Anaesthetic agents inhibit gastrin-stimulated but not basal histamine release from rat stomach ECL cells

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.

Mesencephalic astrocyte-derived neurotrophic factor prevents neuron loss via inhibiting ischemia-induced apoptosis

Mesencephalic astrocyte-derived neurotrophic factor (MANF) has been shown to be up-regulated under the focal cerebral ischemia and protected against ischemic injury in rats. However, the underlying mechanisms are unclear. The aim of this study was to verify the protection of MANF on the cerebral ischemic injury and further investigate the possible mechanisms. Rat focal ischemic model was established by middle cerebral artery occlusion (MCAO). The recombinant human MANF was therapeutically administrated to the ipsilateral ventricle at 2 h after MCAO. MANF decreased the number of the propidium iodide (PI)- and TUNEL-positive neural cells. Contrarily, MANF protected the NeuN-positive cells in hippocampus and cortex from death induced by ischemia. The more interesting results in this study were that MANF repressed the cleavage of caspase-3 triggered by focal cerebral ischemia. MANF also reduced the elevated levels of BIP/Grp78, phosphorylated IRE1, and splicing XBP1 induced by focal cerebral ischemia, but not affect CHOP expression. Meanwhile, focal cerebral ischemia elevated the levels of XBP1 mRNA, including unspliced XBP1 (XBP1u) and spliced XBP1 (XBP1s). However, MANF did not affect the expression of XBP1 mRNA, neither XBP1u nor XBP1s. These results suggest that MANF can prevent the neuron loss via inhibiting ischemia-induced apoptosis and regulating unfolded protein response-related genes.

Immunosuppressive effects of Pteridium aquilinum enhance susceptibility to urethane-induced lung carcinogenesis

Pteridium aquilinum (bracken fern), one of the most important toxic plants in the world, contains the toxic norsequiterpene ptaquiloside that induces cancers in humans and farm animals. Previous studies in the laboratory demonstrated immunotoxic effects produced by ptaquiloside, which are characterized by suppression of natural killer (NK) cell activity (i.e. cytotoxicity and interferon [IFN]-γ production). However, it is unknown whether these immunosuppressive effects could contribute to carcinogenesis in situ in general because of the important function of NK cells in innate killing of tumor cells. This study assessed the impact of P. aquilinum-induced immunosuppression on urethane-induced lung cancer in C57BL/6 mice. Adult mice were treated with an extract of P. aquilinum (30 g/kg/day) by gavage once daily for 14 days, followed by gavage (5 days/week) during an 11-week period that was accompanied by treatment with urethane (1 g/kg) via once-weekly intraperitoneal injection; 20 weeks after the end of the treatment period, all lungs were evaluated. The results indicated there was a significant increase in lung nodule number as well as in multiplicity of lesions in mice treated with both P. aquilinum and urethane (PU group) compared to values in mice treated only with the urethane (U group). In addition, histologic evaluation revealed a 76% increase in the rate of lung adenomas and a 41% increase in rate of bronchiolization of alveoli in the mice from the PU group compared to levels seen in mice within the U group. Taken together, the results here show for the first time that immunosuppressive effects of P. aquilinum could increase the risk of cancer formation in exposed hosts.

