Quantitation and characterization of human plasma neurotensin-like immunoreactivity in response to a meal

Influence of the neurotensin signaling system on feeding and satiety

Neurotensin (Nts) is a peptide that acts via neurotensin receptors and is implicated in multiple aspects of physiology and behavior, including modulating feeding and body weight. How and where the Nts signaling system mediates these effects, and via which of its receptor isoforms is incompletely understood. This review examines the role of Nts signaling via the periphery and central nervous system on feeding and body weight. These data highlight various ways in which the Nts system contributes to feeding and body weight that differ depending on the site, tissue, and the Nts or Nts receptor-expressing cell type in question. Given that the Nts system does not convey the same signaling throughout the body, constitutive approaches modulating the expression or signaling of the Nts signaling system may not provide sufficient resolution to reveal how it impacts feeding. Combining neuropharmacology and site-specific approaches holds promise define the broad range of mechanisms by the Nts system modulates feeding and satiety and its contributions to normal and disrupted feeding states.

Hungry for relief: Potential for neurotensin to address comorbid obesity and pain

Chronic pain and obesity frequently occur together. An ideal therapy would alleviate pain without weight gain, and most optimally, could promote weight loss. The neuropeptide neurotensin (Nts) has been separately implicated in reducing weight and pain but could it be a common actionable target for both pain and obesity? Here we review the current knowledge of Nts signaling via its receptors in modulating body weight and pain processing. Evaluating the mechanism by which Nts impacts ingestive behavior, body weight, and analgesia has potential to identify common physiologic mechanisms underlying weight and pain comorbidities, and whether Nts may be common actionable targets for both.

The effect of a randomised controlled lifestyle intervention on weight loss and plasma proneurotensin

AIMS

Proneurotensin (Pro-NT) is a strong predictor of cardiometabolic disease including type 2 diabetes and obesity, however, the effect of lifestyle change on Pro-NT has not been investigated in this context. Middle Eastern (ME) immigrants represent the largest and fastest growing minority population in Europe and are a high-risk population for obesity and type 2 diabetes. In this randomised controlled lifestyle intervention (RCT) addressing ME immigrants to Sweden where weight-loss was previously studied as the main outcome, as a secondary analysis we aimed to study change in Pro-NT during follow-up and if baseline Pro-NT predicted weight loss.

METHODS

Immigrants from the Middle East at high risk for type 2 diabetes were invited to participate in this RCT adapted lifestyle intervention of four months' duration. The intervention group (N = 48) received a culturally adapted lifestyle intervention comprising seven group sessions and a cooking class addressing healthier diet and increased physical activity. The control group (N = 44) received treatment as usual with information to improve lifestyle habits on their own. Data assessed using mixed effects regression.

OUTCOMES

Primary outcome; change in Pro-NT. Secondary outcome; change in BMI in relation to baseline plasma concentration of Pro-NT.

RESULTS

During the four months follow up, weight was significantly reduced in the intervention (-2.5 kg) compared to the control group (0.8 kg) (β -0.12, 95% CI -0.24 to -0.01, P = 0.028). Pro-NT increased to a significantly greater extent in the intervention compared to the control group during follow up (28.2 vs. 3.5 pmol/L) (β 11.4; 4.8 to 18.02, P < 0.001). Change over time in BMI was associated with baseline Pro-NT (β 0.02; 0.01 to 0.04, P = 0.041).

CONCLUSION

In consistence with data from surgical weight loss, this RCT paradoxically shows increased levels of Pro-NT during a multifactorial lifestyle intervention resulting in weight loss. Long term studies of Pro-NT following weight loss are needed.

TRIAL REGISTRATION

This study is a secondary analysis of the RCT trial registered at www.

CLINICALTRIALS

gov .

REGISTRATION NUMBER

NCT01420198. Date of registration 19/08/2011. The performance and results of this trial conform to the CONSORT 2010 guidelines.

The Role of Central Neurotensin in Regulating Feeding and Body Weight

The small peptide neurotensin (Nts) is implicated in myriad processes including analgesia, thermoregulation, reward, arousal, blood pressure, and modulation of feeding and body weight. Alterations in Nts have recently been described in individuals with obesity or eating disorders, suggesting that disrupted Nts signaling may contribute to body weight disturbance. Curiously, Nts mediates seemingly opposing regulation of body weight via different tissues. Peripherally acting Nts promotes fat absorption and weight gain, whereas central Nts signaling suppresses feeding and weight gain. Thus, because Nts is pleiotropic, a location-based approach must be used to understand its contributions to disordered body weight and whether the Nts system might be leveraged to improve metabolic health. Here we review the role of Nts signaling in the brain to understand the sites, receptors, and mechanisms by which Nts can promote behaviors that modify body weight. New techniques permitting site-specific modulation of Nts and Nts receptor-expressing cells suggest that, even in the brain, not all Nts circuitry exerts the same function. Intriguingly, there may be dedicated brain regions and circuits via which Nts specifically suppresses feeding behavior and weight gain vs other Nts-attributed physiology. Defining the central mechanisms by which Nts signaling modifies body weight may suggest strategies to correct disrupted energy balance, as needed to address overweight, obesity, and eating disorders.

