group (control), four non-anesthetized mares were submitted to 8 h of fasting followed by nasogastric intubation. Gastric juice samples were then collected at T0, T15, T30, T45, T60, T75, and T90. During this period, mares did not receive food or water. After 45 min, mares had free access to Bermuda grass hay and water, and gastric juice samples were collected every hour for four hours (T1 to T4). In a third group comprising ten non-fasted, non-anesthetized mares with free access to Bermuda grass hay and water, gastric juice samples were collected 30 min after concentrate intake (T0). In anesthetized mares, blood gastrin levels were measured prior to anesthesia (8 h fasting; baseline), during recovery from anesthesia, and 4 months after the anesthetic procedure, 90 min after the morning meal. Mean values of gastric juice pH remained acidic during general anesthesia. Mean pH values were within the physiological range (4.52 ± 1.69) and did not differ significantly between time points (T15-T90; p  >  0.05). After recovery from anesthesia, mean gastric pH values increased and remained in the alkaline range throughout the 24 h period of evaluation. Significant differences were observed between T0 (4.88 ± 2.38), T5 (7.08 ± 0.89), T8 (7.43 ± 0.22), T9 (7.28 ± 0.36), T11 (7.26 ± 0.71), T13 (6.74 ± 0.90), and T17 (6.94 ± 1.04) (p  <  0.05). The mean gastric juice pH ranged from weakly acidic to neutral or weakly alkaline in all groups, regardless of food and water intake (i.e., in the fasted, non-fasted, and fed states). Mean gastric pH measured in the control group did not differ from values measured during the 24 h post-anesthesia period or in the non-fasted group. Gastrin concentrations increased significantly during the post-anesthetic period compared to baseline (20.15 ± 7.65 pg/mL and 15.15 ± 3.82 pg/mL respectively; p  <  0.05). General inhalation anesthesia and dorsal recumbency did not affect gastric juice pH, which remained acidic and within the physiological range. Gastric juice pH was weakly alkaline after recovery from anesthesia and in the fasted and fed states. Serum gastrin levels increased in response to general inhalation anesthesia in dorsal recumbency and were not influenced by fasting. Preventive pharmacological measures are not required in horses submitted to general anesthesia and dorsal recumbency.

Simultaneous detection of gastric acid and histamine release to unravel the regulation of acid secretion from the guinea pig stomach

Gastric acid secretion is regulated by three primary components that activate the parietal cell: histamine, gastrin, and acetylcholine (ACh). Although much is known about these regulatory components individually, little is known on the interplay of these multiple activators and the degree of regulation they pose on the gastric acid secretion mechanism. We utilized a novel dual-sensing approach, where an iridium oxide sensor was used to monitor pH and a boron-doped diamond electrode was used for the detection of histamine from in vitro guinea pig stomach mucosal sections. Under basal conditions, gastrin was shown to be the main regulatory component of the total acid secretion and directly activated the parietal cell rather than by mediating gastric acid secretion through the release of histamine from the enterochromaffin-like cell, although both pathways were active. Under stimulated conditions with ACh, the gastrin and histamine components of the total acid secretion were not altered compared with levels observed under basal conditions, suggestive that ACh had no direct effect on the enterochromaffin-like cell and G cell. These data identify a new unique approach to investigate the regulation pathways active during acid secretion and the degree that they are utilized to drive total gastric acid secretion. The findings of this study will enhance our understanding on how these signaling mechanisms vary under pathophysiology or therapeutic management.

Parietal cell activation by arborization of ECL cell cytoplasmic projections is likely the mechanism for histamine induced secretion of hydrochloric acid

BACKGROUND AND AIMS

Enterochromaffin-like (ECL) cells are central in the regulation of acid secretion. G cells release gastrin and activate ECL cell histamine secretion which stimulates parietal cell H(2) receptors initiating acid secretion. It is unclear whether histamine-mediated parietal cell activation is via a vascular or paracrine pathway. To assess this, we utilized immunohistochemistry (IHC) and electron microscopy to examine gastric tissue and used visualization of formalin fixed dispersed gastric cells and glands to investigate and define the anatomical relationship between ECL and parietal cells.

