Gastrins in cat and dog: evidence for a biosynthetic relationship between the large molecular forms of gastrin and heptadecapeptide gastrin

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.

Gastroduodenal Ulceration in Small Animals: Part 1. Pathophysiology and Epidemiology

Gastroduodenal ulceration in small animals is a complex and important comorbidity that occurs when the physiological homeostasis of the gastrointestinal tract is disrupted secondary to administration of medications or the presence of local or systemic diseases. The aim of this article is to provide a comprehensive review of the veterinary literature regarding the pathophysiology, epidemiology, and risk factors associated with gastroduodenal ulceration in small animals. Pertinent concepts from the human literature will be integrated into the discussion. This article serves as an introduction to the second part of this series, which will review current evidence regarding the use of H2-receptor antagonists and proton pump inhibitors in small animals.

The metabolism of gastrin-52 and gastrin-6 in pigs

The kinetics and metabolism in various organs of three bioactive products of progastrin, the small sulfated and nonsulfated gastrin-6 and the large nonsulfated gastrin-52, were examined during intravenous administration in anesthetized pigs. The kidney, hindlimb, liver, head, and gut eliminated the hexapeptides efficiently, with a fractional extraction ranging from 0.50 to 0.28 (P<0.001-0.05). No metabolism was recorded in the lungs, and sulfation was without influence on the extraction of gastrin-6. Gastrin-52 was eliminated only in the kidney and the head, with a fractional extraction between 0.23 and 0.11 (P<0.01-0.05). The half-life of sulfated and nonsulfated gastrin-6 was 1.5+/-0.4 and 1.4+/-0.3 min, the metabolic clearance rate (MCR) was 80.8+/-7.6 and 116.0+/-13.5 ml x kg(-1) x min(-1) (P<0.05), and the apparent volume of distribution (V(dss)) was 199.3+/-70.1 and 231.4+/-37.3 ml/kg, respectively. The decay of gastrin-52 in plasma was biexponential. The half-lives of this biexponential after a bolus injection were 3.9+/-0.5 (T(1/2alpha)) and 25.7+/-1.4 (T(1/2beta)) min, and the MCR and V(dss) were 4.2+/-0.4 ml. kg(-1) x min(-1) and 116.2+/-16.2 ml/kg(1). We conclude that there is a differential elimination of progastrin products in splanchnic and nonsplanchnic tissue, which depends on the chain length of the peptides. Sulfation of gastrin-6 had no influence on the organ-specific extraction but reduced the MCR. Our results are in keeping with previous studies of nonsulfated gastrin-17, which is extracted in the kidney, head, limb, and gut but not in the liver.

Gastrin secretion by ovine antral mucosa in vitro

The effect on gastrin and somatostatin release in sheep of stimulatory and inhibitory peptides and pharmacological agents was investigated using an in vitro preparation of ovine antral mucosa. Carbachol stimulated gastrin release in a dose-dependent manner but had no effect on somatostatin release. As atropine blocked the effect of carbachol, cholinergic agonists appear to stimulate gastrin secretion directly through muscarinic receptors on the G-cell and not by inhibition of somatostatin secretion. Both vasoactive-intestinal peptide (VIP) and gastric-inhibitory peptide (GIP) increased somatostatin release but did not inhibit basal gastrin secretion, although VIP was effective in reducing the gastrin response to Gastrin-releasing peptide (GRP). Porcine and human GRP were stimulatory to gastrin secretion in high doses but bombesin was without effect. The relative insensitivity to GRP (not of ovine origin) previously reported from intact sheep may be caused either by a high basal release of somatostatin or by the ovine GRP receptor or peptide differing from those of other mammalian species.

