Effects of inhibition of gastric secretion on antral gastrin and somatostatin gene expression in rats

Hypergastrinemia in Long-Term Use of Proton Pump Inhibitors

BACKGROUND/AIM

The use of proton pump inhibitors (PPIs) is known to lead to hypergastrinemia; however, the data in patients with atrophic gastritis is still lacking. The aim of this study was to investigate the effects of long-term PPIs use on the gastrin levels in patients with atrophic gastritis and to determine factors affecting hypergastrinemia in long-term users of PPIs.

METHODS

Serum Helicobacter pylori IgG, gastrin and pepsinogen levels were measured. Atrophic gastritis was assessed by upper gastrointestinal endoscopies based on the Kimura-Takemoto classification and pepsinogen levels. CYP2C19 polymorphisms were assessed using DNA extracted from peripheral blood.

RESULTS

A total number of 382 patients (275 men and 107 women) were enrolled. Median serum gastrin levels were higher in PPI users than in non- users (234 vs. 113 pg/mL, p < 0.001) and in women than in men (252 vs. 155 pg/mL, p = 0.006). Gastrin levels were significantly associated with corpus atrophy only in the subgroup of non-users of PPIs. Multivariate analysis revealed that hypergastrinemia (over 150 pg/mL) was significantly associated with PPI use (OR 5.30; 95% CI 3.32-8.47), women (OR 2.22; 95% CI 1.33-3.72) and corpus atrophy (OR 1.82; 95% CI 1.14-2.90).

CONCLUSION

PPI use, women and corpus atrophy were risk factors for hypergastrinemia. Gender, but not corpus atrophy, affected the gastrin levels in long-term users of PPIs.

Acid peptic diseases: pharmacological approach to treatment

Acid peptic disorders are the result of distinctive, but overlapping pathogenic mechanisms leading to either excessive acid secretion or diminished mucosal defense. They are common entities present in daily clinical practice that, owing to their chronicity, represent a significant cost to healthcare. Key elements in the success of controlling these entities have been the development of potent and safe drugs based on physiological targets. The histamine-2 receptor antagonists revolutionized the treatment of acid peptic disorders owing to their safety and efficacy profile. The proton-pump inhibitors (PPIs) represent a further therapeutic advance due to more potent inhibition of acid secretion. Ample data from clinical trials and observational experience have confirmed the utility of these agents in the treatment of acid peptic diseases, with differential efficacy and safety characteristics between and within drug classes. Paradigms in their speed and duration of action have underscored the need for new chemical entities that, from a single dose, would provide reliable duration of acid control, particularly at night. Moreover, PPIs reduce, but do not eliminate, the risk of ulcers in patients taking NSAIDs, reflecting untargeted physiopathologic pathways and a breach in the ability to sustain an intragastric pH of more than 4. This review provides an assessment of the current understanding of the physiology of acid production, a discussion of medications targeting gastric acid production and a review of efficacy in specific acid peptic diseases, as well as current challenges and future directions in the treatment of acid-mediated diseases.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

Control of gastric acid secretion in health and disease

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

Effects of intra-gastric beta-casomorphin-7 on somatostatin and gastrin gene expression in rat gastric mucosa

AIM: To investigate the in vivo effect of beta-casomorphin-7 on the regulation of gastric somatostatin and gastrin messenger RNA in rat gastric mucosa.

METHODS: Somatostatin and gastrin mRNA were quantified by RT-PCR and in situ hybridization (ISH) in 24 rats. The rats were divided into three treatment groups: basal diet + physiological saline (n = 8), basal diet + beta-casomorphin-7 (7.5 × 10^-7 mol) (n = 8), and basal diet + poly-Gly-7 (containing equal mol of N with 7.5 × 10^-7 mol beta-casomorphin-7) (n = 8). After oral administration for 30 days, rats were killed by exsanguinations.

RESULTS: After intra-gastric administration of beta-casomorphin-7 for 30 d, gastrin mRNA increased by 52.8% (P < 0.05, n = 8), and somatostatin mRNA levels decreased by 30.7% compared with the controls (P < 0.01, n = 8). No significant differences in the expression of the two genes were observed in the poly-Gly-treated group, although gastrin mRNA expression was elevated by 35.6% as against the control group (P = 0.15, n = 8). The long-term oral administration of a casomorphin solution significantly decreased the even gray of D-cells, but did not lower the number of D-cells both in the antrum and fundus. Interestingly, the number of G-cells increased in the antrum and fundus, but its average density was augmented only in the antrum.

