Regulation of gastric acid secretion by neurotensin in man. Evidence against a hormonal role.

The present study was designed to evaluate neurotensin as a hormonal regulator of gastric acid secretion in man. After a fat-rich meal, the strongest known stimulus of neurotensin release, plasma neurotensin-like immunoreactivity (NTLI) was elevated from 7.6 +/- 1.9 to 15.5 +/- 2.5 pM. Plasma NTLI was measured with antiserum L170, which requires the biologically active carboxyl-terminal hexapeptide of the neurotensin molecule for recognition and does not crossreact significantly with any known natural catabolite in human plasma. Intravenous infusion of neurotensin at 25 pmol X kg-1 h-1 resulted in a plasma level of 14.7 +/- 2.1 pM, similar to the maximal physiological level observed after the fat-rich meal. Intravenous infusion of neurotensin at 25 pmol X kg-1 h-1 during 2 h, however, failed to significantly inhibit peptone meal-stimulated gastric acid secretion measured by intragastric titration. The 2-h acid output to peptone was 40.8 +/- 6.2 and 41.3 +/- 6.9 mmol during the vehicle and the neurotensin infusion, respectively. Intravenous infusion of neurotensin at 100 or 400 pmol X kg-1 h-1 did not affect acid output, whereas at 1,600 pmol X kg-1 h-1, which resulted in a plasma neurotensin concentration of 725 +/- 80 pM, significantly reduced peptone meal-stimulated gastric acid secretion. The neurotensin-induced inhibition of acid output was independent of the hormone gastrin. The present results provide evidence against a hormonal role for neurotensin in the regulation of meal-stimulated gastric acid secretion in man.

Neuromedin-N inhibits migrating myoelectric complex and induces irregular spiking in the small intestine of rats; comparison with neurotensin.

The effects of neuromedin-N on migrating myoelectric complexes in the small intestine of rats were studied. As neuromedin-N and neurotensin are structurally related peptides a comparison with neurotensin was made. Myoelectric activity was recorded by means of three bipolar electrodes implanted into the wall of the small intestine at 5, 15 and 25 cm distal to the pylorus. The peptides were administered as intravenous infusions to fasted conscious rats. Neuromedin-N at doses of 100-800 pmol kg-1 min-1 caused a dose-dependent disruption of the migrating myoelectric complexes and induced irregular spiking activity (n = 7, P less than 0.05). Neurotensin induced a similar response, but at doses of 1.0-8.0 pmol kg-1 min-1 (n = 5, P less than 0.05). Thus, on a molar basis, neuromedin-N appeared to be about 100-times less potent than neurotensin. Hexamethonium (20 mg kg-1 i.v.) inhibited the migrating motor complexes and induced quiescence, but did not block the effect of neuromedin-N at a dose of 800 pmol kg-1 min-1. Atropine (1 mg kg-1 i.v.) and mepyramine (2 mg kg-1 i.v.) did not affect the migrating motor complexes, nor did they block the effect of neuromedin-N. Simultaneous infusion of neuromedin-N and neurotensin in a 1:1 molar ratio at doses of 2 pmol kg-1 min-1 showed inhibition of the response to neurotensin in eight out of ten experiments. In conclusion, neuromedin-N changes the myoelectric activity in the small intestine from a fasting to a fed pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandins may not mediate inhibition of gastric acid secretion by somatostatin in the rat.

The role of prostaglandins as mediators of the inhibitory effect of somatostatin on gastric acid secretion has been evaluated in conscious and anesthetized rats. The effect of somatostatin on bethanechol-stimulated gastric acid secretion was determined with or without indomethacin pretreatment. Prostaglandin synthesis inhibition (less than 90%) by indomethacin was verified with PGE2-generation assay on gastric mucosal tissue. In both conscious and anesthetized rats somatostatin significantly inhibited the stimulated acid output in the control and indomethacin pretreated groups. The present findings do not support a role for prostaglandins in the inhibition of gastric acid secretion by somatostatin in the rat.

Carbamylcholine, but not somatostatin or neurotensin, stimulates prostaglandin E2 release from the isolated perfused rat stomach.

Prostaglandin E2 release by carbamylcholine (10(-6) M), somatostatin (10(-10)-10(-8) M) and neurotensin (10(-10) - 10(-8) M) has been evaluated in the isolated perfused rat stomach. Carbamylcholine significantly stimulated gastric PGE2 release and increased the perfusion pressure, whereas somatostatin and neurotensin had no effect. Combination of carbamylcholine with somatostatin or neurotensin produced no increase over that found with carbamylcholine alone. The relationship between perfusion-pressure and PGE2 release was not causal. The present findings do not support a role for prostaglandins in the mechanism of somatostatin or neurotensin action in the stomach.

Isolation and characterization of a neurotensin-like decapeptide from a canine upper small intestinal extract.