Nesfatin-1 inhibits gastric acid secretion via a central vagal mechanism in rats

Nesfatin-1, a novel hypothalamic peptide, inhibits nocturnal feeding behavior and gastrointestinal motility in rodents. The effects of nesfatin-1 on gastrointestinal secretory function, including gastric acid production, have not been evaluated. Nesfatin-1 was injected into the fourth intracerebral ventricle (4V) of chronically cannulated rats to identify a nesfatin dose sufficient to inhibit food intake. Nesfatin-1 (2 μg) inhibited dark-phase food intake, in a dose-dependent fashion, for >3 h. Gastric acid production was evaluated in urethane-anesthetized rats. Nesfatin-1 (2 μg) was introduced via the 4V following endocrine stimulation of gastric acid secretion by pentagastrin (2 μg·kg(-1)·h(-1) iv), vagal stimulation with 2-deoxy-D-glucose (200 mg/kg sc), or no stimulus. Gastric secretions were collected via gastric cannula and neutralized by titration to determine acid content. Nesfatin-1 did not affect basal and pentagastrin-stimulated gastric acid secretion, whereas 2-deoxy-D-glucose-stimulated gastric acid production was inhibited by nesfatin-1 in a dose-dependent manner. c-Fos immunofluorescence in brain sections was used to evaluate in vivo neuronal activation by nesfatin-1 administered via the 4V. Nesfatin-1 caused activation of efferent vagal neurons, as evidenced by a 16-fold increase in the mean number of c-Fos-positive neurons in the dorsal motor nucleus of the vagus (DMNV) in nesfatin-1-treated animals vs. controls (P < 0.01). Finally, nesfatin-induced Ca(2+) signaling was evaluated in primary cultured DMNV neurons from neonatal rats. Nesfatin-1 caused dose-dependent Ca(2+) increments in 95% of cultured DMNV neurons. These studies demonstrate that central administration of nesfatin-1, at doses sufficient to inhibit food intake, results in inhibition of vagally stimulated secretion of gastric acid. Nesfatin-1 activates DMNV efferent vagal neurons in vivo and triggers Ca(2+) signaling in cultured DMNV neurons.

Effect of low-protein diet on anthracycline pharmacokinetics and cardiotoxicity

OBJECTIVES

Anthracyclines are broad spectrum anticancer drugs with dose-dependent cardiotoxicity. Protein malnutrition commonly occurs in cancer patients and is considered a risk factor for development of cardiotoxicity. This study was designed to assess the modulatory effect of protein malnutrition on the pharmacokinetics and drug disposition properties of a single dose of doxorubicin and epirubicin and how these possible changes will affect the degree of cardiotoxicity of these drugs.

METHODS

A single interperitoneal dose of 15 mg/kg of either doxorubicin or epirubicin was injected into rats fed with either normal protein diet or low-protein diet. The plasma concentration-time profiles of doxorubicin and epirubicin and their concentrations in different tissues were determined. Serum creatine kinase level was determined at different time intervals and histopathological examination of heart tissue was carried out.

KEY FINDINGS

Protein malnutrition significantly altered the pharmacokinetics of doxorubicin and epirubicin, with a significant decrease in their elimination, and prolonged the exposure of the heart to these drugs. Histopathological examination and serum creatine kinase measurements supported the role of protein malnutrition in enhancement of anthracycline cardiotoxicity.

CONCLUSIONS

If similar alteration in anthracyclines' pharmacokinetics occurs in malnourished cancer patients, protein malnutrition will be a risk factor for development of anthracycline cardiotoxicity and dose adjustment will be required in nutritionally deprived patients.

Measurement of gastric acid secretion in the anaesthetized rat

The protocols described in this unit are designed to assess the effects of substances on gastric acid secretion by the rat stomach, with the animal under general anesthesia. Both stimulatory and inhibitory effects of compounds can be evaluated and specific mechanisms of action can also be investigated. Acid secretion is induced by substances that directly activate parietal cell receptors (histamine and bethanechol), by indirect stimuli, like 2-deoxy-D-glucose, by electrical stimulation of vagal nerves, or by the peptide pentagastrin. Reference antisecretory drugs are represented by histamine H(2) receptor antagonists and proton pump inhibitors. This model allows the evaluation of complete dose-response curves together with a time-course of the secretory/antisecretory effects. Indirect effects involving activation or inhibition of vagal pathways are evaluated in intact animals by means of electrical vagal stimulation or in vagotomized animals.

Tachyphylaxis of the ECL-cell response to PACAP: receptor desensitization and/or depletion of secretory products

BACKGROUND AND PURPOSE

Rat stomach ECL cells secrete histamine and pancreastatin in response to gastrin and pituitary adenylate cyclase-activating peptide-27 (PACAP). This study applies microdialysis to explore how ECL cells in situ respond to PACAP and gastrin.

EXPERIMENTAL APPROACH

Both peptides were administered by microinfusion into the gastric submucosa. The microdialysate was analysed for histamine and pancreastatin (ECL-cell markers) and for somatostatin (D-cell marker).