CONCLUSION: Beta-casomorphin-7 is capable of modulating gene expression of the regulatory peptides from G and D cells. Data from in situ hybridization studies indicate that beta-casomorphin-7 affects gastrin gene expression indirectly by means of the paracrine action of somatostatin, and depends on its intrinsic molecular function.

Gastrin (G) cells and somatostatin (D) cells in patients with dyspeptic symptoms: Helicobacter pylori associated and non-associated gastritis

BACKGROUND

Gastrin G cells and somatostatin D cells are important regulators of gastric acid secretion and alterations in their relative numbers may play a key role in gastroduodenal disease.

AIM

To investigate the effect of Helicobacter pylori infection on the density of immunoreactive G and D cells in gastric antral and corpus biopsies from patients with dyspeptic complaints.

METHODS

One hundred and twenty two patients with dyspeptic complaints had two antrum and two corpus biopsies taken during upper endoscopy. The severity of inflammation and the density of H pylori were evaluated semiquantitatively. In addition, the density and distribution of neuroendocrine cells, especially G and D cells, were examined using immunohistochemistry. Patients were divided into three groups, those with H pylori positive gastritis, H pylori negative gastritis, and histologically normal gastric mucosa.

RESULTS

The number of immunoreactive G cells was significantly higher and the number of immunoreactive D cells lower in patients with H pylori positive gastritis compared with H pylori negative gastritis or histological normal gastric mucosa. The percentage of G cells as a percentage of mucosal endocrine cells was also raised and that of D cells was decreased.

CONCLUSIONS

Helicobacter pylori infection produces alterations in the number of endocrine cells responsible for regulating acid secretion in relation to intragastric pH and feeding. The alterations correlate best with the severity of inflammation and not with H pylori density.

Gastrin: old hormone, new functions

It is exactly a century since the gastric hormone gastrin was first described as a blood-borne regulator of gastric acid secretion. The identities of the main active forms of the hormone (the "classical gastrins") and their cellular and molecular sites of action in regulating acid secretion have all attracted sustained attention. However, recent work on peptides derived from the gastrin precursor that do not stimulate acid secretion ("non-classical gastrins"), together with studies on mice over-expressing the gene, or in which the gastrin gene has been deleted, suggest hitherto unsuspected roles in regulating cell proliferation, migration, and differentiation. Moreover, microarray and proteomic studies have identified previously unsuspected target genes of the classical gastrins. Some of the newer actions have implications for our understanding of the progression to cancer in oesophagus, stomach, pancreas and colon, all of which have recently been linked in one way or another to dysfunctional signalling involving products of the gastrin gene. The present review focuses on recent progress in understanding the biology of both classical and non-classical gastrins.

Hypergastrinemia in response to gastric inflammation suppresses somatostatin

Hypergastrinemia and a reduction in tissue somatostatin occur in Helicobacter pylori-infected patients. We investigated whether the D cell may be a direct target of gastric inflammation and hypergastrinemia. D cells were quantified by morphometry and flow cytometry in 16-wk-old wild-type (G+/+) and gastrin-deficient (G-/-) mice. Hypochlorhydric G-/- mice were treated with either antibiotics for 20 days or infused with gastrin (G-17) for 14 days. G+/+ mice were made hypochlorhydric by treating them with omeprazole for 2 mo. G-/- mice showed significant inflammation compared with the G+/+ mice, which resolved after 20 days of antibiotic treatment. D cell numbers were not significantly different between G-/- and G+/+ mice. After G-17 was infused, fundic and antral D cell numbers decreased in the G-/- mice. G+/+ animals made hypergastrinemic with omeprazole exhibited decreased D cell numbers. When omeprazole-treated mice were treated with antibiotics alone, elevated plasma gastrin levels returned to baseline and D cell numbers returned to resting levels despite persistent hypochlorhydria. Hypergastrinemia, induced by inflammation, results in decreased D cell numbers. Thus the stomach responds to the presence of inflammation by reducing somatostatin levels, thereby releasing the inhibition on the G and parietal cells to maximize gastric acid output.

