Dual inhibitory actions of somatostatin on isolated gastric glands

Gastric Peptides-Gastrin and Somatostatin

Gastric acid secretion (i) facilitates digestion of protein as well as absorption of micronutrients and certain medications, (ii) kills ingested microorganisms, including Helicobacter pylori, and (iii) prevents bacterial overgrowth and enteric infection. The principal regulators of acid secretion are the gastric peptides gastrin and somatostatin. Gastrin, the major hormonal stimulant for acid secretion, is synthesized in pyloric mucosal G cells as a 101-amino acid precursor (preprogastrin) that is processed to yield biologically active amidated gastrin-17 and gastrin-34. The C-terminal active site of gastrin (Trp-Met-Asp-Phe-NH₂ ) binds to gastrin/CCK₂ receptors on parietal and, more importantly, histamine-containing enterochromaffin-like (ECL) cells, located in oxyntic mucosa, to induce acid secretion. Histamine diffuses to the neighboring parietal cells where it binds to histamine H₂ -receptors coupled to hydrochloric acid secretion. Gastrin is also a trophic hormone that maintains the integrity of gastric mucosa, induces proliferation of parietal and ECL cells, and is thought to play a role in carcinogenesis. Somatostatin, present in D cells of the gastric pyloric and oxyntic mucosa, is the main inhibitor of acid secretion, particularly during the interdigestive period. Somatostatin exerts a tonic paracrine restraint on gastrin secretion from G cells, histamine secretion from ECL cells, and acid secretion from parietal cells. Removal of this restraint, for example by activation of cholinergic neurons during ingestion of food, initiates and maximizes acid secretion. Knowledge regarding the structure and function of gastrin, somatostatin, and their respective receptors is providing novel avenues to better diagnose and manage acid-peptic disorders and certain cancers. Published 2020. Compr Physiol 10:197-228, 2020.

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.

Release of regulatory gut peptides somatostatin, neurotensin and vasoactive intestinal peptide by acid and hyperosmolal solutions in the intestine in conscious rats

The impact of exposure of the intestinal mucosa to acid and hyperosmolal solutions on the release of the inhibitory gut peptides somatostatin (SOM), neurotensin (NT) and vasoactive intestinal peptide (VIP) was studied in conscious rats during pentagastrin-stimulated gastric acid secretion. The animals were equipped with a chronic gastric fistula to measure acid secretion and a jejunal Thiry-Vella loop for intestinal challenge with saline, hydrochloric acid (HCl, 200 mmol L(-1)) or hyperosmolal polyethylene glycol (PEG, 1200 mOsm kg(-1)). Gut peptide concentrations were measured in intestinal perfusates, and in plasma samples collected during stimulated acid secretion, and at the end of experiments with luminal challenge of the loops. After pentagastrin-stimulation acid secretion was dose-dependently inhibited by intravenous administration of the gastrin receptor antagonist gastrazole, as well as ranitidine and esomeprazole by maximally 73+/-10%; 95+/-3%; 90+/-10%, respectively. Acid perfusion of the Thiry-Vella loop caused a prominent release of SOM both to the lumen (from 7.2+/-5.0 to 1279+/-580 pmol L(-1)) and to the circulation (from 18+/-5.2 to 51+/-9.0 pmol L(-1)) simultaneously with an inhibition of gastric acid secretion. The release of NT and VIP was not affected to the same extent. PEG perfusion of the loop caused a release of SOM as well as NT and VIP, but less. Simultaneously acid secretion was slightly decreased. In conclusion, intestinal perfusion with acid or hyperosmolal solutions mainly releases SOM, which seems to exert a major inhibitory action in the gut, as shown by inhibition of acid secretion. The other peptides NT and VIP also participate in this action but to a much lesser degree. The operative pathways of these gut peptides hence involve both endocrine (SOM) and paracrine actions (SOM, NT, VIP) in order to exert inhibitory functions on the stomach. The inhibitory action of gastrazole, was in a similar range as that of SOM implying that physiological acid-induced inhibition of gastric acid may primarily be exerted through inhibition of gastrin endocrine secretion.

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.