Responses of python gastrointestinal regulatory peptides to feeding

In the Burmese python (Python molurus), the rapid up-regulation of gastrointestinal (GI) function and morphology after feeding, and subsequent down-regulation on completing digestion, are expected to be mediated by GI hormones and neuropeptides. Hence, we examined postfeeding changes in plasma and tissue concentrations of 11 GI hormones and neuropeptides in the python. Circulating levels of cholecystokinin (CCK), glucose-dependent insulinotropic peptide (GIP), glucagon, and neurotensin increase by respective factors of 25-, 6-, 6-, and 3.3-fold within 24 h after feeding. In digesting pythons, the regulatory peptides neurotensin, somatostatin, motilin, and vasoactive intestinal peptide occur largely in the stomach, GIP and glucagon in the pancreas, and CCK and substance P in the small intestine. Tissue concentrations of CCK, GIP, and neurotensin decline with feeding. Tissue distributions and molecular forms (as determined by gel-permeation chromatography) of many python GI peptides are similar or identical to those of their mammalian counterparts. The postfeeding release of GI peptides from tissues, and their concurrent rise in plasma concentrations, suggests that they play a role in regulating python-digestive responses. These large postfeeding responses, and similarities of peptide structure with mammals, make pythons an attractive model for studying GI peptides.

Manometric and hormonal changes after distal partial gastrectomy

BACKGROUND

Alkaline oesophagitis attributable to duodenal mechanisms may induce oesophageal carcinogenesis in a rat reflux model.

AIM

To investigate the mechanism of the regurgitation after distal partial gastrectomy.

METHODS

Oesophageal manometry was used in 16 patients before and after distal partial gastrectomy with reconstruction by Bilroth methods. Serum concentrations of four gastrointestinal hormones were measured by radioimmunoassay in 10 gastrectomy patients and nine healthy volunteers before and after a standardized meal.

RESULTS

The lower oesophageal sphincter pressure was reduced to 83% after surgery. The amplitude and duration of the peristaltic waves tended to be increased, and the velocity tended to be less after surgery (amplitude 120%, duration 114%, velocity 88%). Interrupted waves appeared more frequently after surgery. The manometric changes in gastrectomized patients are considered to be disadvantageous relative to regurgitation. After surgery, gastrin and pancreatic polypeptide were completely abolished postprandially, whereas cholecystokinin and neurotensin were significantly increased.

CONCLUSION

The hormonal changes should have a suppressive effect on the lower oesophageal sphincter. Both the manometric and the hormonal changes may exacerbate reflux oesophagitis after distal partial gastrectomy.

Radioimmunoassay of regulatory peptides in the presence of acetonitrile: marked improvement of cholecystokinin assays

Radioimmunoassay has made it possible to measure the levels of many hormones. However, samples for some hormones, such as cholecystokinin (CCK), need to be purified by reverse phase chromatography before assay. Usually, samples are eluted from cartridges or HPLC columns in about 50% acetonitrile, dried on a vacuum centrifuge, and then reconstituted in buffer. Drying and reconstituting samples is time consuming and introduces additional sources of error and peptide loss. The present study investigated the effect of acetonitrile on radioimmunoassays for CCK to see if samples containing acetonitrile could be assayed directly. The non-specific binding of a radiolabeled peptide, the zero binding (B0), and the fall in the presence of 2.5 fmol unlabeled CCK were determined in the presence of various proportions of acetonitrile with 0.1% TFA. Additionally, standard curves were compared in the presence and absence of 200microl of 50% acetonitrile, (n = 5). For assays using two separate CCK antisera, increasing amounts of acetonitrile gave progressively higher zero binding and fall, thereby increasing sensitivity and antibody titer. The use of 200microl 50% acetonitrile, chosen to represent typical sample conditions, increased antiserum titers by three to four-fold, as well as increasing sensitivity considerably. For one antiserum (CCK2), the IC20 was 0.36+/-0.02 fmol CCK/tube in the presence of acetonitrile and 1.45+/-0.08 fmol/tube in its absence (P< 0.001). For the other antiserum (Dino 7), the IC20 was 0.40+/-0.02 fmol CCK/tube in the presence of acetonitrile and 0.63+/-0.01 fmol/tube in its absence (P<0.001). A similar increase in sensitivity was seen with a gastrin assay. However, no significant change in the gastrin antibody titer was evident. Assays for several other hormones were unaffected by 200 microl of 50% acetonitrile. At volumes encountered in samples following chromatography, acetonitrile did not adversely affect radioimmunoassays for a number of hormones, and the sensitivity and antibody titer of the CCK assays were improved. Measurement of CCK samples without drying and reconstitution increases assay efficiency and sensitivity.

