Irvin M. Modlin

Pancreastatin: a novel peptide inhibitor of parietal cell signal transduction

The direct inhibition of secretion by pancreastatin was investigated in rabbit isolated parietal cells. Pancreastatin exerted no influence on basal aminopyrine uptake. Pancreastatin inhibited histamine stimulated aminopyrine uptake through a decrease in intracellular cAMP. Pancreastatin inhibition of histamine stimulated uptake was blocked in the presence of pertussis toxin. Pancreastatin also inhibited the carbachol stimulated increase in aminopyrine accumulation. However, the effects of pancreastatin on carbachol stimulation were not reversed by pertussis toxin. Pancreastatin did not alter the carbachol induced increase in cytosolic free calcium. Thus, pancreastatin appears to inhibit parietal cell signal transduction at multiple points along the second messenger pathways.

Stimulation of canine pancreatic polypeptide, gastrin, and gastric acid secretion by ranatensin, litorin, bombesin nonapeptide and substance p

Four dogs with chronic gastric fistulas were give intravenous bombesin nonapeptide (B9), ranatensin, and litorin by constant infusion for 90 min at 1.2 micrograms x kg-1 on separate days. A dose response study with substance P (1.5, 3.0, 60, 18 and 54 micrograms x kg-1 x h-1) was also carried out and all tests compared to a standard protein meal (10g x kg-1). Plasma gastrin and PP were measured by radioimmunoassay and gastric acid by autobiuret titration. Substance P failed to stimulate gastric acid secretion or release either pancreatic polypeptide (PP) or gastrin. Basal gastrin levels were 8 +/-2 fmol/ml. The peak increment of gastrin released by bombesin was 95 +/- 16, ranatensin 22 +/- 6, litorin 18 +/- 4, and meal 39 +/- 5 fmol/ml. Bombesin caused significantly greater release of gastrin than a meal, litorin or ranatensin (P less than 0.01). Basal gastric secretion was 23 +/- 4 microequiv./min. B9 produced a peak acid secretion of 356 +/- 124 muequiv./min. There was no significant difference between the bombesin-like peptides (P less than 0.01). Basal plasma PP was 38 +/- 12 fmol/ml. B9 produced a peak PP increment of 600 +/- 50, litorin 137 +/- 36, ranatensin 98 +/- 11, and a meal 305 +/- 58 fmol/ml. B9 released significantly more PP than either litorin of ranatensin (P less than 0.01). The different amino acid sequences of the peptides are probably responsible for their potency. The substitution of a penultimate phenylalanine residue in litorin and ranatensin for leucine in bombesin does not prevent PP or gastrin release by bombesin-like peptides. Since bombesin-like peptides are widely distributed in the gastrointestinal tract of man and stimulate both acid and gut hormone secretion, it is possible that they might play a physiological role in the modulation of gastrointestinal function.

The mastomys gastric carcinoid : aspects of enterochromaffin-like cell function

The mastomys rodent exhibits a genetic propensity to develop gastric carcinoid tumors. Utilizing acid inhibitory pharmacotherapy (histamine-2 receptor antagonists and proton pump inhibitors), we have demonstrated transformation from normal to neoplastic enterochromaffin-like (ECL) cells in a well-defined fashion over a period of 4 months. In addition, we have demonstrated inhibition of tumor growth with either somatostatin or histamine-1 receptor antagonists (terfenadine and cyproheptadine). In order to define the regulation of growth and secretion of transformed ECL cells, we developed an isolated pure ECL cell system. ECL cells secrete histamine in response to gastrinergic (gastrin), muscarinic (carbachol), and beta-adrenergic (isoproterenol) stimulation. Both cAMP and intracellular calcium-dependent mechanisms are involved in the process of histamine secretion.

Dissociation of pepsinogen and acid secretion in the guinea pig

In vivo observations have suggested that acid secretion may potentiate pepsinogen release. We measured pepsinogen and acid secretion by guinea pig fundic mucosal sheets stimulated by 10(-4) M histamine, 10(-8) and 10(-9) M cholecystokinin, and 3 x 10(-7) M carbamylcholine and then investigated the effects of 10(-4) M omeprazole on basal, carbachol-stimulated, and cholecystokinin-stimulated secretion. Histamine increased basal acid secretion fivefold (p less than 0.01) without altering pepsinogen secretion. Cholecystokinin did not stimulate acid secretion but increased pepsinogen secretion by factors of 23.1 at 10(-8) M and 9.1 at 10(-9) M (both p less than 0.01). The combination of 10(-4) M histamine and 10(-9) M cholecystokinin increased acid secretion 3.5-fold and pepsinogen secretion 6.4-fold, statistically equivalent to the sum of the effects of histamine and cholecystokinin alone. Carbachol increased acid secretion and pepsinogen secretion by factors of 4.0 and 10.9, respectively (both p less than 0.01). Pretreatment with 10(-4) M omeprazole abolished basal and carbachol-stimulated acid secretion. However, pepsinogen secretion was unaffected (p greater than 0.05). Furthermore, omeprazole-treated tissues increased pepsinogen secretion by factors of 10.0 with 3 x 10(-7) M carbachol and 9.1 with 10(-9) M cholecystokinin (both p less than 0.01). Thus, basal and secretagogue-stimulated pepsinogen secretion appear independent of acid secretion in intact guinea pig mucosa.