Neurohormonal regulation of histamine and pancreastatin secretion from isolated rat stomach ECL cells

ECL cells are numerous in the acid-producing part of the rat stomach. They are rich in histamine and pancreastatin, a chromogranin A-derived peptide, and they secrete these products in response to gastrin. We have examined how isolated ECL cells respond to a variety of neuromessengers and peptide hormones. Highly purified (85%) ECL cells were collected from rat stomach using repeated counter-flow elutriation and cultured for 48 h before experiments were conducted. The ECL cells responded to gastrin, sulphated cholecystokinin-8 and to high K+ and Ca2+ with the parallel secretion of histamine and pancreastatin. Glycine-extended gastrin was without effect. Forskolin, an activator of adenylate cyclase, induced secretion, whereas isobutylmethylxanthine, a phosphodiesterase inhibitor, raised the basal release without enhancing the gastrin-evoked stimulation. Maximum stimulation with gastrin resulted in the release of 30% of the secretory products. Numerous neuromessengers and peptide hormones were screened for their ability to stimulate secretion and to inhibit gastrin-stimulated secretion. Pituitary adenylate cyclase activating peptide (PACAP)-27 and -38 stimulated secretion of both histamine and pancreastatin with a potency greater than that of gastrin and with the same efficacy. Related peptides, such as vasoactive intestinal peptide, helodermin and helospectin, stimulated secretion with lower potency. The combination of EC100 gastrin and EC50 PACAP produced a greater response than gastrin alone. None of the other neuropeptides or peptide hormones tested stimulated secretion. Serotonin, adrenaline, noradrenaline and isoprenaline induced moderate secretion at high concentrations. Muscarinic receptor agonists did not stimulate secretion, and histamine and selective histamine receptor agonists and antagonists were without effect. This was the case also with GABA, aspartate and glutamate. Somatostatin and galanin, but none of the other agents tested, inhibited gastrin-stimulated secretion. Our results reveal that not only gastrin but also PACAP is a powerful excitant of the ECL cells, that not only somatostatin, but also galanin can suppress secretion, that muscarinic receptor agonists fail to evoke secretion, and that histamine (and pancreastatin) does not evoke autofeedback inhibition.

Dual inhibitory pathways link antral somatostatin and histamine secretion in human, dog, and rat stomach

BACKGROUND & AIMS

The secretion and function of antral histamine are not known. The aims of this study were to characterize the mechanisms of histamine release from the gastric antrum of humans, dogs, and rats and to determine whether histamine can influence the secretion of somatostatin and gastrin.

METHODS

Somatostatin, gastrin, and histamine secretion from superfused antral segments was measured using radioimmunoassay.

RESULTS

Superfusion with thioperamide (H3 antagonist) increased somatostatin and decreased gastrin and histamine secretion in all three species; superfusion with (r)-alpha-methylhistamine (H3 agonist) had the opposite effect. The pattern implied that endogenous histamine, acting via H3 receptors, exerts an inhibitory paracrine influence on somatostatin secretion, which in turn regulates gastrin secretion. Superfusion with somatostatin antibody increased histamine secretion; the increase was not affected by the gastrin antagonist L-365,260, implying that it was not mediated by the concurrent increase in gastrin but by suppression of an inhibitory pathway linking somatostatin and histamine. Superfusion with methacholine alone and in the presence of either the H3 agonist or antagonist confirmed the existence of reciprocal inhibitory pathways linking somatostatin and histamine.

CONCLUSIONS

Antral histamine in humans, dogs, and rats is linked to antral somatostatin via reciprocal inhibitory paracrine pathways that serve to amplify the regulatory influence of somatostatin.

Release of somatostatin, neurotensin and vasoactive intestinal peptide upon inhibition of gastric acid secretion by duodenal acid and hyperosmolal solutions in the conscious rat

The inhibitory effect of duodenal exposure to acid and hyperosmolal solutions on pentagastrin-stimulated gastric acid secretion was studied in conscious rats equipped with chronic gastric fistula and duodenal Thiry-Vella loop. The loop was challenged with saline, HCl or hyperosmolal polyethylene glycol. Gastric acid secretion was measured in samples from the gastric fistula. Gut peptide concentrations were measured in duodenal perfusates collected each 30 min, and in plasma samples collected both during stimulated acid secretion alone, and at the end of experiments in combination with luminal challenges of the loops. During pentagastrin-stimulated gastric acid secretion, luminal perfusion of the duodenal loop with acid caused inhibition of acid secretion (P < 0.001) and a prominent release of somatostatin both to the lumen (P < 0.001) and to the circulation (P < 0.05). Also, neurotensin (P < 0.01) and vasoactive intestinal peptide (P < 0.01) were released to the lumen, but not to the circulation. Upon perfusion of the duodenal loop with hyperosmolal polyethylene glycol, acid secretion was inhibited (P < 0.05) and somatostatin alone was released to the luminal side (P < 0.01). In conclusion, duodenal exposure to acid inhibits pentagastrin-stimulated gastric acid secretion and releases SOM to the circulation that may directly inhibit acid secretion. Concomitantly, somatostatin (SOM), neurotensin and vasoactive intestinal peptide are released to the lumen. Duodenal exposure to hyperosmolal polyethylene glycol inhibits acid secretion with a luminal release of SOM only. Thus, luminal acid and hyperosmolal solutions inhibit gastric acid secretion by separate mechanisms. After acid or hyperosmolal challenge, the release of SOM to the circulation indicates gastric acid inhibition in an endocrine manner, while a luminal release of gut peptides indicates a local peptide overflow that might be of importance via paracrine regulatory mechanisms in the intact animal.