The new biology of gastrointestinal hormones

The classic concept of gastrointestinal endocrinology is that of a few peptides released to the circulation from endocrine cells, which are interspersed among other mucosal cells in the upper gastrointestinal tract. Today more than 30 peptide hormone genes are known to be expressed throughout the digestive tract, which makes the gut the largest endocrine organ in the body. Moreover, development in cell and molecular biology now makes it feasible to describe a new biology for gastrointestinal hormones based on five characteristics. 1) The structural homology groups the hormones into families, each of which is assumed to originate from a common ancestral gene. 2) The individual hormone gene is often expressed in multiple bioactive peptides due to tandem genes encoding different hormonal peptides, alternative splicing of the primary transcript, or differentiated processing of the primary translation product. By these mechanisms, more than 100 different hormonally active peptides are produced in the gastrointestinal tract. 3) In addition, gut hormone genes are widely expressed, also outside the gut. Some are expressed only in neuroendocrine cells, whereas others are expressed in a multitude of different cells, including cancer cells. 4) The different cell types often express different products of the same gene, "cell-specific expression." 5) Finally, gastrointestinal hormone-producing cells release the peptides in different ways, so the same peptide may act as an acute blood-borne hormone, as a local growth factor, as a neurotransmitter, and as a fertility factor. The new biology suggests that gastrointestinal hormones should be conceived as intercellular messengers of general physiological impact rather than as local regulators of the upper digestive tract.

Identification of gastrin component I as gastrin-71. The largest possible bioactive progastrin product

Gastrin component I is the largest hormonally active form of gastrin. In order to determine its structure, we isolated progastrin-derived peptides from normal human antral tissue. A radioimmunoassay specific for sequence 20-25 of human progastrin was developed to monitor the purifications. After four or five steps of reverse-phase chromatography, the peptides were pure and could be identified by a combination of microsequence, amino acid and mass spectral analysis as well as by a library of sequence-specific immunoassays. In addition to intact progastrin 1-80, fragments 1-71, 1-35, 6-35, 20-35, and 20-36 of progastrin were identified. Only the 71-amino-acid peptide contained at its C-terminus the alpha-amidated bioactive site (Trp-Met-Asp-Phe-NH2). This unoheptacontapeptide amide (gastrin-71) corresponds to component I and is the largest possible bioactive product of progastrin. Its structure shows that progastrin is used in its entirety for biosynthesis of active peptides. The occurrence of fragments 6-35, 20-35, and 20-36 demonstrate that antral progastrin is partially cleaved at two monobasic sites (Arg5 and Arg19) in addition to processing at the three C-terminal dibasic sites. The results show that both the N- and C-terminal parts of antral progastrin undergo extensive processing. The results also suggest that progastrin may follow two different processing pathways of which the less trafficked releases gastrin-71.

The molecular nature of cholecystokinin in plasma. An in vivo immunosorption study in rabbits

The nature of cholecystokinin (CCK) in rabbit plasma was examined by means of a novel in vivo immunosorption procedure. Cholecystokinin (CCK) antibodies in 11 rabbit antisera were denatured, and the released peptides characterized by size and reversed-phase chromatography. Five of six antisera specific for the COOH terminus of CCK contained substantial amounts of CCK-22- and CCK-8-like peptides and small amounts of CCK-33-like peptides (range, 120 to 1140 nmol/l antiserum). In contrast, neither antisera for the NH2-terminus and mid-sequence of porcine CCK-33 nor antisera against the glycine-extended COOH terminus released CCK peptides. Postprandial acidified plasma from non-immunized rabbits concentrated in vitro also contained mainly CCK-22- and -8-like peptides, whereas extracts of rabbit duodenum and jejunum in addition contained forms resembling CCK-58, -39, and/or -33. The results show that mainly small molecular forms of CCK circulate in rabbits, and that NH2-terminal and mid-sequences of porcine and human CCK-33 differ from those of rabbit CCK-33. The results support the contention that plasma in most mammals contains small molecular forms of CCK.

Low tissue gastrin content in the ovine distal duodenum is associated with increased percentage of G34

1. In adult sheep and in lambs, over 95% of gastrin in the abomasal antrum was G17 with small amounts of G34 and lesser amounts of Component I. 2. Low gastrin concentration in the proximal duodenum was associated with a reduced percentage of G17. 3. The proportion of G34 increased progressively down the duodenum from a mean of 7% proximally to 47% in the most distal segment, and correlated negatively in any segment with the gastrin content. 4. In both the antrum and proximal duodenum, 60-70% of the G17 was in the sulphated form. 5. The gastroepiploic venous serum contained less G17 and more G34 than the tissues and up to 19% G14.