Intragastric pH regulates conversion from net acid to net alkaline secretion by the rat stomach

Our previous report showed gastric mucosal surface pH was determined by alkali secretion at intragastric luminal pH 3 but by acid secretion at intragastric pH 5. Here, we question whether regulation of mucosal surface pH is due to the effect of luminal pH on net acid/base secretions of the whole stomach. Anesthetized rats with a gastric cannula were used, the stomach lumen was perfused with weakly buffered saline, and gastric secretion was detected in the gastric effluent with 1) a flow-through pH electrode and 2) a fluorescent pH-sensitive dye (Cl-NERF). During pH 5 luminal perfusion, both pH sensors reported the gastric effluent was acidic (pH 4.79). After perfusion was stopped transiently (stop-flow), net acid accumulation was observed in the effluent when perfusion was restarted (peak change to pH 4.1-4.3). During pH 3 luminal perfusion, both pH sensors reported gastric effluent was close to perfusate pH (3.0-3.1), but net alkali accumulation was detected at both pH sensors after stop-flow (peak pH 3.3). Buffering capacity of gastric effluents was used to calculate net acid/alkaline secretions. Omeprazole blocked acid secretion during pH 5 perfusion and amplified net alkali secretion during pH 3 perfusion. Pentagastrin elicited net acid secretion under both luminal pH conditions, an effect antagonized by somatostatin. We conclude that in the basal condition, the rat stomach was acid secretory at luminal pH 5 but alkaline secretory at luminal pH 3.

The gastrins: their production and biological activities

Gastric epithelial organization and function are controlled and maintained by a variety of endocrine and paracrine mediators. Peptides encoded by the gastrin gene are an important part of this system because targeted deletion of the gene, or of the gastrin-CCKB receptor gene, leads to decreased numbers of parietal cells and decreased gastric acid secretion. Recent studies indicate that the gastrin precursor, preprogastrin, gives rise to a variety of products, each with a distinctive spectrum of biological activity. The conversion of progastrin to smaller peptides is regulated by multiple mechanisms including prohormone phosphorylation and secretory vesicle pH. Progastrin itself stimulates colonic epithelial proliferation; biosynthetic intermediates (Gly-gastrins) stimulate colonic epithelial proliferation and gastric epithelial differentiation; and C-terminally amidated gastrins stimulate colonic proliferation, gastric epithelial proliferation and differentiation, and acid secretion. The effects of progastrin-derived peptides on gastric epithelial function are mediated in part by release of paracrine factors that include histamine, epidermal growth factor (EGF)-receptor ligands, and Reg. The importance of the appropriate regulation of this system is shown by the observation that prolonged moderate hypergastrinemia in transgenic mice leads to remodelling of the gastric epithelium, and in the presence of Helicobacter, to gastric cancer.

Topical review. Gastrin and gastric epithelial physiology

Transepithelial transducing cells, particularly the gastrin (G) cell, co-ordinate gastric acid secretion with the arrival of food in the stomach. Recent work suggests that multiple active products are generated from the gastrin precursor, and that there are multiple control points in gastrin biosynthesis. Biosynthetic precursors and intermediates (progastrin and Gly-gastrins) are putative growth factors; their products, the amidated gastrins, regulate epithelial cell proliferation, the differentiation of acid-producing parietal cells and histamine-secreting enterochromaffin-like (ECL) cells, and the expression of genes associated with histamine synthesis and storage in ECL cells, as well as acutely stimulating acid secretion. Gastrin also stimulates the production of members of the epidermal growth factor (EGF) family, which in turn inhibit parietal cell function but stimulate the growth of surface epithelial cells. Plasma gastrin concentrations are elevated in subjects with Helicobacter pylori, who are known to have increased risk of duodenal ulcer disease and gastric cancer. Studies of the physiology of gastrin may therefore contribute to an understanding of the mechanisms relevant to major upper gastrointestinal tract disease.

Rapid and easy detection of Helicobacter pylori by in situ hybridization

Various in situ hybridization (ISH) methods have been used to identify Helicobacter pylori, a causative organism responsible for chronic gastritis and peptic ulcer disease, but they were hard to perform and time consuming. To detect H. pylori in a rapid and easily reproducible way, we developed synthetic biotinylated oligonucleotide probes which complement rRNA of H. pylori. Formalin-fixed and paraffin-embedded tissues from 50 gastric biopsy specimens were examined. Using a serologic test and histochemical stain (Warthin-Starry silver stain and/or Giemsa stain) as a standard, 40 of them were confirmed to be H. pylori-positive. Our ISH was quickly carried out within one hr and results were compared with those obtained from immunohistochemical stain. The ISH produced a positive reaction in 38 of 40 cases (95%). All H. pylori-negative cases failed to demonstrate a positive signal. The ISH has a sensitivity comparable to those of conventional histochemical and immunohistochemical stain, and has high specificity. In conclusion, ISH with a biotinylated oligonucleotide probe provides a useful diagnostic method for detecting H. pylori effectively in routinely processed tissue sections.

