Intracisternal injection of pancreatic polypeptide stimulates gastric emptying in rats

Supraphysiological effects of pancreatic polypeptide on gastric motor function and nutrient tolerance in humans

Pancreatic polypeptide (PP) is known to affect food intake. In this exploratory study, we set out to investigate its supraphysiological effect on food tolerance, gastric accommodation, and emptying. In 12 healthy volunteers, 0, 3, or 10 pmol*kg-1 *min-1 PP was administered intravenously (PP0, PP3 or PP10). Thirty minutes thereafter, nutrient drink infusion (60 ml*min-1 ) through a nasogastric feeding tube was started until maximum satiation. Gastric accommodation was assessed by measuring the intragastric pressure (IGP; nasogastric manometry). In a separate test, the effect of PP0 or PP10 on gastric emptying was tested in 10 healthy volunteers and assessed using the 13 C breath test. Results are presented as mean ± SEM, and p < 0.05 was considered significant. For the IGP test, PP increased ingested nutrient volume: 886 ± 93, 1059 ± 124, and 1025 ± 125 ml for PP0, PP3, and PP10, respectively (p = 0.048). In all groups, Nadir IGP values were reached upon food intake (transformed values: 1.5 ± 0.2, 1.7 ± 0.3, and 1.6 ± 0.3 mmHg for PP0, PP3, and PP10, respectively; NS) to return to baseline thereafter. For the gastric emptying study, volunteers ingested a similar nutrient volume: 802 ± 119 and 1089 ± 128 ml (p = 0.016), and gastric half-emptying time was 281 ± 52 and 249 ± 37 min for PP0 and PP10, respectively (NS). No significant correlation between tolerated nutrient volume and IGP drop (R² < 0.01; p = 0.88 for PP0 vs. PP3 and R² =0.07; p = 0.40 for PP0 vs. PP10, respectively) or gastric half-emptying time (R² = 0.12; p = 0.32) was found. A supraphysiological PP dose enhances food tolerance; however, this effect is not mediated through gastric motility. CLINICAL TRIAL REGISTRY NUMBER: NCT03854708 is obtained from clinicaltrials.gov.

Pancreatic polypeptide stimulates mouse gastric motor activity through peripheral neural mechanisms

BACKGROUND

Pancreatic polypeptide (PP) is supposed to be one of the major endogenous agonists of the neuropeptide Y4 receptor. Pancreatic polypeptide can influence gastrointestinal motility, acting mainly through vagal mechanisms, but whether PP acts directly on the stomach has not been explored yet. The aims of this study were to investigate the effects of PP on mouse gastric emptying, on spontaneous tone of whole stomach in vitro and to examine the mechanism of action.

METHODS

Gastric emptying was measured by red phenol method after i.p. PP administration (1-3 nmol per mouse). Responses induced by PP (1-300 mmol L^-1 ) on gastric endoluminal pressure were analyzed in vitro in the presence of different drugs. Gastric genic expression of Y4 receptor was verified by RT-PCR.

KEY RESULTS

Pancreatic polypeptide dose-dependently increased non-nutrient liquid gastric emptying rate. In vitro, PP produced a concentration-dependent contraction that was abolished by tetrodotoxin, a neural blocker of Na⁺ voltage-dependent channels. The contractile response was significantly reduced by atropine, a muscarinic receptor antagonist, and by SR48968, an NK2 receptor antagonist, while it was potentiated by neostigmine, an inhibitor of acetylcholinesterase. The joint application of atropine and SR48968 fully abolished PP contractile effect. Reverse transcriptase-polymerase chain reaction analysis revealed the presence of Y4 receptor mRNA in mouse stomach with a greater expression in antrum than in fundus.

CONCLUSIONS & INFERENCES

The present findings demonstrate that exogenous PP stimulates mouse gastric motor activity, by acting directly on the stomach. This effect appears due to the activation of enteric excitatory neurons releasing acetylcholine and tachykinins.

Review article: the role of gastrointestinal hormones in the treatment of delayed gastric emptying in critically ill patients

BACKGROUND

Delayed gastric emptying limits the administration of enteral nutrition, leading to malnutrition, which is associated with higher mortality and morbidity. Currently available prokinetics have limitations in terms of sustained efficacy and side effects.

AIM

To summarise the mechanisms of action and to discuss the possible utility of gastrointestinal hormones to prevent or treat delayed gastric emptying in critically ill patients.