Hormonal response to enteral feeding and the possiblerole of peptide YY in pathogenesis of enteral feeding-related diarrhoea

Diarrhoea is a common complication of enteral feeding. Previous studies have demonstrated a secretion of water and electrolytes in the ascending colon during intragastric but not intraduodenal enteral feeding. The cause of this secretion is likely to be neurohumoral in origin. This study was designed to examine the hormonal responses to enteral feeding. In vivo segmental colonic perfusion studies were undertaken. Before and at hourly intervals during these studies serum was taken for estimations of neurotensin (NT), pancreatic glucagon (PG), peptide YY (PYY) and vasoactive intestinal polypeptide (VIP). During fasting there was a median ascending colonic absorption of water in all groups. During feeding there was a net secretion in the ascending colon in both gastric groups and in the high load duodenal group, but not in the low load duodenal group. During these studies the PYY levels remained unchanged from fasting in the low and high load gastric groups. In the low and high load duodenal groups the PYY levels increased. The NT levels increased only in the high load duodenal group. There were no other changes in NT or in PG or VIP levels either between fasting and feeding, or between the gastric and duodenal groups. PYY is known to stimulate intestinal absorption. The absence of a rise during intragastric feeding may be important in the underlying mechanisms of enteral feeding-induced colonic secretion and hence enteral feeding-related diarrhoea.

Time course of adaptive regulatory peptide changes following massive small bowel resection in the dog

Basal and postprandial concentrations of gastrointestinal hormones were measured in 12 dogs before and at one and three months after a 75% small bowel resection. Five animals were studied again at six months. Concentrations of enteric hormones and neuropeptides, measured in the proximal jejunum and distal ileum adjacent to the anastomotic site at the time of euthanasia, were compared with concentrations in control tissues taken from each animal at the time of resection. Increased basal and postprandial levels of gastrin (P < 0.05), cholecystokinin (CCK, P < 0.05), glucose-dependent insulinotropic peptide (GIP, P < 0.01), peptide YY (PYY, P < 0.001), and enteroglucagon (P < 0.001), were seen at one month after small bowel resection. In contrast, no significant changes were seen in concentrations of secretin, motilin, neurotensin, somatostatin, PP, or glucagon. Concentrations of enteroglucagon, GIP, and PYY remained high throughout the six-month study period. In contrast, gastrin and CCK had normalized by three months. Thus, only enteroglucagon, PYY, and GIP showed sustained elevations following enterectomy; the gastrin and CCK changes were transient. Following enterectomy, concentrations of vasoactive intestinal polypeptide (VIP) were reduced by about 50% in mucosal (P < 0.001) and muscle (P < 0.05) layers of proximal and distal gut. In contrast, calcitonin gene-related peptide (CGRP) was increased by about twofold in jejunal and ileal mucosa (P < 0.05), and CGRP elevations were even more marked in the muscle layers (P < 0.001). Somatostatin and neuropeptide Y (NPY) concentrations were similar to controls in all areas except for a small decrease in NPY in ileal mucosa (P < 0.05). These findings suggest that the increased motilin and PP concentrations previously reported after bowel resection in man are more likely to reflect underlying inflammatory bowel disease rather than enterectomy. The normalization of hypergastrinemia explains why the increased acid secretion after small bowel resection is transient. These results provide evidence for independent secretory control of enteroglucagon and PYY, which are both products of intestinal L cells. In addition, these studies reveal marked changes in enteric neuropeptide concentrations following bowel resection. VIP, which is thought to be a major inhibitory transmitter in the gut, is markedly reduced, while CGRP, which is mainly localized in sensory afferent fibers, is increased. These major neuropeptide changes are likely to be of importance in the adaptive responses to massive small bowel resection.

Effects of jejunoileal autotransplantation on gastrointestinal regulatory peptides

Plasma gastrointestinal hormones were measured before and during feeding in eight dogs, more than one year after total autotransplant of the entire jejunoileum, and in controls. At sacrifice, tissues were taken from the transplanted segment and intact bowel for measurement of hormone and enteric neuropeptide content. Gastrin levels were reduced in autotransplanted dogs (fasting 63% of control, incremental response 67% of control, both P < 0.05), reflecting the loss of acid inhibitory reflexes. Secretin and cholecystokinin responses were identical between the two groups. Postprandial levels of gastric inhibitory peptide (incremental response 175% of control, P < 0.005), insulin, and peptide YY (158% of control, P < 0.05) were elevated following denervation, the former suggesting more rapid gastric emptying while the latter may reflect malabsorption. The neurotensin meal response was obtunded by denervation (incremental response 43% of control, P < 0.05), providing evidence for a neural pathway for its release. Pancreatic polypeptide responses were identical between the groups, suggesting intact pancreatic innervation. Abnormal hormone secretion may contribute to the impaired fed motor responses seen following extrinsic denervation of the small bowel. In contrast, the neuropeptide content of the autotransplanted small intestine is normal, suggesting that extrinsic denervation has no long-term effects on peptide content of the enteric nervous system.