Effect of Helicobacter pylori infection on antral gastrin and somatostatin cells and on serum gastrin concentrations

OBJECTIVES

Helicobacter pylori infection induces selective reduction of the number of antral D-cells and results in abnormal regulation of serum gastrin secretion. The purpose of this study was to investigate the relationship between H. pylori infection and the numbers of G-cells and D-cells.

METHODS

The numbers of antral G-cells and D-cells, the ratio of G-cells to D-cells and fasting serum gastrin concentrations were compared between 37 patients with (29 with duodenal ulcers and 8 with gastric ulcers) and 33 without H. pylori infection (22 with duodenal ulcers and 11 with gastric ulcers). Serum gastrin concentrations were measured using the radioimmunoassay technique. Antral mucosal biopsy specimens were examined using immunohistochemical staining with antibodies specific for gastrin and somatostatin and the numbers of G-cells and D-cells per gastric gland were counted.

RESULTS

Fasting serum gastrin concentrations were significantly higher in patients with H. pylori infection compared to patients without infection (80.3 +/- 23.5 vs 47.6 +/- 14.1 pg/ml, p < 0.001). The number of G-cells per gastric gland was similar in infected and uninfected patients (7.1 +/- 3.1 vs 7.3 +/- 3.9, respectively, p > 0.5). The number of D-cells was significantly lower in patients with H. pylori infection than in uninfected patients in both duodenal and gastric ulcer patients (1.3 +/- 0.4 vs 2.5 +/- 1.6, respectively, p < 0.001). The ratio of G-cells to D-cells was also significantly higher in infected patients compared with uninfected patients for both gastric and duodenal ulcers (5.7 +/- 2.7 vs 3.5 +/- 1.9, respectively, p < 0.001).

CONCLUSIONS

These results strongly suggest that Helicobacter pylori infection induces reduction of the number of antral D-cells. The resulting relative hypofunction of the inhibitory action of D-cells against G-cells may be responsible for increased serum gastrin secretion.

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

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

Regulation by gastric acid of the processing of progastrin-derived peptides in rat antral mucosa

1. Inhibition of gastric acid secretion by proton pump inhibitors like omeprazole increases the synthesis and secretion of the pyloric antral hormone gastrin. We report here how omeprazole influences the conversion of the gastrin precursor to its final products, and the abundance of mRNAs encoding proteins associated with progastrin processing in rat antral mucosa. 2. Progastrin processing was studied using a pulse-chase protocol in antral mucosa, incubated in vitro, from rats treated with omeprazole for up to 5 days. Labelled peptides were detected by on-line scintillation counting after immunoprecipitation and HPLC. The mRNAs encoding prohormone-processing enzymes were identified by Northern blot, polymerase chain reaction or ribonuclease protection assay, and their cellular origins identified by immunocytochemistry. 3. Cleavage of [3H]- and [35S]-labelled progastrins at Arg-94-95 or Arg-57-58, and amidation at Phe-92 were not influenced by pretreatment with omeprazole. In contrast, cleavage of G34 (the thirty-four amino acid amidated gastrin) at Lys-74-75 to give G17 (the seventeen amino acid amidated gastrin), and of G34-Gly to G17-Gly (G34 and G17 with COOH-terminal glycine), was increased 3-fold after treatment with omeprazole for either 1 or 5 days. 4. Approximately 20% of newly synthesized amidated and Gly-extended gastrins were secreted within 240 min of the labelling period in omeprazole-treated samples, but secretion of labelled gastrins from control tissue was undetectable over a comparable period. 5. The amidating enzyme, peptidyglycine alpha-amidating mono-oxygenase (PAM), the prohormone convertases PC1/3, PC2, PC5 and the PC2 chaperone 7B2 were localized to rat antral gastrin cells by immunocytochemistry. The relative abundance of mRNA species encoding 7B2, PC5 and PAM were unchanged after treatment with omeprazole for 5 days, whereas gastrin, PC1/3 and PC2 mRNAs are known to increase at this time. 6. The main consequence of increased cleavage at Lys-74-75 is the production of G17 and G17-Gly at the expense of G34 and G34-Gly, respectively. The latter have longer plasma half-lives, and so their increased cleavage may serve to limit the rise in plasma gastrin concentration after inhibition of acid secretion. Changes in the abundance of mRNAs encoding prohormone-processing enzymes cannot account for the rapidity of the changes in cleavage of progastrin at Lys residues after omeprazole.