Effects of hypochlorhydria and hypergastrinemia on structure and function of gastrointestinal cells. A review and analysis

Since hypochlorhydria can induce hypergastrinemia, and gastrin has a trophic effect on some gastrointestinal cells, states that cause elevated plasma gastrin levels are of interest in terms of effects on cell growth and function. This article reviews the relationship between gastric mucosal cells during periods of acid stimulation and inhibition and analyses the effects of hypochlorhydria and hypergastrinemia on gastric and colonic cells and tumors. Hypochlorhydria releases the inhibitory effect of antral gastrin cells, inducing them to release gastrin in the presence of peptides or amino acids in the gastric lumen or in response to antral distension. Gastrin stimulates the oxyntic mucosa, which may lead to hyperplasia of enterochromaffin-like cells, resulting in enterochromaffin-like carcinoid tumors in aged rats and, rarely, in patients with chronic atrophic gastritis or gastrinomas. In addition to hypergastrinemia, other factors appear to be required for the progression of enterochromaffin-like hyperplasia to carcinoids; genetic factors may be involved. Gastrin elevations due to antisecretory drug therapy are indirectly proportional to the degree of acid inhibition and are reversible upon cessation of therapy. The gastrin levels during omeprazole therapy are similar to those caused by gastric vagotomy. Available evidence does not support a relationship between hypergastrinemia and the occurrence or growth of gastric carcinoma or colonic tumors.

Secretion of pancreastatins from the porcine digestive tract

Pancreastatin, a 49-amino acid peptide with a COOH-terminal glycine amide, was originally isolated from porcine pancreas, but pancreastatin immunoreactivity has been found in several neuroendocrine tissues. There are strong indications that pancreastatin is derived from chromogranin A, since the amino acid sequence 240-288 in porcine chromogranin A corresponds to pancreastatin flanked by typical signals for proteolytic processing. We have studied the effect of electric stimulation of the nervous supply to perfused porcine pancreas, antrum, nonantral stomach, and small intestine on the release of immunoreactive pancreastatin, and we characterized the molecular nature of the secreted immunoreactivity by using a radioimmunoassay specific for the COOH-terminal glycine amide of porcine pancreastatin in combination with chromatography. In all tissues nerve stimulation significantly increased the release of immunoreactive pancreastatin. The secreted immunoreactive pancreastatin was heterogeneous, consisting of pancreastatin itself, a COOH-terminal pancreastatin fragment, and NH2-terminally extended pancreastatin forms. Pancreastatin predominated in the perfusate from pancreas and antrum, whereas mainly NH2-terminally extended molecular forms were secreted from the antrectomized stomach and small intestine. The different molecular forms of pancreastatin were secreted from the perfused organs in the same molar ratio as they occur in extracts of the corresponding tissues. Thus, pancreastatin and other chromogranin A-derived peptides in organ-specific proportions regularly accompany the secretion of the peptide hormones from the gastrointestinal tissues on appropriate stimulation.

Somatostatin inhibits gastrin-induced histamine secretion and synthesis in the rat

Somatostatin is a potent inhibitor of gastric acid secretion. However, the effect of somatostatin on gastric histamine secretion and synthesis has not been well understood, despite the fact that histamine plays a key role in the regulation of gastric acid secretion. This study was designed to determine the effect of somatostatin on gastric histamine mobilization and acid secretion in conscious rats. In conscious rats with a gastric fistula, a 4 h intravenous infusion of gastrin-17 I (1 nmol/kg/h) evoked a marked increase in fundic histidine decarboxylase activity (the sole histamine-forming enzyme) and reduced fundic histamine content with a concomitant increase in gastric acid secretion. Somatostatin-14 (10 nmol/kg/h) significantly inhibited gastrin-induced gastric acid secretion and fundic histidine decarboxylase activity and prevented a gastrin-induced decrease in fundic histamine content. In conscious rats with a vesical fistula, somatostatin-14 (10 nmol/kg/h) significantly inhibited the urinary histamine excretion induced by a gastrin-17 I (1 nmol/kg/h) infusion. These findings suggest that the inhibitory action of somatostatin on gastrin-induced acid secretion is mediated by the inhibition of histamine mobilization.