Effect of meal composition and sham feeding on duodenojejunal motility in humans

The disruptive effect of meals of different fat content and caloric value and of sham feeding on the interdigestive migrating motor complex (IDMMC) was studied in eight healthy subjects using an ambulatory recording system that allowed continuous monitoring of small bowel motility for three consecutive days. The durations of fed pattern were not significantly different between meals of 800 kcal/50% fat, 400 kcal/50% fat, and 800 kcal/25% fat, but were significantly longer compared to IDMMC cycle length and sham feeding. The latter two were not significantly different. On a separate day, five subjects consumed a meal of 400 kcal/9% fat and a second one of 800 kcal/50% fat. The duration of the fed pattern following the high fat meal was significantly longer than the low fat one. Sham feeding significantly increased plasma concentrations of gastrin and neurotensin (NT), but did not affect those of cholecystokinin (CCK), insulin, and peptide YY (PYY). The various variables of the IDMMC were not different during the two nights of the study, and velocity of migration of phase III during the first day and both nights was similar. We conclude that the duration of the fed pattern depends, in part, on the composition of the meal. Sham feeding, resulting in an increase in both plasma gastrin and NT concentrations, does not disrupt the IDMMC. When using thin probes, IDMMC is stable during prolonged recording.

Gut regulatory peptides and intestinal permeability in acute infantile gastroenteritis

The plasma concentrations of seven gut regulatory peptides were measured in 11 infants suffering from acute gastroenteritis. Samples were taken at the time of the acute illness, upon reintroduction of feeding, and three months after recovery. These results were compared with controls. In the infants with diarrhoea, a massive increase in the fasting plasma mean (SEM) concentrations of enteroglucagon was found at the time of illness (1292 (312) v 79 (27) pmol/l), with concentrations of pancreatic glucagon, peptide tyrosine tyrosine, and motilin also being increased (17.8 (3.1) v 6.3 (1.1) pmol/l, 114.6 (15.2) v 37.0 (11.0) pmol/l, 217.6 (44.1) v 98.5 (18.3 pmol/l) respectively). The preprandial concentrations of motilin were found to be still increased at recovery (183.9 (35.4) pmol/l), but the concentrations of the other three peptides had returned to normal values. No differences in plasma concentrations of vasoactive intestinal polypeptide, neurotensin, or pancreatic polypeptide were found. An increased intestinal permeability was demonstrated at the time of diarrhoea by the urinary ratio of lactulose to mannitol, suggesting simultaneous gut damage. The effects of regulatory peptides may be relevant to the pathophysiology of gastroenteritis in infants.

Effect of duodenal switch procedure on gastric acid production, intragastric pH, gastric emptying, and gastrointestinal hormones

The duodenal switch operation preserves the pylorus and the proximal 3 to 7 cm of duodenum in continuity with the stomach while diverting pancreaticobiliary secretions. We compared it with the Roux-en-Y without vagotomy or antrectomy in 12 dogs with innervated gastric pouches. Acid secretion was inhibited between tests using ranitidine in the Roux-en-Y group only, but two of the six dogs still developed stomal ulcers and the remainder showed stomal hyperemia. This may be due to a significant increase in gastric acid output after Roux-en-Y, but gastric emptying and plasma gastrin, cholecystokinin, secretin, gastric inhibitory polypeptide, peptide YY, and neurotensin were similar after both procedures. In 12 patients and a further 6 dogs, the duodenal switch caused no significant change in the intragastric pH environment as assessed by intragastric pH monitoring. The duodenal switch is a suitable procedure for pancreaticobiliary diversion.

Plasma levels of human neurotensin: methodological and physiological considerations

The ingestion of a meal high in fat content is known to increase circulating levels of neurotensin (NT) in humans. However, the magnitude of the postprandial rise of NT in the general circulation and its physiological significance have been subject of much debate. The present study examines circulating levels of NT in male volunteers prior to and following each of their three daily meals (ca. 31 g fat/meal). The response observed are also compared to that elicited by the direct instillation of intralipid (ca. 44 g fat) into the duodenum. NT levels were determined by radioimmunoassay of acid/acetone extracted plasma fractionated by high pressure liquid chromatography. Meals caused a significant but modest increase in NT levels, with the largest increment (ca. 4 fmol/ml) occurring after breakfast. In contrast, NT levels increased ca. 20 fmol/ml with intraduodenal instillation of lipid. The meal-stimulated increases in circulating NT measured here are 4- to 5-fold less than those reported by others, the difference most likely reflecting the lesser amount of lipid ingested. Previous studies provided subjects with single meals containing in excess of 120 g of fat; the 30 g of fat ingested by our subjects, ca. 33% of total caloric intake, is near that recommended by the U.S. Senate, Select Committee on Nutritional and Human Needs. These data show that diets with a reasonable fat content have only a modest effect on circulating levels of NT.