EGF receptor activation stimulates endogenous gastrin gene expression in canine G cells and human gastric cell cultures

Gastrin release from the antral gastrin-expressing cell (G cell) is regulated by bombesin and luminal factors. Yet, these same extracellular regulators do not stimulate expression of the gene. Since the gastric mucosa expresses large quantities of EGF receptor ligands such as TGFalpha, we examined whether EGF receptor ligands stimulate gastrin gene expression in gastrin-expressing cell cultures. EGF receptor activation of primary cultures stimulated gastrin gene expression about twofold; whereas bombesin treatment of antral G cell cultures stimulated gastrin release but not gene expression. EGF and TGFalpha were weak stimulants of gastrin release. EGF receptor activation of AGS human gastric adenocarcinoma cell line stimulated gastrin gene expression nearly fourfold; and gastrin reporter constructs transfected into AGS cells were stimulated more than fourfold by EGF. EGF induction was conferred by the previously defined GC-rich gastrin EGF response element (gERE) element located at -68 to -53 bp upstream from the cap site since a mutation of the gERE element abolished both basal and EGF induction. Moreover, EGF treatment of AGS cells stimulated binding of the transcription factor Sp1 to this element. Collectively, these results demonstrate that gastrin gene expression and gastrin release are regulated by different signaling pathways: gene expression by EGF receptor activation and gastrin secretion by neuropeptides and luminal factors.

Interaction of nicotine and a H2-receptor antagonist, famotidine, on gastrin and chromogranin A expression

The purpose of this study is to examine the effect of nicotine on famotidine-induced hypergastrinemia in the rat. In addition, the effects of nicotine on gene expression for gastrin and chromogranin A (CGA) in the stomach were examined. Famotidine treatment alone (20 mg/kg. 2 x/day for 14 days) increased serum gastrin levels significantly (P < 0.05) but not antral levels of gastrin mRNA and peptide. Nicotine treatment (12 mg/kg/d) alone did not affect serum gastrin levels; however, nicotine potentiated the hypergastrinemic action of famotidine. The hypergastrinemic action of nicotine was not mediated by a downregulation of stomach somatostatin (SRIF) since stomach SRIF mRNA levels were unaffected by nicotine treatment. Administration of nicotine and famotidine also upregulated stomach CGA gene expression (i.e., mRNA and protein levels) significantly.

Post-translational processing of progastrin: inhibition of cleavage, phosphorylation and sulphation by brefeldin A

The precursor for the acid-stimulating hormone gastrin provides a useful model for studies of post-translational processing because defined sites of cleavage, amidation, sulphation and phosphorylation occur within a dodecapeptide sequence. The factors determining these post-translational processing events are still poorly understood. We have used brefeldin A, which disrupts transport from rough endoplasmic reticulum to the Golgi complex, to examine the mechanisms of cleavage, phosphorylation and sulphation of rat progastrin-derived peptides. Biosynthetic products were detected after immunoprecipitation using antibodies specific for the extreme C-terminus of progastrin, followed by reversed-phase and ion-exchange h.p.l.c. Gastrin cells incorporated [3H]tyrosine, [32P]phosphate and [35S]sulphate into both progastrin and its extreme C-terminal tryptic (nona-) peptide. Ion-exchange chromatography resolved four forms of the C-terminal tryptic fragment of progastrin which differed in whether they were phosphorylated at Ser96, sulphated at Tyr103, both or neither. The specific activity of [3H]tyrosine in the peak that was both phosphorylated and sulphated was higher than in the others. Brefeldin A inhibited the appearance of [3H]tyrosine-labelled C-terminal tryptic fragment but there was an accumulation of labelled progastrin and a peptide corresponding to the C-terminal 46 residues of progastrin. Brefeldin A also inhibited incorporation of 32P and 35S into both progastrin and its C-terminal fragment. Thus phosphorylation of Ser96, sulphation of Tyr103 and cleavage at Arg94-Arg95 depend on passage of newly synthesized progastrin along the secretory pathway; as brefeldin A is thought to act proximal to the trans-Golgi, these processing steps would appear to occur distal to this point. The data also indicate that the stores of unphosphorylated C-terminal tryptic fragment are not available for phosphorylation, implying that this modification occurs proximal to the secretory granule; cleavage is known to occur in the secretory granule which suggests that it occurs after phosphorylation.