KEY RESULTS

Microinfusion of PACAP (0.01-0.3 nmol microl(-1)) raised microdialysate histamine and pancreastatin dose-dependently. The response was powerful but short-lived. The response to gastrin was sustained at all doses tested. It is unlikely that the transient nature of the histamine response to PACAP reflects inadequate histamine synthesis, since the pancreastatin response to PACAP was short-lived too, and both gastrin and PACAP activated ECL-cell histidine decarboxylase. Unlike gastrin, PACAP mobilized somatostatin. Co-infusion of somatostatin abolished the histamine-mobilizing effect of PACAP. However, pretreatment with the somatostatin receptor type-2 antagonist (PRL-2903) did not prolong the histamine response to PACAP, suggesting that mobilization of somatostatin does not explain the transient nature of the response. Repeated administration of 0.1 nmol microl(-1) of PACAP (1 h infusions, 1 h intervals) failed to induce a second histamine response. Pretreatment with a low dose of PACAP (0.03 nmol microl(-1)) abolished the response to a subsequent near-maximal PACAP challenge (0.3 nmol microl(-1)).

CONCLUSION

The transient nature of the histamine response to PACAP reflects desensitization of the PACAP receptor and/or exhaustion of a specific storage compartment that responds to PACAP but not to gastrin.

The vagus regulates histamine mobilization from rat stomach ECL cells by controlling their sensitivity to gastrin

The ECL cells in the oxyntic mucosa secrete histamine in response to gastrin, stimulating parietal cells to produce acid. Do they also operate under nervous control? The present study examines histamine mobilization from rat stomach ECL cells in situ in response to acute vagal excitation and to food or gastrin following vagal or sympathetic denervation. Applying the technique of microdialysis, we monitored the release of histamine by radioimmunoassay. Microdialysis probes were placed in the submucosa on either side of the stomach, 3 days before experiments. The rats were awake during microdialysis except when subjected to electrical vagal stimulation. One-sided electrical vagal stimulation raised serum gastrin and mobilized gastric histamine. However, gastrin receptor blockade prevented the histamine mobilization, indicating that circulating gastrin accounts for the response. Vagal excitation by hypoglycaemia (insulin) or pylorus ligation did not mobilize either gastrin or histamine. The histamine response to food was almost abolished by gastrin receptor blockade, and it was halved on the denervated side after unilateral subdiaphragmatic vagotomy. While the histamine response to a near-maximally effective dose of gastrin was unaffected by vagotomy, the response to low gastrin doses was reduced significantly. Abdominal ganglionic sympathectomy failed to affect the histamine response to either food or gastrin. In conclusion, gastrin is responsible for most of the food-evoked mobilization of ECL-cell histamine. The histamine response to electrical vagal stimulation reflects the effect of circulating gastrin rather than a direct action of the vagus on the ECL cells. Vagal denervation was accompanied by an impaired histamine response to food intake, probably reflecting the right-ward shift of the serum gastrin concentration-histamine response curve. The results suggest that the vagus controls the sensitivity of the ECL cells to gastrin.

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.

Submucosal microinfusion of endothelin and adrenaline mobilizes ECL-cell histamine in rat stomach, and causes mucosal damage: a microdialysis study

Rat stomach ECL cells release histamine in response to gastrin. Submucosal microinfusion of endothelin or adrenaline, known to cause vasoconstriction and gastric lesions, mobilized striking amounts of histamine. While the histamine response to gastrin is sustainable for hours, that to endothelin and adrenaline was characteristically short-lasting (1-2 h). The aims of this study were to identify the cellular source of histamine mobilized by endothelin and adrenaline, and examine the differences between the histamine-mobilizing effects of gastrin, and of endothelin and adrenaline. Endothelin, adrenaline or gastrin were administered by submucosal microinfusion. Gastric histamine mobilization was monitored by microdialysis. Local pretreatment with the H1-receptor antagonist mepyramine and the H2-receptor antagonist ranitidine did not prevent endothelin- or adrenaline-induced mucosal damage. Submucosal microinfusion of histamine did not cause damage. Acid blockade by ranitidine or omeprazole prevented the damage, suggesting that acid back diffusion contributes. Gastrin raised histidine decarboxylase (HDC) activity close to the probe, without affecting the histamine concentration. Endothelin and adrenaline lowered histamine by 50-70%, without activating HDC. Histamine mobilization declined upon repeated administration. Endothelin reduced the number of histamine-immunoreactive ECL cells locally, and reduced the number of secretory vesicles. Thus, unlike gastrin, endothelin (and adrenaline) is capable of exhausting ECL-cell histamine. Microinfusion of alpha-fluoromethylhistidine (known to deplete ECL cells but not mast cells of histamine) reduced the histamine-mobilizing effect of endothelin by 80%, while 1-week pretreatment with omeprazole enhanced it, supporting the involvement of ECL cells. Somatostatin or the prostanoid misoprostol inhibited gastrin-, but not endothelin-stimulated histamine release, suggesting that endothelin and gastrin mobilize histamine via different mechanisms. While gastrin effectively mobilized histamine from ECL cells in primary culture, endothelin had no effect, and adrenaline, a modest effect. Hence, the striking effects of endothelin and adrenaline on ECL cells in situ are probably indirect, possibly a consequence of ischemia.