METHODS

We searched PubMed for articles discussing 'delayed gastric emptying', 'enteral nutrition', 'treatment', 'gastrointestinal hormones', 'prokinetic', 'agonist', 'antagonist' and 'critically ill patients'.

RESULTS

Motilin and ghrelin receptor agonists initiate the migrating motor complex in the stomach, which accelerates gastric emptying. Cholecystokinin, glucagon-like peptide-1 and peptide YY have an inhibiting effect on gastric emptying; therefore, antagonising these gastrointestinal hormones may have therapeutic potential. Other gastrointestinal hormones appear less promising.

CONCLUSIONS

Manipulation of endogenous secretion, physiological replacement and administration of gastrointestinal hormones in pharmacological doses is likely to have therapeutic potential in the treatment of delayed gastric emptying. Future challenges in this field will include the search for candidates with improved selectivity and favourable kinetic properties.

Gut peptides and the regulation of appetite

There is a growing worldwide epidemic of obesity. Obese people have a higher incidence of type 2 diabetes and cardiovascular disease, and hence present increasing social, financial and health burdens. Weight loss is always difficult to achieve through lifestyle changes alone, and currently licensed anti-obesity drug treatments, such as orlistat and sibutramine, if tolerated, only achieve modest weight loss. Therefore, there is a need to identify more potent pharmacological targets. In the last 10 years, discoveries of new hormones such as leptin and ghrelin, together with greater understanding of previously described hormones such as cholecystokinin (CCK), pancreatic polypeptide (PP), peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), have led to a rapid increase in our knowledge of the regulation of energy balance. Among the most important factors, controlling appetite and satiety are peptide hormones released from the gut. In this paper, we provide a full up-to-date overview of the current state of knowledge of this field, together with the potential of these peptides as drugs, or as other therapeutic targets, in the treatment of obesity. Finally, we propose an integrated model to describe the complex interplay of these hormones in the broader physiology of energy balance.

Synergistic effects of Y2 and Y4 receptors on adiposity and bone mass revealed in double knockout mice

Neuropeptide Y regulates numerous physiological processes via at least five different Y receptors, but the specific roles of each receptor are still unclear. We previously demonstrated that Y2 receptor knockout results in a lean phenotype, increased cancellous bone volume, and an increase in plasma pancreatic polypeptide (PP), a ligand for Y4 receptors. PP-overexpressing mice are also known to have a lean phenotype. Deletion of the Y4 receptor also produced a lean phenotype and increased plasma PP levels. We therefore hypothesized that part of the Y2 phenotype results from increased PP action on Y4 receptors and tested this in PP transgenic Y4(-/-) and Y2(-/-) Y4(-/-) double knockout mice. Bone mass was not altered in Y4 knockout mice. Surprisingly, despite significant hyperphagia, Y2(-/-) Y4(-/-) mice retained a markedly lean phenotype, with reduced body weight, white adipose tissue mass, leptinemia, and insulinemia. Furthermore, bone volume was also increased threefold in Y2(-/-) Y4(-/-) mice, and this was associated with enhanced osteoblastic activity. These changes were more pronounced than those observed in Y2(-/-) mice, suggesting synergy between Y2 and Y4 receptor pathways. The lack of bone changes in PP transgenic mice suggests that PP alone is not responsible for the bone mass increases but might play a major role in the lean phenotype. However, a synergistic interaction between Y2 and Y4 pathways seems to regulate bone volume and adiposity and could have important implications for possible interventions in obesity and for anabolic treatment of osteoporotic bone loss.

Roles of pancreatic polypeptide in regulation of food intake

Pancreatic polypeptide (PP) is produced in pancreatic islets of Langerhans and released into the circulation after ingestion of a meal. Peripherally administered PP suppresses food intake and gastric emptying. On the other hand, central administration of PP elicits food intake and gastric emptying. Therefore, PP actions on food intake may be, in part, attributable to gastric emptying. PP transgenic mice exhibit decreases in both food intake and gastric emptying rate that were clearly reversed by anti-PP antiserum. PP is an anorexigenic signal in the periphery and an orexigenic signal in the central nervous system.