Histamine secretion from rat enterochromaffinlike cells

BACKGROUND

In vivo studies have suggested an important role for gastric enterochromaffinlike (ECL) cells in mediating acid secretion. Direct evidence for this function is lacking and requires a preparation of highly purified ECL cells. This work investigates the possible role and mechanism of histamine release from the ECL cell in the peripheral regulation of acid secretion, using purified ECL cells from rat fundic mucosa.

METHODS

A combination of elutriation and density-gradient centrifugation was used to purify rat fundic ECL cells. Enrichment was determined by the presence of acidic vacuoles containing a V type adenosine triphosphatase, electron microscopy, immunostaining, and histamine content and release.

RESULTS

ECL cells were enriched at least 65-fold with respect to the fundic epithelium. Gastrin (EC50 0.2 nmol/L) and cholecystokinin octapeptide (nonsulfated, EC50 0.04 nmol/L) stimulated histamine release in a time- and dose-dependent manner, suggesting a CCK-B receptor subtype, confirmed by the inhibition of gastrin/CCK stimulation with the CCK-B antagonist L365,260. Somatostatin also inhibited gastrin-mediated histamine release. Single cell imaging showed that gastrin elevated intracellular cytosolic calcium concentration biphasically. Carbachol and the C kinase activator 120-tetradecanoylphorbol-13-acetate also stimulated histamine release. Epinephrine (blocked by propranolol), forskolin, and dibutyryl-5'-cyclic adenosine monophosphate were also effective, implicating a beta-adrenergic pathway. The H3 agonist R-alpha-methyl-histamine inhibited, whereas the H3-antagonist thioperamide potentiated gastrin/CCK stimulated histamine release.

CONCLUSIONS

These in vitro results support a central role for the ECL cell in the peripheral regulation of gastric acid secretion.

Negative feedback regulation of acid secretion: linkage of acid secretion and mucosal defense

To regulate inappropriate or excessive acid secretion that may be harmful not only to surrounding mucosal cells but also parietal cells themselves, versatile mechanisms operate to control acid secretion. In this article elaborate mechanisms that participate in the control of acid secretion are described with our data and a review of relevant findings in the literature concerning the action or secretion of key regulatory molecules. The analysis of the regulatory mechanisms revealed that appropriate secretory control can be achieved by multiple feedback regulations, which are closely coordinated with mucosal defense. In the feedback circuits, the regulatory factors cannot be simply categorized into either aggressive or defensive factors. Therefore we propose that the mucosal integrity and tissue homeostasis would be better understood by a feedback loop model rather than by a dualistic balance model.

Pharmacology of gastric acid inhibition

Gastric acid secretion is precisely regulated by neural (acetylcholine), hormonal (gastrin), and paracrine (histamine; somatostatin) mechanisms. The stimulatory effect of acetylcholine and gastrin is mediated via increase in cytosolic calcium, whereas that of histamine is mediated via activation of adenylate cyclase and generation of cAMP. Potentiation between histamine and either gastrin or acetylcholine may reflect postreceptor interaction between the distinct pathways and/or the ability of gastrin and acetylcholine to release histamine from mucosal ECL cells. The prime inhibitor of acid secretion is somatostatin. Its inhibitory paracrine effect is mediated predominantly by receptors coupled via guanine nucleotide binding proteins to inhibition of adenylate cyclase activity. All the pathways converge on and modulate the activity of the luminal enzyme, H+,K(+)-ATPase, the proton pump of the parietal cell. Precise information on the mechanisms involved in gastric acid secretion and the identification of specific receptor subtypes has led to the development of potent drugs capable of inhibiting acid secretion. These include competitive antagonists that interact with stimulatory receptors (e.g. muscarinic M1-receptor antagonists and histamine H2-receptor antagonists) as well as non-competitive inhibitors of H+,K(+)-ATPase (e.g. omeprazole). The histamine H2-receptor antagonists (cimetidine, ranitidine, famotidine, nizatidine and roxatidine acetate) continue as first-line therapy for peptic ulcer disease and are effective in preventing relapse. Although they are generally well tolerated, histamine H2-receptor antagonists may cause untoward CNS, cardiac and endocrine effects, as well as interfering with the absorption, metabolism and elimination of various drugs. The dominance of the histamine H2-receptor antagonists is now being challenged by omeprazole. Omeprazole reaches the parietal cell via the bloodstream, diffuses through the cytoplasm and becomes activated and trapped as a sulfenamide in the acidic canaliculus of the parietal cell. Here, it covalently binds to H+,K(+)-ATPase, the hydrogen pump of the parietal cell, thereby irreversibly blocking acid secretion in response to all modes of stimulation. The main potential drawback to its use is its extreme potency which sometimes leads to virtual anacidity, gastrin cell hyperplasia, hypergastrinaemia and, in rats, to the development of carcinoid tumours. The cholinergic receptor on the parietal cell has recently been identified as an M3 subtype and that on postganglionic intramural neurones of the submucosal plexus as an M1 subtype.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell biology of gastric acid secretion