ECL cell histamine mobilization studied by gastric submucosal microdialysis in awake rats: methodological considerations

The ECL cells are endocrine/paracrine cells in the acid-producing part of the stomach. They secrete histamine in response to circulating gastrin. Gastric submucosal microdialysis has been used to study ECL-cell histamine mobilization in awake rats. In the present study we assess the usefulness and limitations of the technique. Microdialysis probes were implanted in the gastric submucosa. Histological analysis of the stomach wall around the probe revealed a moderate, local inflammatory reaction 1-2 days after implantation; the inflammation persisted for at least 10 days. Experiments were conducted 3 days after the implantation. The "true" submucosal histamine concentration was determined by perfusing at different rates (the zero flow method) or with different concentrations of histamine at a constant rate (the no-net-flux method): in fasted rats it was calculated to be 87+/-5 (means+/-S.E.M.) nmol/l and 76+/-9 nmol/l, respectively. The corresponding histamine concentrations in fed rats were 93+/-5 and 102+/-8 nmol/l, respectively. With a perfusion rate of 74 microl/hr the recovery of submucosal histamine was 49%, at 34 microl/hr the recovery increased to 83%. At a perfusion rate below 20 microl/hr the microdialysate histamine concentration was close to the actual concentration in the submucosa. The ECL-cell histamine mobilization was independent of the concentrations of Ca2+ in the perfusion medium (0-3.4 mmol/l Ca2+). In one experiment, histamine mobilization in response to gastrin (10 nmol/kg/hr subcutaneously) was monitored in rats pretreated with prednisolone (60 mg/kg) or indomethacin (15 mg/kg). The two antiinflammatory agents failed to affect the concentration of histamine in the microdialysate either before or during the gastrin challenge, which was in accord with the observation that the inflammatory reaction was modest and that inflammatory cells were relatively few around the probe and in the wall of the probe. In another experiment, rats were given aminoguanidine (10 mg/kg) or metoprine (10 mg/kg) 4 hr before the start of gastrin infusion (5 nmol/kg/hr intravenously). Metoprine (inhibitor of histamine N-methyl transferase) did not affect the microdialysate histamine concentration, while aminoguanidine (inhibitor of diamine oxidase) raised both basal and gastrin-stimulated histamine concentrations. We conclude that microdialysis can be used to monitor changes in the concentration of histamine in the submucosa of the stomach, and that the inflammatory reaction to the probe is moderate and does not affect the submucosal histamine mobilization.

Metabolism of carcinogenic urethane to nitric oxide is involved in oxidative DNA damage

Carcinogenic urethane (ethyl carbamate) forms DNA adduct via epoxide, whereas carcinogenic methyl carbamate can not. To clarify a mechanism independent of DNA adduct formation, we examined DNA damage induced by N-hydroxyurethane, a urethane metabolite, using 32P-5'-end-labeled DNA fragments. N-hydroxyurethane induced Cu(II)-mediated DNA damage especially at thymine and cytosine residues. DNA damage was inhibited by both catalase and bathocuproine, suggesting a role for H(2)O(2) and Cu(I) in DNA damage. Free (*) OH scavengers did not inhibit the DNA damage, although methional did inhibit it. These results suggest that reactive species, such as the Cu(I)-hydroperoxo complex, cause DNA damage. Formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) was increased by N-hydroxyurethane in the presence of Cu(II). When treated with esterase, N-hydroxyurethane induced 8-oxodG formation to a similar extent as that induced by hydroxylamine. Enhancement of DNA cleavages by endonuclease IV suggests that hydroxylamine induced depurination. Furthermore, hydroxylamine induced a significant increase in 8-oxodG formation in HL-60 cells but not in its H(2)O(2)-resistant clone HP 100 cells. o-Phenanthroline significantly inhibited the 8-oxodG formation in HL-60 cells, confirming the involvement of metal ions in the 8-oxodG formation by hydroxylamine. Electron spin resonance spectroscopy, utilizing Fe[N-(dithiocarboxy)sarcosine](3), demonstrated that nitric oxide (NO) was generated from hydroxylamine and esterase-treated N-hydroxyurethane. It is concluded that urethane may induce carcinogenesis through oxidation and, to a lesser extent, depurination of DNA by its metabolites.