Neurotensin-like immunoreactivity can be detected in extracts of canine upper gastrointestinal mucosa when measured by carboxyl terminal but not by amino terminal antibodies to neurotensin. The nature of this immunoreactive material was characterized by complete purification on gel filtration and HPLC followed by peptide microsequence analysis. The structure obtained was Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-(Leu), identical in structure to the carboxyl terminal decapeptide of neurotensin. It cannot, however, be excluded that this neurotensin decapeptide was generated from a larger neurotensin-like peptide during the extraction procedure by a physiological or artificial enzymatic process. Since carboxyl terminal neurotensin fragments containing eight or more residues have full biological activity, this peptide may be responsible for neurotensin-like biological activities within the mucosa of, or after release from, the upper gut.

Neurotensin-like immunoreactivity generated by pepsin from human plasma and gastric tissue.

The present study demonstrates precursors of neurotensin-like immunoreactivity (NTLI) endogenous to human gastric tissue and plasma, and the existence of a gastric NTLI-generating enzyme system. The molecular size of the NTLI-precursors in plasma and gastric tissue were estimated by gel permeation chromatography to be ca 50,000-60,000 and 60,000-70,000 Da, respectively. The neurotensin-like peptide generated from the precursor was detected with a carboxyl-terminally directed antiserum but did not cross-react with an amino-terminally directed antiserum. A neurotensin-like peptide isolated from pepsin-treated human plasma was characterized by mass spectrometry and its amino acid sequence determined. This novel nonapeptide, referred to as kinetensin, failed to affect pentagastrin-stimulated acid secretion or blood pressure in the rat. Sequence homologies between neurotensin, kinetensin and proteins of the serum albumin family suggest a common evolutionary origin and raise questions regarding albumin-like proteins as precursors of regulatory peptides.

Stimulation of histamine release by the peptide kinetensin.

The peptide kinetensin isolated from pepsin-treated human plasma induced a dose-dependent release of histamine when exposed to rat peritoneal mast cells. The threshold concentration was around 10(-6) M, the ED50 was 10(-5) M, and the optimal concentration of between 10(-4) to 10(-3) M released 80% of the total histamine. Kinetensin was 10 to 100 times less potent than neurotensin and equipotent with the opioid peptide dynorphin. The histamine release was clearly temperature-dependent, with no release occurring at 0 degrees or 45 degrees C and with an optimum around 37 degrees C. The histamine release was significantly reduced in the absence of extracellular calcium. Kinetensin also induced a dose-dependent increase in vascular permeability when injected intradermally into rats. The findings indicate that kinetensin is a potent histamine releaser in the rat and may serve as an inflammatory mediator.

Fat inhibits meal-stimulated gastric acid secretion after prostaglandin synthesis inhibition by indomethacin in man.

The effect of cyclooxygenase inhibition by indomethacin on the mechanism by which fat inhibits gastric acid secretion was studied in eight healthy men. Oral 200-mg doses of indomethacin administered 15 h and 2 h before testing is known to inhibit prostaglandin synthesis by 90%, as determined by prostaglandin E2 generation assay on endoscopically obtained gastric mucosal biopsy specimens. Fat-induced inhibition of gastric acid secretion was evaluated by intragastric administration of 200 ml vegetable oil or glucose (control) for 30 min, followed by intragastric titration with peptone at pH 5.5 for 2 h. The mean acid output was significantly lower after fat (8 +/- 0.2 mmol 30 min-1) as compared with control (12 +/- 0.5 mmol 30 min-1). The fat-induced inhibition was unaffected by indomethacin treatment, and the plasma gastrin responses were similar in all groups. It is concluded that the participation of cyclooxygenase products such as prostaglandins in the mechanisms by which fat inhibits the gastric phase of acid secretion in man is likely to be minor.

Histamine secretion induced by neuromedin-N.

Neuromedin-N dose-dependently stimulated the release of histamine from rat serosal mast cells and was 10 to 100 times less potent than neurotensin. The threshold concentration was 10(-6) M, and 10(-3) M neuromedin-N released 31% of the total cell histamine content. The histamine release induced by neuromedin-N was temperature-dependent with an optimum around 30-37 degrees C. Skin vascular permeability increased dose-dependently in response to intradermal injections of neuromedin-N and this peptide was 10 to 100 times less potent than neurotensin. Mepyramine inhibited the effect on vascular permeability suggesting that the effect of neuromedin-N was mediated via the release of histamine.

Gastric emptying and intestinal transit of hyperosmolar solutions in relation to indomethacin and certain gut polypeptides in the rat.

The present study in the rat demonstrates an inhibitory mechanism of gastric emptying, sensitive to the osmolality of a liquid meal. Gastric emptying and intestinal transit were studied in groups that differed with regard to the osmolality of the gastric or duodenal instillation, experimental time, and indomethacin treatment. By intragastric instillation animals were fed an aqueous solution containing the nonabsorbable marker 51CrO2-4. After certain fixed time intervals the rats were killed and the stomach and small intestine were dissected out en bloc. The distribution of the isotope along the gastrointestinal canal was then determined. Intestinal transit was evaluated in a similar manner. The marker solution was introduced into the duodenum. A hyperosmolar solution, 1200 mOsm kg-1, was emptied from the stomach significantly more slowly than was an iso-osmolar solution, 300 mOsm kg-1. Of the iso- and hyper-osmolar solutions 87% and 74%, respectively, were emptied in 1 h. The isotope distribution along the intestinal canal after intraduodenal instillation was not affected by the osmolality of the installation. The osmotic inhibition of gastric emptying was not affected by indomethacin treatment (4 mg kg-1) or related to elevated plasma levels of gastrin, neurotensin, somatostatin, or gastric inhibitory polypeptide.