A specific histamine-stimulated phosphoprotein in isolated parietal cells

Histamine-stimulated phosphorylation was studied in isolated rabbit parietal cells. Secretion of acid, as assessed by aminopyrine uptake, was linear at 15 min of stimulation with histamine. By utilizing two dimensional gels, a specific 30,000-Da protein (pp30) was identified whose phosphorylation was prominently stimulated by histamine after 15 min of incubation. The pp30 protein displayed an isoelectric point of 6.0. Furthermore, cAMP-dependent pp30 phosphorylation could also be demonstrated in vitro in a preparation of parietal cell cytosol. The results suggest that pp30 may represent an important histamine-stimulated cAMP-dependent phosphoprotein involved in the initiation or maintenance of parietal cell secretion.

Genotoxicity, carcinogenicity and acid-suppressing medications

With the availability of increasingly potent acid-suppressing medications, questions continue to rise concerning the safety of these compounds in regards to carcinogenetic potential. In this review, we examine current concepts and procedures relating to genotoxicity, the potential for a chemical agent to interact with and alter the genomic information of the cell, and carcinogenesis. A description and discussion of commonly utilized techniques for the determination of (a) in vitro mutagenicity, (b) in vitro and in vivo DNA damage and repair, (c) in vitro and in vivo chromosomal damage and (d) chronically dosed animal tumorigenesis development is presented. Observations from these procedures as they have been applied to a review of the safety of acid-suppressing medications will be discussed. An evaluation of reports relating to potential genotoxic and carcinogenic hazards of therapeutically relevant acid-suppressing medications (cimetidine, ranitidine, omeprazole) is presented. Information related to the effect of prolonged administration of acid-suppressing medications, alterations of serum gastrin levels, and the potential for tumor promotion is discussed.

Implications of sustained suppression of gastric acid secretion

The implications of profound and sustained suppression of acid secretion are of increasing concern. Short-term inhibition of acid secretion by H2-receptor blockade or proton pump inhibition alters the gastric luminal flora and increases the risk of nosocomial pneumonia in critically ill patients who are receiving prophylaxis for stress gastritis. Long-term suppression alters gut flora, carcinogen levels in the gastric lumen, and the hormonal milieu, leading to proliferative changes in the fundic mucosa. Previous reports have noted a significant incidence of gastric malignancies in the achlorhydric environment of atrophic gastritis and pernicious anemia. Concern has also been expressed regarding the possibility of gastric neoplasia that arises after vagotomy and distal gastrectomy. The exact risk of gastric epithelial and endocrine hyperplasia or neoplasia in patients receiving potent antisecretory agents is not yet known, but such risks cannot be dismissed until long-term follow-up studies are available. The relationship between sustained suppression of acid secretion and the proliferation of epithelial and endocrine elements may provide insight into processes that regulate replication and growth of cells in the gastric mucosa.

A novel protein kinase activity in rabbit gastric gland cytosol.

A novel protein kinase activity was characterized from the cytosolic fraction of isolated rabbit gastric glands. The kinase phosphorylated a major 33,000 Da endogenous protein (pp33) and was stimulated by Zn2+ and Mn2+ with Kact of 1.0 and 7.5 mM, respectively. Mg2+ and Ca2+ failed to stimulate any pp33 kinase activity. The kinase utilized both ATP and GTP as phosphate donors with a Km of 10 microM for both. The pp33 protein displayed an isoelectric point of 7.5 to 7.8 and was phosphorylated predominantly on threonine residues. The kinase activity is clearly differentiable from all reported kinase activities and appeared to be enriched in rabbit gastric fundic mucosa. The results indicate that gastric fundic mucosa contains a novel protein kinase activity.

A calmodulin dependent protein kinase in parietal cells

An enriched population of isolated rabbit gastric parietal cells, from the fundic mucosa of New Zealand White rabbit, contained an active cytosolic calmodulin-dependent protein kinase activity with a prominent 100 kDa substrate (pp100). The latter focused as a doublet with isoelectric point of 6.8-7.0. The pp100 protein was phosphorylated only on threonine residues on a single tryptic peptide. Trifluoperazine inhibited the pp100 kinase activity with a KI of 10-15 microM. Addition of exogenous calmodulin was able to restore activity to uninhibited levels. A protein band with a molecular weight and phosphopeptide map identical to pp100, phosphorylated by calcium-dependent kinase, was also observed in rabbit pancreatic cytosol. The data suggest that a type III calmodulin-dependent kinase is present in parietal cell cytosol.