Hormonal control of intestinal Fc receptor gene expression and immunoglobulin transport in suckling rats

Hormonal control of immunoglobulin (Ig) absorption and of intestinal Fc receptor mRNA expression were investigated in rats to assess its potential role in the normal postsuckling inhibition of this transport system. Corticosterone and L-thyroxine therapy caused premature inhibition of the absorption of orally administered murine monoclonal antibody and of Fc receptor mRNA expression in a dose- and time-dependent manner. Low-dose corticosterone had no effect on Fc receptor mRNA synthesis after 3 d but decreased Ig transport fivefold after 7 d. High dose corticosterone resulted in a threefold reduction in Fc receptor after 3 d, and there was almost complete inhibition (> 30-fold) of transport and of Fc receptor transcript levels after 7 d. Similarly, 7 d of high-dose thyroxine decreased both serum Ig transport and Fc receptor (> 30-fold). However, adrenalectomy did not prevent the normal post-suckling declines in Ig transport or receptor synthesis. This study demonstrates that exogenous corticosteroids and thyroxine hormone inhibit Ig transport and steady-state duodenal Fc receptor mRNA levels in suckling rats. Endogenous adrenal steroids however, do not appear to be entirely responsible for the age-dependent decline in this transport system.

Food stimulation of histidine decarboxylase messenger RNA abundance in rat gastric fundus

1. Histidine decarboxylase in the enterochromaffin-like cells of the gastric corpus mucosa converts histidine to histamine which in turn stimulates gastric acid secretion. The control of histidine decarboxylase activity is poorly understood. We have examined how fasting and refeeding influence the abundance of the messenger RNA encoding histidine decarboxylase in the gastric corpus of the rat. 2. The polymerase chain reaction was used to generate a probe for detection of histidine decarboxylase messenger RNA in Northern and slot blots of total RNA from the gastric corpus of rats fasted for up to 48 h, or fasted and then refed. A gastrin monoclonal antibody was used to neutralize the action of endogenous gastrin. 3. Fasting progressively reduced histidine decarboxylase messenger RNA abundance by 3- to 4-fold after 48 h. Refeeding induced a rapid increase in histidine decarboxylase messenger RNA abundance which was detectable after 30 min. 4. There was a significant correlation between histidine decarboxylase messenger RNA abundance and plasma gastrin. Administration of gastrin antibody inhibited the increase in histidine decarboxylase activity after 6 h refeeding, but not after refeeding for 30 min. 5. The results suggest that histamine-mediated changes in postprandial acid secretion depend on control of histidine decarboxylase mRNA levels, and that gastrin regulates production of this enzyme in the rat over periods of a few hours.

Differential control of somatostatin messenger RNA in rat gastric corpus and antrum. Role of acid, food, and capsaicin-sensitive afferent neurons

Somatostatin messenger RNA in the antrum and corpus of rat stomach was quantified by Northern and slot blotting using a probe generated by the polymerase chain reaction. Fasting for 48 h enhanced the abundance of somatostatin mRNA in the pyloric antral region, but not in the acid-secreting region of the stomach. In fasted rats, somatostatin mRNA in antrum, but not corpus, was decreased by inhibition of acid secretion with omeprazole. In contrast, in rats treated with capsaicin to lesion small diameter afferents there was a significant decrease in somatostatin mRNA abundance in the corpus but not antrum. The effects of capsaicin cannot be attributed to nonspecific changes in gastric endocrine cell gene expression, since the abundance of histidine decarboxylase mRNA (which is a functionally regulated marker for a different gastric endocrine cell type) did not change with capsaicin. Gastric capsaicin-sensitive afferents are rich in calcitonin gene-related peptide, and in rats with antibodies to this peptide there was reduced corpus somatostatin mRNA. Moreover, infusion of calcitonin gene-related peptide in control rats produced a significant increase in somatostatin mRNA in the gastric corpus. The results indicate that somatostatin mRNA abundance is controlled by the gastric luminal contents and the extrinsic afferent innervation, but the relative importance of these factors differs in antrum and corpus: luminal contents are relatively more important in antrum and primary afferents using calcitonin gene-related peptide in the corpus.