Metabolism of neurotensin by isolated perfused rat kidney

Studies in humans and conscious animals have established that the kidney is a key organ involved in the clearance of the brain-gut peptide neurotensin (NT). However, these in vivo studies cannot determine the mechanisms involved in the renal elimination of NT. We have therefore used the isolated perfused rat kidney preparation, which enables renal elimination to be studied in isolation from other organs. NT was measured with both COOH-terminal (biologically active end) and NH2-terminal (biologically active end) directed antisera. NT was stable in control perfusions (no kidney) with a disappearance half-life of greater than 250 min. The disappearance half-life in filtering kidneys was 52 +/- 3 min (COOH terminal) and 67 +/- 3 min (NH2-terminal). High-performance liquid chromatography of perfusate revealed a pattern similar to that seen in vivo with the NT being metabolized to NH2-terminal fragments. When the kidneys were rendered nonfiltering, NH2-terminal clearance was abolished, whereas COOH-terminal clearance was reduced by 75%. There was no release of NT peptidases into the perfusate. These data demonstrate that NT is metabolized directly by the kidney, predominantly by a filtration and reabsorption mechanism and with only a minor role for peritubular metabolism.

Distribution of neurotensin-like immunoreactivities in porcine and human gut

The distribution of neurotensin-like immunoreactivity (NTLI) in porcine and human intestine was studied by extraction of mucosal and muscular layers of esophagus, stomach, duodenum, jejunum, ileum, and colon. NTLI was quantitated and characterized by radioimmunoassays and gel filtration chromatography. Porcine tissue was obtained in anesthetized animals (n = 6) and human tissue during surgery (n = 28). Concentrations of NTLI increased gradually from the distal esophagus to the ileum. Highest concentrations were found in 2.0 M acetic acid extracts of proximal ileal mucosa (150 (131-223) and 525 (500-729) pmol/g wet tissue, respectively (medians and interquartile range]. After acid extraction, concentrations of intact NT and COOH-terminal and NH2-terminal NTLI were similar, but in water concentrations of NH2-terminal NTLI were high and intact NT and COOH-terminal NTLI low. The distribution of NTLI was similar in the two species. Gel chromatography of ileal, jejunal, and duodenal mucosa indicated that in these tissues NTLI consisted primarily of intact NT. In antral mucosa COOH-terminal immunoreactivity different from NT was detected. The chemical identity is unknown, but it may represent precursor forms of NT.

Inhibitory effect of neurotensin on gastric acid secretion in rats. Development of a bioassay model

Neurotensin has been shown to inhibit gastric acid secretion when administered in pharmacological doses, but no information has been available concerning its possible dose-related effect during intravenous infusion. In this study, a dose-related and reversible inhibitory effect of neurotensin was demonstrated in pentobarbital-anesthetized female Sprague-Dawley rats. The rats underwent continuous gastric perfusion with saline, 1 ml/min, and intravenous infusion of both pentagastrin and neurotensin. Inhibition of acid secretion did not depend upon the occurrence of hypotension, and ranged from 35 +/- 7% of maximal acid output at 0.24 nmol/kg/hr to 60 +/- 10% at 7.2 nmol/kg/hr of neurotensin. Blood levels of C-terminal neurotensin-like immunoreactivity were proportional to the dose of peptide infused and were 52 fmol/ml during infusion of 0.24 nmol/kg/hr, a dose that significantly inhibited pentagastrin-induced acid secretion. Thus, a model has been developed to study the effect of neurotensin infusion on acid secretion; the concentration of plasma neurotensin-like immunoreactivity at which inhibition occurs in this model is similar to the concentration reported to occur after a nutrient stimulus.

Do normal plasma neurotensin concentrations increase ileal output?

Infusions of neurotensin increase ileal secretion in experimental animals, and the volume of ileal effluent in patients with ileostomies. The aim of the present study was to determine whether normal postprandial plasma concentrations of neurotensin increase the volume of fluid leaving the ileum. Basal and peak postprandial plasma neurotensin concentrations were 23 (17-36) and 39 (25-43) pmol/l (median and range) respectively in five subjects with ileostomies and 15 (3-27) and 32 (15-82) pmol/l respectively in nine normal subjects. Infusion of neurotensin for 30 min at a rate of 6.3 pmol/kg/min into six patients with ileostomies increased ileostomy output about 10-fold, and produced a significant decrease in the concentration of solid material, but plasma neurotensin concentrations rose to 237 (82-422) pmol/l during infusion at this rate. Infusion of neurotensin at 2.3 pmol/kg/min, producing plasma levels of 60 (16-108), had no significant effect the amount or nature of ileostomy effluent. We conclude that normal postprandial plasma concentrations of neurotensin are unlikely to influence the volume of fluid leaving the ileum.