The parietal cells, which are responsible for the production of gastric HCl acid, are uniquely equipped for high-gradient ion transport. Adequate energy is supplied by oxidative metabolism in the mitochondria, which occupy an exceptionally high proportion of the cytoplasmic volume. Another characteristic feature is the secretory canaliculi. These are tortuous small channels lined by microvilli which penetrate all parts of the cytoplasm and which expand during stimulation of secretion. The activity of the parietal cell is controlled by receptors for acetylcholine, histamine and gastrin on the basolateral cell membrane. Stimulation of these receptors modulates the levels of protein kinases in the cell and brings about the changes from resting to stimulated structure. A key role in the production of acid is played by the gastric acid pump, also known as the H+, K(+)-ATPase, which exports hydrogen ions in 1:1 exchange for potassium ions. This protein is a member of the P-type ATP-driven ion pumps and appears to be uniquely located in the parietal cell. The gastric acid pump is found in the tubulovesicular membranes of the resting cell and moves to the membrane lining the secretory canaliculus when acid secretion is stimulated. Functional acid secretion also requires the presence of KCl pathways in the secretory membrane in order to supply the acid pump with a source of potassium ions. For each hydrogen ion secreted across the secretory membrane, one bicarbonate ion is generated in the cytoplasm and is transported across the basolateral membrane in exchange for chloride. The movement of ions across the apical membrane is followed osmotically by water, resulting in the secretion of 160 mM HCl from the parietal cell.

Neurohormonal regulation of histamine release from isolated rabbit fundic mucosal cells

Histamine-containing cells isolated from rabbit fundic mucosa were found in a small cell elutriation fraction (cells with diameter about 9-12 microns) enriched in mucus and endocrine cells and containing less than 1% mast cells (F1 cells). Gastrin (HG-17), pentagastrin and CCK-8 (C-terminal octapeptide of cholecystokinin) dose-dependently stimulated histamine release (EC50, respectively, 0.126 +/- 0.03, 0.92 +/- 0.15 and 0.211 +/- 0.025 nM) and somatostatin inhibited this release. PGE1, PGE2 and PGD2 alone were unable to enhance histamine release even at high concentrations but, when used in combination with gastrin of CCK-8, the release of histamine caused by these peptides was potentiated (about 1.5- to 2-fold). Carbachol also enhanced the liberation of histamine but with a weaker potency and efficacy than the gastrointestinal peptides (EC50: 1.50 +/- 0.06 microM). The use of specific muscarinic antagonists for M1-, M2- and M3-type receptors led us to conclude that an M1 receptor might be involved in the muscarinic-induced stimulation of histamine release. Activators of protein kinase C, 12-O-tetradecanoylphorbol-13-acetate (TPA) and 1-oleyl-2-acetyl-glycerol (OAG) as well as the calcium ionophore, A23187, induced histamine release, whereas agents which increased intracellular cAMP content were devoid of effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Enterochromaffin-like cells in rat stomach respond to short-term infusion of high doses of cholecystokinin but not to long-term, sustained, moderate hyperCCKemia caused by continuous cholecystokinin infusion or pancreaticobiliary diversion

The histamine-producing enterochromaffin-like (ECL) cells in the oxyntic mucosa are controlled by gastrin. An acute gastrin challenge induces release and accelerated resynthesis of ECL cell histamine. Long-term stimulation with gastrin causes ECL cell hyperplasia. We set out to study whether the ECL cells respond not only to gastrin but also to cholecystokinin (CCK). A wide dose range of gastrin-14 sulfated and -17 non-sulfated and CCK-8 sulfated (CCK-8s) and non-sulfated (CCK-8) was infused intravenously to rats for 3 h. The activity of the histamine-forming enzyme was measured at termination of infusion. Gastrins and CCK-8s were equally effective in activating the enzyme, whereas sulfated CCK-8 was notably less potent than the other three peptides. Clearly, the receptor responsible for activation of the ECL cells distinguishes poorly between gastrin-17 and CCK-8s, which is in line with the characteristics of the CCK-B receptor. Moreover, neither the response to gastrin-17 nor that to CCK-8s was affected by concomitant infusion of devazepide (200 micrograms/kg/h), a selective CCK-A-receptor antagonist. One group of rats received CCK-8s continuously via a minipump. Another group of rats was subjected to pancreaticobiliary diversion (PBD), which increases the plasma CCK concentration 10- to 20-fold. The rats were killed 7 or 10 weeks later, respectively, and the stomachs were analyzed with regard to mucosal growth and ECL cell hyperplasia. HyperCCKemic rats had increased pancreatic weights but showed no signs of growth stimulation in the stomach and no ECL cell hyperplasia.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-endocrine applications of somatostatin and octreotide acetate: facts and flights of fancy