MATERIAL AND METHODS

Sprague-Dawley rat stomachs were instilled with formalin. Thereafter fixed mucosal cells and whole gastric glands were dispersed by mechanical and chemical dissolution and enzymatic digestion. Smears with fixed isolated cells and whole glands were stained by IHC with histidine decarboxylase (HDC) and H+/K+-ATPase antibodies. Whole tissue samples of Sprague-Dawley and cotton rat oxyntic mucosa were investigated with IHC using HDC, VMAT2 and H+/K+-ATPase antibodies, and electron microscopy was performed to further delineate the precise anatomic relationship between ECL cells and parietal cells.

RESULTS

Each ECL cell generated a network of HDC- and VMAT2-positive dendritic-like elongations that were in direct contact with several parietal cells. Thus, ECL cells at the base of the gland were in communication with parietal cells in the middle of the gland. Electron microscopy confirmed that the cytoplasmic ECL cell elongations containing secretory vesicles were in direct juxtaposition to parietal cells.

CONCLUSIONS

These findings indicate that ECL cells directly regulate parietal cell function in a neurocrine manner via slender neuron-like elongations.

Hormonal regulation of gastric acid secretion

Although gastric acid is not essential for life, it facilitates the digestion of protein and the absorption of iron, calcium, vitamin B(12), and thyroxin. It also prevents bacterial overgrowth and enteric infection. Gastric acid secretion must be precisely regulated, as too much acid may overwhelm mucosal defense mechanisms and lead to ulceration and maldigestion. The pathways regulating gastric acid secretion may be categorized as neural, paracrine, and hormonal; the hormonal pathways are the focus of this review. During meal ingestion, the main hormone responsible for stimulating acid secretion is gastrin, which acts primarily by releasing histamine from enterochromaffin-like cells. Ghrelin and orexin may also function as stimulatory hormones. Nutrients within the intestine, mainly lipid and protein, release peptide hormones such as cholecystokinin, secretin, neurotensin, and glucagon-like peptide, which may act in concert to inhibit acid secretion.

The development of multiple probe microdialysis sampling in the stomach

A multiple probe approach of implanting microdialysis probes into each separate tissue layer would better represent sampling from the stomach. Presently, microdialysis sampling experiments are performed with only a single probe in the submucosa to represent sampling from the stomach tissue. The focus of this research was to develop a four-probe microdialysis sampling design to simultaneously monitor the stomach lumen, mucosa, submucosa and in the blood of the rat. Due to the small outer diameter of the microdialysis probe (350 microm), implantation into each separate layer was achieved with confirmation of probe location from histological examination. To assess the significance of sampling by this approach, multiple probe microdialysis sampling was used to monitor drug absorption in the stomach. Salicylic acid, caffeine and metoprolol were individually dosed to the ligated stomach. Analysis of the dialysate samples was performed by HPLC-UV and concentration-time curves and pharmacokinetics analysis were used to determine differences between the different probe locations.

Gastric secretion

PURPOSE OF REVIEW

This review summarizes the past year's literature regarding the regulation of gastric exocrine and endocrine secretion.

RECENT FINDINGS

Gastric acid secretion is tightly regulated by overlapping neural, hormonal, paracrine, and intracellular pathways in order to achieve the correct amount of acid secretion required by the specific situation. Too little acid can interfere with the absorption of iron, calcium, vitamin B12, and certain drugs as well as predispose to enteric infection, bacterial overgrowth, and gastric malignancy. Too much acid can induce esophageal, gastric, and duodenal injury. Gastrin, histamine, acetylcholine, and ghrelin stimulate whereas somatostatin, cholecystokinin, atrial natriuretic peptide, and nitric oxide inhibit acid secretion. Most patients infected with Helicobacter pylori manifest a pangastritis and produce less than normal amounts of acid; those with antral predominant gastritis, however, are hypergastrinemic and produce increased amounts of acid. Improved understanding of the channels and receptors that are required for and regulate H+K+-ATPase activity should lead to the development of novel antisecretory agents.

SUMMARY

A better understanding of the pathways regulating gastric secretions should lead to new strategies to prevent and treat a variety of gastric disorders such as gastroesophageal reflux disease, autoimmune gastritis, gastric cancer, and functional dyspepsia.