Multiple effects of trichloroethanol on calcium handling in rat submandibular acinar cells

The effect of trichloroethanol (TCEt), the active metabolite of chloral hydrate, on the intracellular concentration of calcium ([Ca(2+)](i)) was investigated in rat submandibular glands (RSMG) acini loaded with fura-2. TCEt (1 - 10 mM) increased the [Ca(2+)](i) independently of the presence of calcium in the extracellular medium. Dichloroethanol (DCEt) and monochloroethanol (MCEt) reproduced the stimulatory effect of TCEt but at much higher concentrations (about 6 fold higher for DCEt and 20 fold higher for MCEt). TCEt mobilized an intracellular pool of calcium, which was depleted by a pretreatment with thapsigargin, an inhibitor of the sarcoplasmic and endoplasmic reticulum calcium-dependent ATPases, but not with FCCP, an uncoupler of mitochondria. TCEt 10 mM inhibited by 50% the thapsigargin-sensitive microsomal Ca(2+)-ATPase. DCEt 10 mM and MCEt 10 mM inhibited the ATPase by 20 and 10%, respectively. TCEt inhibited the increase of the [Ca(2+)](i) and the production of inositol phosphates in response to carbachol, epinephrine and substance P. TCEt inhibited the uptake of calcium mediated by the store-operated calcium channel (SOCC). ATP and Bz-ATP increased the [Ca(2+)](i) in RSMG acini and this effect was blocked by extracellular magnesium, by Coomassie blue and by oxydized ATP (oATP). TCEt potentiated the increase of the [Ca(2+)](i) and of the uptake of extracellular calcium in response to ATP and Bz-ATP. TCEt had no effect on the uptake of barium and of ethidium bromide in response to purinergic agonists. These results suggest that TCEt, at sedative concentrations, exerts various effects on the calcium regulation: (1) it mobilizes a thapsigargin-sensitive intracellular pool of calcium in RSMG acini; (2) it inhibits the uptake of calcium via the SOCC; (3) it inhibits the activation by G protein-coupled receptors of a polyphosphoinositide-specific phospholipase C. It does not interfere with the activation of the ionotropic P2X receptors. The use of chloral hydrate should be avoided in studies exploring the in vivo responses to sialagogues.

ECL-cell histamine mobilization in conscious rats: effects of locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators

1. The ECL cells control gastric acid secretion by mobilizing histamine in response to circulating gastrin. In addition, the ECL cells are thought to operate under nervous control and to be influenced by local inflammatory processes. 2. The purpose of the present study was to monitor histamine mobilization from ECL cells in conscious rats in response to locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators. 3. Microdialysis probes were implanted in the submucosa of the acid-producing part of the rat stomach. Three days later, the agents to be tested were administered via the microdialysis probe and their effects on basal (48 h fast) and stimulated (intravenous infusion of gastrin-17, 3 nmol kg(-1) h(-1)) mobilization of ECL-cell histamine was monitored by continuous measurement of histamine in the perfusate (radioimmunoassay). 4. Locally administered gastrin-17 and sulfated cholecystokinin-8 mobilized histamine as did pituitary adenylate cyclase-activating peptide-27, vasoactive intestinal peptide, peptide YY, met-enkephalin, endothelin and noradrenaline, adrenaline and isoprenaline. 5. While gastrin, sulfated-cholecystokinin-8, met-enkephalin and isoprenaline induced a sustained elevation of the submucosal histamine concentration, endothelin, peptide YY, pituitary adenylate cyclase activating peptide, vasoactive intestinal peptide, noradrenaline and adrenaline induced a transient elevation. 6. Calcitonin gene-related peptide, galanin, somatostatin and the prostanoid misoprostol inhibited gastrin-stimulated histamine mobilization. 7. The gut hormones neurotensin and secretin and the neuropeptides gastrin-releasing peptide, neuropeptide Y and substance P failed to affect ECL-cell histamine mobilization, while motilin and neuromedin U-25 had weak stimulatory effects. Also acetylcholine, carbachol, serotonin and the amino acid neurotransmitters aspartate, gamma-aminobutyric acid, glutamate and glycine were inactive or weakly active as was bradykinin. 8. In summary, a range of circulating hormones, local hormones, catecholamines, neuropeptides and inflammatory mediators participate in controlling the activity of rat stomach ECL cells in situ.