Effect of fixation conditions on the granule morphology of rat antral gastrin cells: an ultrastructural and immunocytochemical study

After fixation at pH = 7 or pH = 5 the gastrin (G) cells in the rat pyloric antrum were investigated by conventional and immunoelectron microscopy. After fixation at pH = 7 G cells contained numerous electron-lucent granules, a few electron-dense and intermediate granules while G cells had numerous electron-dense and intermediate granules, and a few electron-lucent granules after fixation at pH = 5. Preembedding immunoelectron microscopy revealed that in G cells after fixation at pH = 7, gastrin immunoreactivity was mainly seen in the cytoplasm, cores of a few electron-dense and intermediate granules and in the periphery but not within the numerous electron-lucent granules; in G cells after fixation at pH = 5, gastrin immunoreactivity was mainly visible in the cores of numerous intermediate and electron-dense granules, in the periphery of a few electron-lucent granules, but weakly in the cytoplasm. Therefore, the variable electron density of G cells may reflect differences in the degree of the leakage of contents from the gastrum-containing granules into the cytoplasm during fixation. This phenomenon may be related to the acidity of the granule substances during intragranular maturation.

Bovine and feline gastrin cDNA sequences and the amino acid and nucleotide sequence homologies among mammalian species

The complete nucleotide sequences of cDNAs encoding bovine and feline preprogastrins have been cloned from the antral mucosa mRNA. The gastrin mRNA of each animal encodes a preprogastrin of 104 amino acids consisting of a signal peptide, a prosegment of 37 amino acids, and a gastrin 34 sequence, followed by a glycine (the amide donor). The cleavage following a pair of lysine residues yields gastrin 17. We found that pairs of arginine residues flanking gastrin 34, the typical processing site sequence of all other preprogastrins and many peptide hormones, were arginines in the bovine preprogastrin, but the first basic amino acid pair had changed to Arg-Trp (57-58 residues) instead of Arg-Arg in the feline preprogastrin. Comparison of these amino acid and nucleotide sequences with published mammalian sequences showed extensive homology in the coding (63 to 73% amino acid identity) and in the untranslated regions (67 to 89% identity). Prosequence, the most variable region, shows greater amino acid difference between bovine and human preprogastrin (54% identity), and between bovine and rat preprogastrin (54% identity) than between other species (62 to 82% identity).

Biologically active peptides in the gastrointestinal lumen

The release of a variety of biologically active peptides into the gastrointestinal lumen via gastric, duodenal and intestinal secretions, as well as in the saliva, pancreatic juice and bile, has been explored. The key features of luminal secretion of peptides such as secretion at high concentrations, neurohormonal regulation, luminal orientation of stimulated secretion, stability of peptides in the gastrointestinal lumen, altered secretion under pathophysiological conditions, and biological activity of luminally administered peptides are discussed. This review develops a detailed picture of the current understanding of luminal secretion of peptides and their possible biological functions under normal and pathophysiological conditions.

Total small bowel resection inhibited bombesin-stimulated release of cholecystokinin and pancreatic polypeptide in anesthetized cats

In anesthetized cats, immunoreactive cholecystokinin (CCK), pancreatic polypeptide (PP), and gastrin were released in response to bombesin both before and after small bowel resection. Total small bowel resection significantly decreased bombesin-stimulated release of cholecystokinin and pancreatic polypeptide without affecting the release of gastrin. Integrated analysis showed that CCK, pancreatic polypeptide, and gastrin were released in significant quantities after small bowel resection. The results show that total small bowel resection caused significant inhibition of bombesin-stimulated release of cholecystokinin and pancreatic polypeptide; in contrast, gastrin release remained unaffected. The data further indicate that extra bowel sources of cholecystokinin exist in cats and the release of CCK from those sources occurred following bombesin stimulation.