Gastric acidification inhibits meal-stimulated gastric acid secretion after prostaglandin synthesis inhibition by indomethacin in humans.

The effects of cyclooxygenase inhibition by indomethacin on gastric acid secretion were studied in 8 healthy men. Oral doses of indomethacin (200 mg), administered 15 and 2 h before testing, were known to inhibit prostaglandin synthesis by 90% in 3 of the subjects as determined by prostaglandin E2 generation assay on endoscopically obtained gastric mucosal biopsy specimens. Acid-induced inhibition of gastric acid secretion was evaluated in a randomized and blinded study in which acid output was measured for 2 h during basal conditions by aspiration, for the next 2 h by intragastric titration during distention with isotonic glucose, and for the following 2 h by intragastric titration during meal stimulation with peptone. The studies were done on separate days, and intragastric pH was maintained at either 2.5 or 5.5 after administration of indomethacin or placebo. Basal acid output was not altered by indomethacin treatment. Distention of the stomach stimulated acid output significantly to a similar degree in all groups, without affecting plasma gastrin. Meal stimulation increased plasma gastrin and acid output significantly more at pH 5.5 (47 +/- 12 pM, 13 +/- 2 mmol/30 min) than at pH 2.5 (30 +/- 8 pM, 6 +/- 2 mmol/30 min). No effect of indomethacin treatment was observed. It is concluded that the participation of cyclooxygenase products in the mechanisms by which acid inhibits the gastric phase of acid secretion in humans is likely to be minor. These results also cast doubt on an important physiologic role for cyclooxygenase products in the regulation of basal acid secretion or of acid secretion stimulated by distention or a peptone meal.

Somatostatin may not be a hormonal messenger of fat-induced inhibition of gastric functions.

The present study was designed to evaluate somatostatin as a hormonal inhibitor of gastric functions in humans. Seven healthy volunteers were investigated on 6 separate days. Peptone meal-stimulated gastric acid secretion was measured by intragastric titration for 2 h and gastric emptying was estimated with a dye-dilution technique. The effect of intravenous administration of somatostatin at 0, 12.5, 50, 100, and 200 pmol/kg.h was investigated and related to the effect of intragastric administration of 100 ml of vegetable oil. Plasma somatostatinlike immunoreactivity was elevated during intravenous administration of somatostatin at 100 and 200 pmol/kg.h, whereas no increase was detected in response to the oil. Somatostatin infusion at 100 and 200 pmol/kg.h significantly inhibited the acid secretion by 25% and 65%, and the oil reduced the acid output by 41%. Somatostatin at 100 and 200 pmol/kg.h significantly enhanced gastric emptying, whereas the oil inhibited gastric emptying. These observations suggest that somatostatin may not be an important hormonal messenger of fat-induced inhibition of acid secretion or gastric emptying.

Somatostatin inhibits gastric acid secretion after gastric mucosal prostaglandin synthesis inhibition by indomethacin in man.

The inhibitory effect of indomethacin, 200 + 200 mg administered per os over 24 hours, on the prostaglandin E2 generative capacity of gastric mucosal tissue was determined in healthy male volunteers. The effect of prostaglandin synthesis inhibition on somatostatin induced suppression of food-stimulated acid secretion was tested. Peptone meal stimulated acid secretion was quantified in five healthy volunteers by intragastric titration with and without indomethacin pretreatment. Somatostatin doses of 200, 400, and 800 pmol/kg/h each significantly inhibited the peptone stimulated acid output. Indomethacin treatment, resulting in 90% inhibition of prostaglandin E2 synthesis, did not affect glucose- or peptone-stimulated acid output or modify the inhibitory action of somatostatin. Clinically, acid inhibition by somatostatin has been used to treat bleeding peptic ulcers. Ulcer haemorrhage may be preceded by an excessive use of drugs that inhibit prostaglandin synthesis such as aspirin or other non-steroidal anti-inflammatory agents. Recent observations in the rat indicate that prostaglandins mediate the inhibitory action of somatostatin on gastric acid secretion. The present results suggest that prostaglandins are not required for inhibition of gastric acid secretion by somatostatin in man.

The amino acid sequence of kinetensin, a novel peptide isolated from pepsin-treated human plasma: homology with human serum albumin, neurotensin and angiotensin.

A novel nonapeptide with neurotensin-like immunoreactivity was isolated from pepsin-treated human plasma by dialysis, ion-exchange chromatography and high performance reversed-phase liquid chromatography. The amino acid sequence was determined by automated gas-phase sequence analysis as Ile-Ala-Arg-Arg-His-Pro-Tyr-Phe-Leu. Sequence homology with human serum albumin and with the biologically active peptides neurotensin and angiotensin is demonstrated. The name proposed for this peptide is kinetensin.