Gastric secretion

The influence of central and peripheral stimuli on gastric acid secretion is mediated via activation of histaminergic, gastrinergic, and cholinergic pathways coupled to intracellular second-messenger systems that determine the trafficking and activity of H+ K+-ATPase, the proton pump of the parietal cell. Histamine, released from enterochromaffin-like cells stimulates the parietal cell directly via H-2 receptors coupled to generation of cAMP. Gastrin, acting via cholecystokinin-2 receptors on enterochromaffin-like cells coupled to an increase in intracellular calcium, stimulates the parietal cell indirectly by activating histidine decarboxylase, releasing histamine, and inducing enterochromaffin-like cell hypertrophy and hyperplasia. Acetylcholine, released from gastric postganglionic intramural neurons, stimulates the parietal cell directly via M-3 receptors coupled to intracellular calcium release and calcium entry. The second-messenger systems activated in the parietal cell converge on H+ K+-ATPase that catalyzes the exchange of luminal K+ for cytoplasmic H+ and is responsible for gastric luminal acidification. The main inhibitor of acid secretion is somatostatin which, acting via sst2 receptors, exerts a tonic inhibitory influence on parietal, enterochromaffin-like, and gastrin cells. Acute infection with Helicobacter pylori results in hypochlorhydria, whereas chronic infection may be associated with either hypo- or hyperchlorhydria. Although prostaglandins are thought to play a physiologic role in the regulation of acid secretion and maintenance of gastric mucosal integrity, the precise roles of cyclooxygenase-1 and cyclooxygenase-2 in these processes still eludes us.

Gastrin and the neuropeptide PACAP evoke secretion from rat stomach histamine-containing (ECL) cells by stimulating influx of Ca2+ through different Ca2+ channels

1. Gastrin and PACAP stimulate secretion of histamine and pancreastatin from isolated rat stomach ECL cells. We have examined whether or not secretion depends on the free cytosolic Ca2+ concentration ([Ca2+]i) and the pathways by which gastrin and PACAP elevate [Ca2+]i. Secretion was monitored by radioimmunoassay of pancreastatin and changes in [Ca2+]i by video imaging. The patch clamp technique was used to record whole-cell currents and membrane capacitance (reflecting exocytosis). 2. In the presence of 2 mM extracellular Ca2+, gastrin and PACAP induced secretion and raised [Ca2+]i. Without extracellular Ca2+ (or in the presence of La3+) no secretion occurred. The extracellular Ca2+ concentration required to stimulate secretion was 10 times higher for gastrin than for PACAP. Depletion of intracellular Ca2+ pools by thapsigargin had no effect on the capacity of gastrin and PACAP to stimulate secretion. 3. Gastrin-evoked secretion was inhibited 60-80 % by L-type channel blockers and 40 % by the N-type channel blocker omega-conotoxin GVIA. Combining L-type and N-type channel blockers did not result in greater inhibition than L-type channel blockers alone. Whole-cell patch clamp measurements confirmed that the ECL cells are equipped with voltage-dependent inward Ca2+ currents. A 500 ms depolarising pulse from -60 mV to +10 mV which maximally opened these channels resulted in an increase in membrane capacitance of 100 fF reflecting exocytosis of secretory vesicles. 4. PACAP-evoked secretion was reduced 40 % by L-type channel blockers but was not influenced by inhibition of N-type channels. SKF 96365, a blocker of both L-type and receptor-operated Ca2+ channels, inhibited PACAP-evoked secretion by 85 %. Combining L-type channel blockade with SKF 96365 abolished PACAP-evoked secretion. 5. The results indicate that gastrin- and PACAP-evoked secretion depends on Ca2+ entry and not on mobilisation of intracellular Ca2+. While gastrin stimulates secretion via voltage-dependent L-type and N-type Ca2+ channels, PACAP acts via L-type and receptor-operated Ca2+ channels.