Progastrin maturation during ontogenesis. Accumulation of glycine-extended gastrins in rat antrum at weaning

The post-translational maturation of antral progastrin was studied in the developing rat. While N-terminal proteolysis remained unchanged and tyrosine O-sulphation varied only slightly during ontogenesis, major changes were observed in the degree of alpha-carboxyamidation. In the third week of life the immediate precursor of amidated gastrin, glycine-extended gastrin, accumulated, and at weaning (day 21) the concentrations exceeded those of amidated gastrin. Our results confirm that weaning is accompanied by an increased synthesis of gastrin and imply that alpha-carboxyamidation is the rate-limiting step during the biosynthetic maturation of gastrin.

Cell-specific processing of pro-cholecystokinin and pro-gastrin

The present review argues that the gastrin-cholecystokinin family is a suitable model for the study of cell-specific processing of pro-hormones. First, the homologous active site of the hormones is a precisely defined tetrapeptide amide, which is well preserved during evolution. Second, the genes of both hormones are translated in a variety of cells (neurons, endocrine cells, paracrine cells, lymphocytes, etc,), but to a varying degree during ontogenesis and pathogenesis of various diseases. Third, each pro-hormone contains multiple processing sites (mono- and dibasic cleavage sites, amidation sites and consensus sequences for seryl phosphorylation and tyrosyl sulfation) leaving ample room for variations in the post-translational processing. The review discusses examples of cell-specific processing that appears to be functionally expedient.

Immunochemical and biochemical characterization of gastrin/cholecystokinin-like peptides in Palaemon serratus (Crustacea Decapoda): intermolt variations

Gastrin/cholecystokinin (G/CCK)-like peptides cross-reacting with an antiserum specific for the carboxyamide terminal pentapeptide of gastrin and CCK have been detected in the eyestalks and in the stomach of the prawn Palaemon serratus using immunocytochemical methods. In the eyestalks, immunoreactivity is present in the neuroendocrine cells, the X organ-sinus gland tractus and the neurohemal organ itself. This suggests, for the first time, the existence of a neuroendocrine secretion of G/CCK-like peptides. Hemolymph G/CCK level is about 18 pM. In the stomach, G/CCK-like material has been observed in epithelial cells in the cuticle and in the lumen. Molecular sieving of crude extracts of the medulla terminalis from the eyestalks, the stomach, and the hemolymph samples on a Sephadex G-50 filtration column exhibited a molecular heterogeneity of the G/CCK immunoreactive material. Large components were observed principally in the medulla terminalis and in the hemolymph, and smaller forms in the stomach. A fraction common for the three tissues had an apparent molecular weight of 2500 Da. That fraction was characterized further by HPLC and shown to be more hydrophobic than human G17 I. By radioimmunoassay relatively low levels were detected in all the aforementioned organs. Although the concentration of the G/CCK-like components varies during the intermolt cycle, this was the case mainly in the hemolymph and in the stomach. These observations suggest a possible role of G/CCK-like peptides in molting processes.

Gastric acid, plasma gastrin, and somatostatin responses to feeding and exogenous gastrin alone and in combination in conscious cats

Meat in the stomach or duodenum potentiates pentagastrin-induced acid secretion in cats, presumably by a humoral mechanism. In the present study on cats, a meat meal significantly augmented the maximal acid response from a Heidenhain pouch (HP) to pentagastrin or to human synthetic gastrin I by 31 and 30%, respectively. The maximal HP acid response to pentagastrin was augmented also by peptone instilled into the stomach through a gastric fistula. Intravenous infusion of amino acids stimulated acid secretion but did not augment the maximal acid response to pentagastrin. The plasma concentrations of gastrin and somatostatin increased during infusion of pentagastrin and gastrin I and were not further altered by simultaneous feeding. The present results indicate that the mechanism for potentiation of gastrin-induced acid secretion is of physiological significance, since feeding augmented also the acid response to heptadecapeptide gastrin, the only gastrin secreted from the antrum and duodenum in cats. The potentiation of acid secretion is not dependent on the vagal excitation induced by oral feeding, since potentiation was demonstrated also by intragastric peptone instillation. The mechanism for the potentiation is not due to absorbed amino acids or a decrease of plasma somatostatin.