Effect of oral fat on plasma levels of neurotensin and neurotensin fragments in humans. Characterization by high-pressure liquid chromatography

The effect of ingestion of fat (Lipomul 1 g/kg) on the circulating levels of neurotensin (NT1-13) and amino-terminal fragments (NT1-8, NT1-11) and carboxy-terminal fragment (NT8-13) of NT were investigated in six healthy male volunteers. NT and NT fragments were extracted from plasma collected at 0, 15, 30, and 60 min after ingestion of fat, and the plasma levels of NT1-13 and NT fragments were characterized using high-pressure liquid chromatography and radioimmunoassay techniques. Significant elevations of plasma levels of NT1-8, NT1-11, and NT1-13 were observed at 15, 30, and 60 min after fat ingestion. The maximum elevations were 273% for NT1-8, 234% for NT1-11, and 54% for NT1-13. NT8-13 levels failed to rise significantly when compared to basal levels. These findings indicate that both the amino-terminal and carboxy-terminal fragments of NT are either released along with intact NT or are formed as metabolites from NT1-13 in response to ingestion of fat in man.

Action of neurotensin on meal-stimulated gastric acid secretion in the dog

In six to nine mongrel dogs the effect of graded doses of intravenous neurotensin (188, 375, 750, and 1500 pmol/kg h) on acid secretion basally or stimulated by distention (by isotonic glucose), peptone (0.5, 1, and 4 g%), and pentagastrin was studied. Neurotensin did not affect acid secretion basally, stimulated by distention, or the maximal peptone dose. However, when submaximal doses (0.5 and 1 g%) of peptone were instilled in the stomach, neurotensin stimulated the secretory response to intragastric peptone. This effect was observed in doses of intravenous neurotensin which mimicked circulating neurotensin concentrations after a standard test meal. Thus, neurotensin could be considered a physiologic stimulant of acid secretion when protein is present in the stomach. The mechanism for this action of neurotensin is unknown but could be partly explained by an enhanced release of gastrin. The potentiating effect of neurotensin on peptone-stimulated acid secretion could play a major role in gastric secretory function of the dog.

Biological potency of neurotensin metabolites in vivo: importance of alcohol 'fixation' of blood

Neurotensin (NT), a 13-amino acid peptide, is released from the ileum following a meal. It is metabolized principally by the kidney and in the circulation to N-terminal fragments and apparently rapidly degraded C-terminal fragments. The present study was designed to compare the biological activity (plasma pancreatic polypeptide response) and the clearance kinetics of NT(1-13), the N-terminal fragment NT(1-11) and the C-terminal fragment NT(8-13). To measure accurately the circulating concentrations of short-lived NT fragments in the circulation, a method was devised of collecting blood directly into alcohol ('alcohol fixation'). The alcohol fixation procedure prevented the post-collection losses of the C-terminal fragment NT(8-13) and established that, based on blood levels achieved, NT(8-13) had 70% of the pancreatic polypeptide stimulating potency of NT(1-13). Nevertheless, the metabolic clearance rate of NT(8-13) was about sevenfold higher than the intact molecule, NT(1-13), suggesting that circulating C-terminal fragments have a minor physiological role. When the N-terminal fragment NT(1-11) was infused, there was no sustained effect on pancreatic polypeptide secretion although it was cleared at a rate similar to that of NT(1-13). It is concluded that the use of alcohol fixation prevents post-collection losses of NT fragments and enables true biological potencies of short-lived fragments to be assessed. The biological activity of NT resides in the C-terminus which, once split from the protective N-terminus, is rapidly degraded in the circulation. The remaining intact but inactive N-terminus is relatively stable.

Neurotensin metabolism in the rat: contribution of the kidney

The kidney plays a key role in the metabolism of neurotensin (NT). We have examined the renal mechanisms of NT clearance by measuring plasma NT basally and after 45 min infusion of NT(1-13) in intact rats, anephric rats (no glomerular filtration, no peritubular metabolism) and ureteral ligated rats (reduced filtration). Plasma NT was measured by radioimmunoassay with both C (biologically active end) and N terminal directed antisera. In anephric and ureteral ligated rats, basal plasma NT like immunoreactivity measured with either antisera was increased 3-fold compared with unoperated rats. C terminal concentrations were higher than N indicating that a C terminal variant of NT was present in basal plasma. Infusion of NT(1-13) increased N terminal NT from 36 +/- 3 to 249 +/- 35 pmol/l (p less than 0.01) in unoperated rats with significantly larger increases in the renally compromised groups. This was reflected in the reduced metabolic clearance rates (measured with the N terminal directed antisera) in the anephric (16 +/- 1 ml/kg/min) and ureteral ligated (17 +/- 3 ml/kg/min) rats when compared with the control rats (26 +/- 4 ml/kg/min). The similar reductions in the anephric and ureteral ligated rats suggested that the decrease in N terminal NT metabolism was from the absence of filtration. Infusion of NT did not increase C terminal NT immunoreactivity in intact, anephric and ureteral ligated rats showing that the C terminal end was extremely labile. However when endogenous converting enzyme activity was blocked by captopril administration there was a significant increase in C terminal immunoreactivity suggesting a role for converting enzyme like proteases in the clearance of the biologically active end of NT.(ABSTRACT TRUNCATED AT 250 WORDS)