Somatostatin, originally detected by Krulich and ultimately isolated by Brazeau, was initially described as a growth hormone release-inhibiting factor. Subsequent investigation into the use of native somatostatin and the development of long-acting somatostatin analogues, especially octreotide acetate, have fostered increasing uses of these compounds. Though the clinical and investigational uses of somatostatin and its analogues are varied, one central theme remains constant: the ability of these agents to suppress circulating peptide levels. This article, a review of the current non-endocrine applications of somatostatin and its analogues, covers a wide range of potential applications for somatostatin-like compounds. These include use in cirrhosis and variceal bleeding, peptic ulcer disease, pancreatic fistulas, acute and chronic pancreatitis, dumping syndrome, cancer therapy, small bowel fistulas, psoriasis, pain control, and autonomic hypotension. Somatostatin may also play a role in the development and potential treatment of neurologic disease and may have profound found influence on behavior.

Effects of some gastrointestinal peptides on isolated human and rabbit gastric glands

The isolated gastric gland preparation, with aminopyrine accumulation as an index of the parietal cell response, has been used to study the effects of somatostatin (S-14), gastrin-releasing peptide (GRP), cholecystokinin (CCK-8), vasoactive intestinal peptide (VIP), and peptide YY (PYY) on the in vitro acid secretion in human and rabbit oxyntic mucosa. Somatostatin was able to inhibit the parietal cell response to histamine in both human and rabbit isolated gastric glands (maximal inhibition, 22% and 34%, respectively) but failed to inhibit the parietal cell response to db-cAMP. However, other peptides capable of inhibiting gastric acid secretion in vivo, such as CCK, VIP, and PYY, were unable to induce any inhibition of the parietal cell response to db-cAMP or histamine in the isolated gastric gland preparation irrespective of the species studied. GRP was not able to induce a parietal cell response, a finding that is in accord with the assumption that the stimulatory effect of GRP on gastric acid secretion in vivo is by releasing gastrin from antral G-cells.

Aspects of the regulation of gastric histamine release

Histamine is found in large amounts in the gastric mucosa and plays an essential role in the regulation of acid secretion. It is thought to stimulate acid secretion directly after being released by the other two major secretagogues (gastrin and acetylcholine) (the mediator hypothesis) or to potentiate the action of the other two secretagogues (the interaction hypothesis). Recent studies with isolated, vascularly perfused rat stomach have shown that gastrin in physiologic concentrations elicits a release of histamine sufficient to explain its acid-stimulatory effect. Vagal nerve stimulation, on the other hand, only gives a faint histamine release, indicating that the vagal acid stimulation is mainly mediated by a direct stimulation of the parietal cell. Furthermore, the gastrin-stimulated histamine release seems to be mediated by a calcium-dependent mechanism. Somatostatin inhibits gastrin-stimulated histamine release via a paracrine mechanism, and a prostaglandin E1 analogue (misoprostol) has been shown to be a potent inhibitor of base-line and gastrin-stimulated histamine release. These studies show that the modulation of histamine release may be a central regulatory mechanism of gastric acid secretion. Although these studies have been done in rats, there are indications that these results are of a general nature nd valid for other species as well.

Thapsigargin potentiates histamine-stimulated HCl secretion in gastric parietal cells but does not mimic cholinergic responses