Immunohistochemistry and morphometry of gastrin cells in the rat pyloric antrum during starvation

The gastrin cells (G cells) in the rat pyloric antrum after 7, 14, 21 and 28 days of starvation were investigated by immunohistochemistry and electron microscopy. In the peroxidase anti-peroxidase method for light microscopy, gastrin immunoreactive cells during starvation markedly decreased in number and size. Quantitative electron microscopy revealed that during starvation the number of electron-lucent granules were greatly reduced, but the number of electron-dense granules increased; the number of intermediate granules were not remarkably changed in G cells. These results may suggest that the synthesis of gastrin and granule maturation were greatly inhibited during long-term starvation.

Species variation in the tyrosine sulfation of mammalian gastrins

The degree of tyrosine sulfation and the distribution between gastrin-17- and gastrin-34-like immunoreactivity (LI) were studied in the antra of ten mammalian species. Specific radioimmunoassays, gel-, and ion-exchange chromatography as well as enzymatic cleavage with trypsin and arylsulfatase were used. The percentage of sulfation varied from 24.4 +/- 4.2 (mean +/- SEM) in dogs to 80.1 +/- 2.6 in sheep, 46.8 +/- 3.3 in humans, 50.1 +/- 3.2 in cows, 55.9 +/- 2.3 in rats, 57.4 +/- 3.1 in pigs, 61.3 +/- 2.2 in guinea pigs, 64.1 +/- 4.7 in cats, 64.8 +/- 2.1 in mice and 68.2 +/- 2.8 in rabbits. Gastrin-34-LI in antral extracts could be converted to gastrin-17-LI by trypsin in all species. Five percent of antral gastrins eluted as gastrin-34-LI in all species. We conclude that while the ratio of gastrin-34-LI to gastrin-17-LI varies little in mammals, large differences occur in the degree of sulfation.

Abnormal processing of antral gastrin in active duodenal ulcer disease

The concentrations of gastrins containing the active C-terminal tetrapeptide amide (mainly gastrin-34 and gastrin-17) and the N-terminal tridecapeptide fragment of gastrin-17 were measured in antral and duodenal biopsy specimens. The antral concentration of the N-terminal gastrin fragment was much higher in patients with active duodenal ulcer (33.4 +/- 6.8 nmol g-1, mean +/- SEM, n = 15) than in controls (5.6 +/- 2.9 nmol g-1, n = 10), patients with gastric ulcer (5.6 +/- 1.8 nmol g-1, n = 10) or patients with pernicious anaemia (7.7 +/- 2.5 nmol g-1, n = 6). No differences were found between the groups regarding gastrin-34 and gastrin-17 concentrations. In duodenal extracts, the N- and C-terminal gastrin concentrations were similar in all groups of patients. These data suggest that the posttranslational processing of antral gastrin is abnormal in patients with active duodenal ulcer disease.

The molecular nature of cholecystokinin in the feline pancreas and related nervous structures

Nerve terminals in pancreatic islets and ganglia containing cholecystokinin (CCK)/gastrin-like peptides are particularly abundant in the cat. In order to elucidate the possible origin and molecular nature of the peptides in these nerves, extracts of the feline pancreas, vagus, sympathetic trunk, and celiac-superior mesenteric ganglion were examined by gel chromatography monitored by sequence-specific radioimmunoassays. Small amounts of CCK-33 and CCK-8 were present in the pancreatic terminals. In the vagus and the sympathetic trunk, CCK, mainly as CCK-8, occurred in concentrations of 3.5 and 3.7 pmol/g. The celiac-superior mesenteric ganglion contained 40 pmol CCK/g distributed in five forms, including a predominant CCK-8-like component and a component eluting like CCK-4. Gastrins were not detected in the nervous structures. The results suggest that the celiac-superior mesenteric ganglia, the vagal nerves and the sympathetic trunks all may contribute to the CCK nerve terminals in the feline pancreas.