Extraction of neurotensin by the liver

1. Neurotensin is released from the intestine into the portal circulation and to exert a systemic effect it must traverse the liver intact. 2. The role of the liver in neurotensin clearance was examined using the isolated perfused rat liver preparation. Two concentrations of neurotensin were used to determine the extraction capacity of the liver. 3. Approximately 10% of the added neurotensin (with either dose) was extracted in a single pass through the liver. This extraction rate was low when compared to previous studies with cholecystokinin (60% extraction in a single pass) and vasoactive intestinal peptide (100%). 4. It is concluded that there is a small but high capacity for direct extraction of neurotensin. This low direct extraction percentage supports our previous contention that the major influence of the liver on the metabolism of neurotensin is by the release of neurotensin degrading peptidases into the circulation.

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.

Extraction of neurotensin-like immunoreactivities from porcine ileal mucosa

The extractability of neurotensin (NT) from porcine ileal mucosa was studied by comparison of eight extraction procedures. Tissue content of neurotensin-like immunoreactivity was quantitated and characterized by sequence-specific radioimmunoassays and gel filtration chromatography. Homogenization prior to boiling in extraction solvent produced higher levels of the intact peptide than the reverse procedure. N-terminal immunoreactivity was not influenced by the sequence of these steps. Tissue levels of intact NT were highest after extraction with 2.0 M acetic acid (mean 79.1 pmol/g, N = 6) and lowest with distilled water (mean 6.5 pmol/g, N = 6). The opposite was the case with levels of N-terminal immunoreactivity (mean 55.2 pmol/g and 105.7 pmol/g respectively, N = 6). Recovery experiments with addition of synthetic NT 1-13 and the N-terminal fragment NT 1-8 indicated that these differences could be explained by differences in recovery of intact NT and N-terminal immunoreactive components in tissue. Gel chromatography confirmed that in acetic acid almost only the intact peptide was extracted from ileal mucosa, and showed that after extraction in water or phosphate buffer several N-terminal components were present. The results suggest that a molecular heterogeneity may be present in ileal tissue. If this concept is supported by further studies differential extraction procedures may be needed in the future.

Renal function during bovine neurotensin infusion in man

The peptide hormone neurotensin (NT) is found mainly in gut endocrine cells of the ileum, but has also been identified as a putative neurotransmitter in the central and peripheral nervous systems. It may have a dual role as a circulating gastrointestinal hormone and peripheral neurotransmitter. Its predominant effects are to reduce oesophageal sphincter tone, inhibit gastric secretion and emptying and inhibit intestinal motility, but stimulate intestinal and pancreatic exocrine secretion; NT-like immunoreactivity has been found in kidney and therefore NT may influence renal function. When infused i.v. in rabbits it causes antinatriuresis. We have studied its renal effects in 11 healthy males by i.v. infusion under conditions of altered dietary sodium. Postprandial circulating neurotensin levels were reproduced by infusion. There were no consistent systemic or renal haemodynamic effects. Plasma electrolytes and renin did not change. Only renal chloride excretion changed significantly, falling by ca. 30%, and recovering after infusion. There is no evidence for a specific renal tubular chloride transport mechanism, but coupled cotransport, Na+:K+:2CI-, may be hormonally regulated. NT might stimulate this process and contribute to the renal response to changes in dietary composition, especially sodium intake.

Effect of renal failure or portacaval shunt on release of neurotensin in man

Neurotensin-like immunoreactivity (NTLI) is released into the circulation following a meal. However, the factors influencing its degradation and metabolism are uncertain, but it is presumed that the kidneys and liver are involved. This study has determined the release of NTLI after a liquid fat meal (200 ml 20% Intralipid) in 10 healthy control subjects, five patients with stable chronic renal failure (CRF), and five patients after a portacaval shunt (PCS), to assess the possible role of the liver and kidney in NTLI metabolism. NTLI was measured by radioimmunoassay using both specific C and N terminally directed antisera. The results showed: Basal NTLI levels are similar in controls, after PCS, and in CRF. A liquid fat meal produces a single early peak of NTLI and levels are greater after PCS than in controls or CRF. Levels return much more slowly to basal in CRF and after PCS. Thus it is likely that both the liver and kidney are involved in the metabolism and clearance of NTLI.