The role of calcium in control of HCl secretion by the gastric parietal cell was examined using a recently available intracellular calcium-releasing agent, thapsigargin, which has been shown, in some cell types, to induce sustained elevation of intracellular calcium ([Ca2+]i), an action that appears to be independent of inositol lipid breakdown and protein kinase C activation and to be mediated, at least partially, by selective inhibition of endoplasmic reticulum Ca2(+)-ATPase. Using the calcium-sensitive fluorescent probe, fura-2, in combination with digitized video image analysis of single cells as well as standard fluorimetric techniques, we found that thapsigargin induced sustained elevation of [Ca2+]i in single parietal cells and in parietal cells populations. Chelation of medium calcium led to a transient rise and fall in [Ca2+]i, indicating that the sustained elevation in [Ca2+]i in response to thapsigargin was due to both intracellular calcium release and influx. Although thapsigargin appeared to affect the same calcium pool(s) regulated by the cholinergic agonist, carbachol, and the pattern of thapsigargin-induced increases in [Ca2+]i were similar to the plateau phase of the cholinergic response, thapsigargin did not induce acid secretory responses of the same magnitude as those initiated by carbachol (28 vs 600% of basal). The protein kinase C activator, 12-O-tetradecanoyl phorbol-13-acetate (TPA) potentiated the secretory response to thapsigargin but this combined response also did not attain the same magnitude as the maximal cholinergic response. In the presence but not the absence of medium calcium, thapsigargin potentiated acid secretory responses to histamine, which elevate both cyclic AMP (cAMP) and [Ca2+]i in parietal cells, as well as forskolin and cAMP analogues but had no effect on submaximal and an inhibitory effect on maximal cholinergic stimulation. Furthermore, thapsigargin did not fully mimic potentiating interactions between histamine and carbachol, either in magnitude or in the pattern of temporal response. Assuming that the action of thapsigargin is specific for intracellular calcium release mechanisms, these data suggest that 1) sustained influx of calcium is necessary but not sufficient for cholinergic activation of parietal cell HCl secretion and for potentiating interactions between cAMP-dependent agonists and carbachol; 2) mechanisms in addition to elevated [Ca2+]i and protein kinase C activation may be involved in cholinergic regulation; and 3) increases in [Ca2+]i in response to histamine are not directly involved in the mechanism of histamine-stimulated secretion.

Histamine metabolism in human gastric mucosa. Effect of pentagastrin stimulation

The metabolism of histamine in the human gastric mucosa was studied in the basal state and during pentagastrin stimulation. Studies were made in healthy volunteers and in patients with peptic ulcer disease. Mucosal biopsies were taken from antral and oxyntic gland areas whereupon histamine content, histidine decarboxylase activity, and histamine methyltransferase activity were simultaneously assayed. Histamine content of the oxyntic gland mucosa was decreased as a consequence of pentagastrin administration in all groups studied, and this decrease was numerically largest in patients with duodenal ulcer disease. Pentagastrin induced a significant increase in histidine decarboxylase activity of the oxyntic gland mucosa with the most profound increase seen in patients with duodenal ulcer. The highest rates of histamine formation were present in the oxyntic mucosa of patients with Zollinger-Ellison syndrome. The activity of histamine methyltransferase was the same in all groups studied and was not changed by pentagastrin. In conclusion, pentagastrin administration in humans is followed by a significant mobilization of histamine only from the oxyntic gland mucosa, an effect that is more pronounced in patients with duodenal ulcer disease.

Antiulcer activity of the calcium antagonist propyl-methylenedioxyindene--I. Effect on cold/restraint stress-induced ulcers in rats

1. Propyl-methylenedioxyindene (pr-MDI) is an intracellularly acting calcium antagonist with H2-receptor blocking properties. Stimulus-secretion coupling is inhibited by much lower concentrations of pr-MDI than is excitation-contraction coupling. 2. Since the processes leading to gastric ulceration are calcium-dependent, the aim of this study was to determine if pr-MDI could provide useful antiulcer activity at doses below those required to produce cardiovascular effects. 3. The antiulcer activity of pr-MDI (10-30 mg/kg) was examined in the cold (4 degree C)/restraint (3 hr) stress-induced ulcer model in male rats, and compared with the effects of the H2-blocker cimetidine (10-30 mg/kg) and the calcium channel blocker verapamil (11-32 mg/kg). The drugs were administered intraperitoneally 10 min prior to the cold/restraint stress. 4. All three drugs significantly reduced the number of ulcers and the cumulative length of ulcerated stomach surface in a roughly dose-dependent and equivalent manner. However, whereas the antiulcer doses of verapamil were extremely high, those of pr-MDI were one-sixth to one-half of its antiarrhythmic ED50 in rodents.

Inhibition of acid formation and stimulation of somatostatin release by cholecystokinin-related peptides in rabbit gastric glands

1. The purpose of the present study was to investigate the role of somatostatin in the inhibition of acid production induced by caerulein and cholecystokinin (CCK) in isolated rabbit gastric glands. Acid production was estimated by the aminopyrine technique. 2. Exogenous somatostatin 14 and somatostatin 28 (10(-7) M) reduced to a similar extent the aminopyrine uptake produced by 5 x 10(-5) M-histamine during the course of 40 min incubation. 3. Significant inhibition of histamine-stimulated aminopyrine accumulation occurred at a somatostatin 14 concentration of 10(-9) M. 4. Caerulein and CCK octapeptide (10(-13)-10(-7) M) were found to release somatostatin from isolated gastric glands in a dose-dependent manner. The dose-response relationships for somatostatin release and inhibition of aminopyrine uptake were similar. Thus, the half-maximal dose approximations for somatostatin release and inhibition of aminopyrine uptake were 0.5 and 1.4 x 10(-9) M respectively for CCK octapeptide and 0.9 and 2.5 x 10(-11) M for caerulein. Heptadecapeptide gastrin proved to be a very poor releaser of somatostatin in the system used. The CCK octapeptide-induced somatostatin release was time dependent and the concentrations of somatostatin that accumulated in the incubation medium were similar to those of exogenous somatostatin that were needed to evoke inhibition. 5. The present results support the concept that cholecystokinin inhibits gastric acid secretion by releasing somatostatin from endocrine-like cells in the gastric mucosa. It is suggested that cholecystokinin-related peptides may play a physiological role in inhibiting gastric acid secretion. A similar role for gastrin is not supported by the present study.