Gastrin and cholecystokinin in pituitary neurons

Gastrins occur in the hypothalamo-hypophyseal neurons of all mammalian species examined. In addition, human, bovine, and murine hypothalamo-hypophyseal neurons contain the homologous cholecystokinins (CCKs). CCK also occurs in neurons innervating bovine melanotrophs. Although the concentration of gastrin is of the same magnitude (15-30 pmol/g) in all neural lobes, the concentration of CCK varies from undetectable in pig and cat to 1 nmol/g in the cow. The constant occurrence of neurohypophyseal gastrin suggests a role different from that of the species-dependent CCK.

Structure of a human gastrin gene

A gastrin gene was isolated from a genomic library of human DNA. The human gastrin gene is about 4100 base pairs long and contains two intervening sequences. Thus, a 3500-base-pair intervening sequence is located 5 base pairs proximal to the ATG initiator codon, while a 129-base-pair intervening sequence separates the region coding for the principal hormonal form of gastrin, the heptadecapeptide, from the region coding for the major amino-terminal portion of the gastrin precursor. The 5' flanking region of the gene contains the conserved sequences, T-A-T-A-A and G-A-C-T-C-A-T-A-T, in positions similar to those of other eukaryotic genes.

Abnormally processed gastrins in active duodenal ulcer disease

In contrast to healthy subjects, duodenal ulcer patients in the active phase contain large amounts of a peptide in serum and antrum which react with antiserum specific for the N-terminus, but not the C-terminus of gastrin-17. The immunochemical and chromatographic properties were similar to that of the N-terminal tridecapeptide sequence of gastrin-17. The peptide follows the clinical course of duodenal ulcer disease, as it disappears when the ulcer heals. The N-terminal tridecapeptide - lacking the bioactive tetrapeptide of gastrin-17 - is a potent inhibitor of gastric acid secretion, presumably by way of competitive antagonism to gastrin. It is suggested to participate in the regulation of gastric acid secretion in patients with active duodenal ulcer disease. To confirm the chemical structure of the peptide, antral and gastrinoma extracts were used for isolation, purification and amino acid analysis. We found two different peptides with the same N-terminus as gastrin-17, namely the previously known N-terminal tridecapeptide fragment of gastrin-17 and a new gastrin component, identical with a C-terminal glycine extended gastrin-17. Furthermore, a C-terminal glycine extended component, corresponding to each of the other molecular forms of gastrin were present. Thus, a variety of abnormally processed gastrins are synthesized and released to the circulation during the active period of duodenal ulcer disease.

Intracellular topography of immunoreactive gastrin demonstrated using electron immunocytochemistry

Gastrin (G)-producing cells from the mammalian gastric antrum have been investigated using computer-assisted morphometry and a novel double colloidal gold-labeled-immunoglobulin electron immunocytochemical procedure. Correlation analysis of human antral G-cells indicates (p less than 0.001) that a single population of granules exists with small (160 nm) electron-dense and large (240 nm) electron-lucent forms representing the extremes. Non-crossreacting region-specific antisera have been used to visualize G-17 and G-34 (progastrin) to the small electron-dense granules and G-17 to the other intermediate forms. From the results we propose a topographic segregation of immunoreactive gastrins within 2 apparently distinct granule subclasses and suggest that this may represent the pathway of granule maturation.