Mobilization of rat stomach ECL-cell histamine in response to short- or long-term treatment with omeprazole and/or YF 476 studied by gastric submucosal microdialysis in conscious rats

1. Mobilization of histamine from the ECL cells was monitored by gastric submucosal microdialysis in conscious rats. The ECL cells are known to operate under gastrin control and the purpose of the present study was to examine their in situ response to short-term (12 h) as well as long-term (28 days) hypergastrinaemia, induced by treatment with the proton pump inhibitor omeprazole. 2. Hypergastrinaemia promptly raised the histamine concentration in the microdialysate. The effect was prevented by CCK(2) receptor blockade (YF476). On day 7 of omeprazole treatment the microdialysate histamine concentration reached a peak, five times higher than before treatment. Subsequently (14 and 28 days), less histamine was mobilized. 3. Gastrin infusion (4 h) raised the microdialysate histamine concentration in a dose-dependent manner in fasted rats and freely fed rats and in rats treated with omeprazole for a week. However, while fasted and fed rats responded to low doses of gastrin, the omeprazole-treated rats required large doses of gastrin to respond. 4. When the amount of histamine mobilized was related to the serum gastrin concentration the following EC(50) values could be calculated: fasted rats 2.3 x 10(-10) M, freely fed rats 2.5 x 10(-10) M, omeprazole-treated rats 8.7 x 10(-10) M. The maximal histamine responses in the three groups were 18.4 pmol 4 h(-1)+/-0.8, 21.9 pmol 4 h(-1)+/-1.2 and 68.0 pmol 4 h(-1)+/-3.5, respectively. 5. The results suggest that ECL cells, exposed to a high gastrin concentration for a week, respond with a shift in the receptor-ligand binding affinity from high to low. Apparently, CCK(2) receptors of the ECL cells are subject to dynamic changes with respect to ligand-binding affinity.

alpha-fluoromethylhistidine depletes histamine from secreting but not from non-secreting rat stomach ECL cells

Histamine in the oxyntic mucosa of the rat stomach occurs in mast cells (10%) and ECL cells (90%). Unlike the mast cells, the ECL cells operate under the control of gastrin. alpha-Fluoromethylhistidine, an irreversible inhibitor of the histamine-forming enzyme, histidine decarboxylase depletes ECL-cell but not mast-cell histamine. This report shows that the effectiveness by which histidine decarboxylase inhibition depletes ECL-cell histamine depends on the rate of histamine secretion. Rats received alpha-fluoromethylhistidine by continuous subcutaneous infusion for 24 h. Maximally effective doses (>/=3 mg/kg/h) inhibited histidine decarboxylase and reduced oxyntic mucosal histamine in fed rats by 80-90%. In fasted rats, the reduction was 50%. alpha-Fluoromethylhistidine greatly reduced the number of histamine-immunoreactive ECL cells (immunocytochemistry) and of secretory vesicles in the ECL cells (electron microscopy) in fed but not in fasted rats. The half-life of oxyntic mucosal histamine (determined upon histidine decarboxylase inhibition) was 2.6 h in fed rats and 19.4 h in fasted rats. The amount of histamine secreted in response to gastrin (monitored by gastric submucosal microdialysis) was greatly reduced by alpha-fluoromethylhistidine in fed rats but not in fasted rats. ECL cells were isolated from rat stomach by elutriation (80% purity). Their histamine content was determined after culture, with or without alpha-fluoromethylhistidine, in the presence of varying concentrations of gastrin. In a medium containing 10 nM gastrin, ECL cells responded to a maximally effective concentration of alpha-fluoromethylhistidine (0.1 nM) with 80% reduction in histamine content. In the absence of gastrin, ECL cells responded to alpha-fluoromethylhistidine with 45% reduction of histamine; the releasable histamine pool was unaffected. In conclusion, the combination of histidine decarboxylase inhibition and a high rate of histamine secretion will promptly exhaust the ECL-cell histamine pool, while histidine decarboxylase inhibition and a low secretion rate will affect the histamine pool much less.