Characterization of in vivo acid secretory responses of rabbit with comparison to dog and rat

Even though rabbit gastric glands are a commonly used model for the study of gastric physiology, little is known about the secretion of gastric acid in the rabbit in vivo. Gastric acid secretion in response to pentagastrin, histamine, and bethanechol stimulation, and the effect of specific cholinergic and histamine H2-receptor blockers was studied in 62 urethane-anesthetized rabbits. Additionally, the responses of eight conscious rabbits prepared with chronic gastric fistula were compared with similarly prepared conscious dogs (n = 10) and conscious rats (n = 18). In anesthetized rabbits, maximal pentagastrin stimulation resulted in acid outputs only 22% that of maximal histamine stimulation, but this was enhanced by restoring cholinergic tone with a subthreshold infusion of bethanechol. In conscious rabbits, pentagastrin was an effective stimulant, resulting in maximal acid output 70% that of maximal histamine stimulation. This is in contrast to the in vitro findings reported reported by others utilizing isolated gastric glands and cells. Histamine was a potent stimulant of acid secretion in both the anesthetized and conscious rabbit, an observation that parallels the in vitro findings. Unlike the dog and rat, atropine was ineffective as an inhibitor of histamine-stimulated acid secretion in the conscious rabbit, although it was marginally effective against histamine in anesthetized rabbits and against pentagastrin in conscious rabbits. It is concluded that cholinergic tone plays a crucial role in pentagastrin-stimulated acid secretion in the rabbit. This may be an explanation for the poor response to pentagastrin described in isolated rabbit gastric gland preparations.

Histamine-mediated hyposmotic stimulation of gastric acid secretion

Gastric glands incubated in hyposmotic medium (200 mOsm) accumulated aminopyrine, a measure of acid secretion, to the same extent as that of paired glands in isomotic medium containing histamine (10(-4) M). These maximal responses to hyposmolality and histamine were not additive. The hyposmotic response peaked earlier than the histamine response. Hyposmotic stimulation was nearly abolished by preincubation of the glands with metiamide and cimetidine, H-2 histamine antagonists. In the presence of histaminase, no hyposmotic stimulation occurred. The response to forskolin, a stimulant of adenylate cyclase, was equivalent in hyposmotic and isosmotic media. These results indicate that hyposmolality releases histamine from a paracrine cell in the gastric gland and that histamine binds to H-2 receptors on the parietal cell to initiate a cyclic AMP-mediated stimulation of acid secretion.

The effect of somatostatin on baseline and stimulated acid secretion and vascular histamine release from the totally isolated vascularly perfused rat stomach

The effect of somatostatin-(1-14) (S1-14) on the gastrin- and histamine-induced acid secretion and gastrin-evoked vascular histamine release was studied in isolated vascularly perfused rat stomachs being continuously perfused by a gassed buffer containing 10% ovine erythrocytes and 50 microM isobutyl methylxanthine (IMX). Concentrations of gastrin (520 pM) and histamine, (0.5 microM) were chosen to give acid secretion in the same range (61.5 +/- 7.0 and 49.4 +/- 9.4 mumol/60 min). S1-14 induced a concentration-dependent decrease in acid secretion stimulated by both gastrin and histamine. Even at the lowest concentration examined (0.1 nM) somatostatin gave a significant inhibition of both gastrin- and histamine-stimulated acid secretion. The inhibitory effect was, however, most marked for gastrin-stimulated acid secretion (P less than 0.05 at 1 nM concentration of S1-14). Gastrin gave an immediate and marked vascular histamine release which was inhibited by somatostatin in the higher concentrations (1.0 and 5.0 nM). Somatostatin at the lowest concentration tested (0.1 nM) did not inhibit the gastrin-induced vascular histamine release although it did inhibit acid secretion. Furthermore, baseline histamine release was not affected by somatostatin. This study suggests that somatostatin inhibits acid secretion both via a direct effect of the parietal cell and by inhibiting gastrin-induced histamine release. Baseline histamine release is regulated by a mechanism not sensitive to somatostatin.