Molecular cloning of human gastrin cDNA: evidence for evolution of gastrin by gene duplication

An oligo(dT)-primed cDNA copy of the mRNA coding for the human gastrin precursor was constructed from poly(A)-containing RNA from a human pancreatic, gastrin-producing tumor (a gastrinoma). The cDNA was inserted into the Pst I endonuclease site of plasmid pBR322 by the use of the poly(dC) and poly(dG) tailing procedure. Clones containing gastrin sequences were selected by hybridization to a purified single-stranded 32P-labeled gastrin cDNA probe. This probe was constructed with gastrinoma mRNA as template. As primer for the cDNA synthesis, we used a synthetic oligonucleotide mixture, d(AG-A-A-AG-T-C-C-A-T-C-C-A), corresponding to the gastrin-specific amino acid sequence Trp-Met-Asp-Phe. In this way we determined the nucleotide sequence of the entire coding region (303 nucleotides), the entire 3' untranslated region (102 nucleotides), and 8 nucleotides of the 5' untranslated region. A striking homology between parts of the coding region suggests that evolution of the gastrin gene has involved a gene duplication.

Molecular forms of gastrin in canine duodenum after antrectomy

Extracts of the proximal third of the duodenum from 8 antrectomized dogs with the gastrointestinal continuity restored by gastroduodenostomy (n = 4) or gastrojejunostomy (n = 4) as well as from 4 unoperated controls were subjected to gel chromatography. The eluates were all assayed using two different gastrin antisera, one directed against the COOH-terminal end of gastrin and the other directed against the NH2-terminal end of gastrin-17. The gastrin component pattern was very similar in all antrectomized dogs regardless if they had a gastroduodenostomy or a gastrojejunostomy. Gastrin-17 was found to dominate while the amount of gastrin-34 was at most one tenth of that of gastrin-17. Using the COOH-terminal directed antiserum approximately 15% (mean value) of the total gastrin-like immunoreactivity eluted in a peak appearing in the same region as the COOH-terminal octapeptide of cholecystokinin. In unoperated control dogs the corresponding peak constituted approximately 70% (mean value) of the total gastrin-like immunoreactivity. In two of the control dogs small amounts of gastrin-like immunoreactivity appeared at the elution volumes of gastrin-34 and gastrin-17. In the duodenal extracts of all dogs gastrin-like immunoreactivity was found between the elution sites of gastrin-34 and gastrin-17. This material probably represents cholecystokinin-33. The present results show that the increase in duodenal gastrin found after antrectomy, which we have reported previously, is due mainly to an increase in gastrin-17.

The effect of antrectomy on the number of gastrin-immunoreactive cells in the canine duodenum

The number of gastrin-immunoreactive cells in the duodenum was assessed by immunohistochemistry in 10 dogs that had been subjected to antrectomy with gastroduodenostomy (Billroth I), in 4 dogs in which an antrectomy with gastrojejunostomy (Billroth II) had been performed and in 4 unoperated controls. Gastrin-immunoreactive cells were found only in dogs that had been subjected to antrectomy ad modum Billroth I and then only within the first 30 mm of the duodenum, i.e. in the duodenal bulb. The gastrin cells occurred scattered on the villi, in the crypts and within the glands of Brunner. In 3 of the dogs patches of antral-type mucosa occurred within the first 10 mm of the duodenum. All dogs in which gastrin-immunoreactive cells were found have in a previous study been shown to have a markedly increased tissue concentration of gastrin in the proximal third of the duodenum compared to unoperated controls. In the dogs subjected to antrectomy ad modum Billroth II in which no cells were observed the level of gastrin in duodenal tissue has been found to be moderately elevated compared to that of control dogs. The results indicate that the increased gastrin concentration in the proximal third of the duodenum following antrectomy ad modum Billroth I corresponds to an increase in the number of gastrin-immunoreactive cells in the duodenal bulb.