Effect of neurotensin and neurotensin fragments on gastric acid secretion in man

The effect of intravenous infusion of neurotensin (NT) and NT-fragments on pentagastrin stimulated gastric acid secretion was investigated in healthy subjects. Neurotensin was infused in three doses (72, 144 and 288 pmol/kg per h). An N-terminal fragment (NT 1-8), a C-terminal fragment (NT 8-13) and an NT-analogue, substituted at the C-terminal tyrosine residue (Phe11-NT) were infused in two doses (72 and 144 pmol/kg per h). Concentrations of the infused peptides were measured in peripheral venous blood by radioimmunoassay. Plasma levels of NT 1-13, NT 1-8 and Phe11-NT increased in a dose-dependent manner; NT 1-13 to 50 (34-69), 78 (54-113) and 143 (112-242) pmol/l (medians and range) at 72, 144 and 288 pmol/kg per h, NT 1-8 to 405 (340-465) and 1215 (915-1300) pmol/l, and Phe11-NT to 200 (110-245) and 390 (250-410) pmol/l at 72 and 144 pmol/kg per h, respectively. Increases in plasma levels of NT 8-13 could not be detected during the infusion, suggesting that the fragment is rapidly metabolized in man. Neurotensin 1-13 inhibited gastric acid secretion in a dose-dependent manner and the decrease in gastric acid secretion was linearly related to plasma levels of NT 1-13. Neurotensin 1-8 and NT 8-13 inhibited gastric acid secretion only at 144 pmol/kg per h, while the analogue Phe11-NT had no effect. The results showed that the inhibition of gastric acid secretion produced by NT was dose-dependent and linearly related to circulating levels of NT, and that under physiological conditions this effect presumably is elicited by the C-terminal part of the peptide.

In vitro degradation of neurotensin in human plasma

To study the degradation of neurotensin in plasma in vitro, fresh human plasma was incubated with neurotensin in the presence and absence of the peptidase inhibitors pepstatin A, EDTA, PMSF and aprotinin. The half-time of disappearance of neurotensin at 37 degrees C was calculated to be 226 min in vitro as opposed to 1.4 min in vivo when measured by radioimmunoassay with a C-terminally directed neurotensin antiserum. Both gel filtration and reversed phase high-pressure liquid chromatography (HPLC) showed that the main degradation product of neurotensin in human plasma in vitro was chromatographically and immunologically identical to neurotensin 1-8 and HPLC also demonstrated the formation of neurotensin 1-11. The loss of neurotensin incubated in human plasma in vitro was greatly reduced by EDTA but not by the other peptidase inhibitors tested. In this respect peptidase(s) responsible for the degradation of neurotensin in plasma differ from those present in brain homogenates. EDTA may be of importance in the preservation of neurotensin in plasma samples.

Neurotensin-like immunoreactivities in human plasma: feeding responses and metabolism

Feeding responses and day and night levels of plasma concentration of neurotensin (NT) and NT-fragments were studied in healthy subjects. Plasma levels were measured by three radioimmunoassays recognizing intact NT in addition to C- and N-terminal immunoreactivity. The metabolism of NT was studied following intravenous administration. In 106 subjects fasting levels of intact NT (median 18 pmol/l), C-terminal (median 30 pmol/l) and N-terminal immunoreactivity (median 95 pmol/l) were unrelated to sex or age. Postprandially plasma levels in seven subjects measured with all assays increased by a factor 1-3. Following a mixed meal the increase was biphasic, whereas the response to dairy cream was monophasic. Repetitive measurements during 24 hours showed that levels of N-terminal immunoreactivity fluctuated in a manner related to meal ingestion and were elevated throughout the daytime, whereas intact NT and C-terminal immunoreactivity changed little. Following intravenous infusion of 2.4 pmol/kg/min NT in 5 subjects the chromatographic pattern was similar to that seen postprandially. The plasma half life of intact NT and C-terminal immunoreactivity was 1.5 and 1.2 min, whereas that of N-terminal immunoreactivity was 10.0 min. The differences in circulating levels could be explained by these differences in metabolism, but the physiological significance remains to be elucidated.

Chromatographic characterisation of neurotensin-like immunoreactivity in plasma and tissue extracts from hepatoma patients

To determine the nature of the neurotensin-like immunoreactivity produced by some cases of primary liver carcinoma, plasma and tumour neurotensin-like immunoreactivities were characterised in patients with neurotensin-producing hepatoma by chromatographic analysis in conjunction with radioimmunoassay with C- and N-terminally directed antisera. Gel filtration revealed that the majority of neurotensin-like immunoreactivity extracted from the tumour was co-eluted with synthetic neurotensin, whereas a substantial amount of the plasma immunoreactive neurotensin emerged at the void volume. Tumour neurotensin-like immunoreactivity showed a degree of heterogeneity on gel filtration and high pressure liquid chromatography that contrasted with neurotensin-like immunoreactivity extracted from ileal mucosa, suggesting a different post-translational processing of neurotensin within tumour tissue. The ratio of neurotensin 1-8 to neurotensin 1-13 in the patients' plasma suggested that neurotensin was metabolised in a manner similar to that in healthy subjects. The clinical and biological significance of the ectopic production of neurotensin in some hepatomas remains unclear.