The life cycle of the gastrin granule

The ultrastructure of gastrin cells in the rat antrum was analyzed with standardized and quantitative planimetric methods. Resting and active cells were compared. The gastrin cells were activated by removal of the acid-producing part of the stomach (fundectomy). As a result of the serum gastrin concentrations were greatly elevated. Compared with gastrin cells in fasted control rats the gastrin cells in fundectomized rats were increased in number, contained fewer cytoplasmic granules, increased amount of endoplasmic reticulum, and an enlarged Golgi area. Generally, the secretory granules of the gastrin cell displayed a wide range of electron density from highly electron-dense to electron-lucent. They exhibited certain characteristic features: 1) Electron-dense granules made up a greater proportion of the total granule population in active gastrin cells than in resting cells. 2) Electron-dense granules were more frequent near the Golgi stacks than in the periphery of the cell. 3) Electron-dense granules were smaller in size than the electron-lucent granules; hence, small electron-dense granules probably represent young granules (progranules), while large, electron-lucent granules represent mature (old) granules. 4) Electron-dense granules invariably displayed a more intense immunoreactivity than electron-lucent granules. The gastrins are generated from a large precursor molecule. The post-translational processing of this precursor is reflected in the gastrin-component pattern. The gastrin-component pattern in antral extracts of fundectomized and normal fasting rats differed in that the proportion of the gastrin-4-like component was reduced, whereas the gastrin-34-like component was increased in the fundectomized rats. The results suggest a greater proportion of small gastrin components in the mature granules than in the newly formed ones, presumably due to more extensive conversion of larger forms into smaller forms with a longer granule half-life. As a result gastrin-17- and gastrin-34-like components make up a larger proportion of total gastrin in active gastrin cells than in resting gastrin cells.

Pituitary gastrins occur in corticotrophs and melanotrophs

The gut hormone gastrin was identified in pituitary cells containing adrenocorticotropic hormone and alpha-melanocyte--stimulating hormone by region-specific immunocytochemistry and radioimmunoassay. Smaller amounts of gastrin were found in nerve fibers of the neural lobe and pituitary stalk. Since adrenocorticotropic hormone--like peptides occur in antropyloric gastrin cells, these data indicate a considerable similarity in peptide composition of pituitary and gastrointestinal endocrine cells and reinforces questions of multiple hormone production.

Effect of intraluminal pH on the release of somatostatin and gastrin into antral, bulbar and ileal pouches of conscious dogs

Experiments were performed on conscious dogs with chronic pouches of the antrum, the duodenal bulb or the ileum, which were perfused with solutions of varying pH. Gastrin and somatostatin levels were measured in the perfusates. When the pouches were perfused with 0.15 M NaCl only small amounts of gastrin and somatostatin (1 pmol/min) were released into the lumen of the antrum and of the duodenal bulb. By lowering pH of the perfusion fluid a pH dependent release of somatostatin was induced into the lumen of the antrum and the duodenal bulb. Perfusion with 0.1 M HCl caused a large output of somatostatin (6--60 pmol/min) into the pouches. The upper pH limit for stimulation of the intraantral or intrabulbar somatostatin release appeared to be approximately pH 3--4. Somatostatin was also released into ileal perfusates at intraluminal pHs below 3--4. Lowering of pH in the antral pouches caused an increased intraluminal gastrin release, which was quantitatively less impressive than that of somatostatin. Occasionally also the gastrin release into the duodenal bulb increased during perfusion with 0.1 M HCl, whereas no such release was induced by acidification of the lumen of the ileum. It is suggested that the inhibition of gastrin release observed at low intraantral pH is mediated by a local effect of somatostatin, since this peptide is released in a pH dependent manner in the antropyloric region. It is also suggested that acidification of any region of the gastrointestinal tract will stimulate the release of peptides from all endocrine cells of the open type, probably by an unspecific effect on the membrane. Thus both gastrin and somatostatin are released by acidification of the antrum, but in the presence of high local levels of somatostatin, the release of gastrin is substantially inhibited.

Release of gastrin and somatostatin into the gastric lumen of healthy subjects and patients with duodenal ulcer and achlorhydria

Gastrin and somatostatin were measured in alkaline gastric instillates in normal subjects, patients with duodenal ulcer disease in quiescent state and in patients with achlorhydria. Both peptides were released into the lumen. The gastrin-somatostatin ratio (G/S) in healthy subjects was approximately three. Duodenal ulcer patients had significantly lower G/S ratio due to lower gastrin and higher somatostatin levels whereas in patients with achlorhydria, the G/S ratio did not differ from normal subjects.