Peptidergic regulation of gastrointestinal motility in rodents

Peptides involved in the endocrine and enteric nervous systems as well as in the central nervous system exert concerted action on gastrointestinal motility. Mechanical and chemical stimuli which induce peptide release from the epithelial endocrine cells are the earliest step in the initiation of peristaltic activities. Gut peptides exert hormonal effects, but peptide-containing stimulatory (Ach/substance P/tachykinin) and inhibitory (VIP/PACAP/NO) neurons are also involved in the induction of ascending contraction and descending relaxation, respectively. The dorsal vagal complex (DVC), located in the medulla of the brainstem, constitutes the basic neural circuitry of vago-vagal reflex control of gastrointestinal motility. Several gut peptides act on the DVC to modify vagal cholinergic reflexes directly (PYY and PP) or indirectly via afferent fibers in the periphery (CCK and GLP-1). The DVC is also a primary site of action of many neuropeptides (such as TRH and NPY) in mediating gastrointestinal motor activities. The identification over the last few years of a number of neuropeptide systems has greatly changed the field of feeding and body weight regulation. By exploring the brain and gut systems that employ recently identified peptidergic molecules, it will be possible to elaborate on the central and peripheral pathways involved in the regulation of gastrointestinal motility.

Mouse pancreatic polypeptide modulates food intake, while not influencing anxiety in mice

This study was designed to investigate the effects of synthetic mouse pancreatic polypeptide (mPP) on feeding and anxiety in mice. The intracerebroventricular (i.c.v.) injection of mPP (0.003-3 nmol) dose-dependently increased food intake. A significant increase was observed 20 min after i.c.v. injection and continued for 4 h. The intraperitoneal (i.p.) injection of mPP (0.03-30 nmol) dose-dependently decreased food intake. A significant decrease was observed 20 min after i.p. injection and continued for 4 h. In the elevated plus maze test, the i.c.v. injection of mPP (0.003-3 nmol) did not affect anxiety behavior. These results suggest that mPP modulates food intake and the Y4 receptor in the brain may contribute to the regulation of feeding, whereas appearing not to influence anxiety in mice.

Decreased food intake and body weight in pancreatic polypeptide-overexpressing mice

BACKGROUND & AIMS

Pancreatic polypeptide (PP) is a 36-amino acid hormone produced by F cells within the pancreatic islets and the exocrine pancreas. The definitive function of PP in mammalian physiology remains to be determined. This study examined the effects of chronic overexpression of PP through the development of PP transgenic mice.

METHODS

PP transgenic mice were created by using mouse PP complementary DNA under the control of the cytomegalovirus immediate early enhancer-chicken beta-actin hybrid promoter (pCAGGS expression vector).

RESULTS

A unique line of transgenic mice was created that overexpresses PP in the pancreatic islets with low levels of expression in other tissues including the brain. Plasma PP concentrations were more than 20 times higher than those of control littermates. However, PP overproduction led to postnatal lethality in half of the pups because of markedly decreased milk intake. The remaining PP transgenic mice gained less weight with specifically reduced food intake and fat mass compared with controls, a result that was more evident in male than in female mice. The transgenic mice exhibited a reduced rate of gastric emptying of a solid meal but had normal oxygen consumption and fasting leptin levels. Immunoneutralization with anti-PP antiserum reversed the phenotypic changes of transgenic animals.

CONCLUSIONS

PP could be involved in feeding and body weight regulation partly through regulation of gastric emptying.

Distribution of pancreatic polypeptide receptors in the rat brain

Pancreatic polypeptide (PP) is a regulatory peptide that modulates gastrointestinal function. Previously we demonstrated PP receptors in the brainstem and interpeduncular nucleus, and the PP receptors in the brainstem appear to modulate gastric motility and pancreatic exocrine secretion. The purpose of this study is to extend our understanding of the distribution of PP receptors in the rat brain in order to determine the systems that are potentially modulated by PP. Rat brains were studied using 125I-PP receptor autoradiography on cryostat sections of the entire brain cut in three planes (horizontal, sagittal, and coronal). Brain regions exhibiting PP binding sites were confirmed when identified in all three planes of section. Saturable PP binding was identified in the hypothalamus (arcuate and paraventricular n), the rostral forebrain (medial preoptic area, anterior olfactory nucleus, islands of Calleja, the dorsal endopiriform n, piriform cortex, and the bed n of the stria terminalis), medial amygdaloid n; the thalamus (anteromedial thal. n; reuniens thal. n; and paraventricular thal n), the interpeduncular red nucleus, substantia nigra, parabrachial n; locus coeruleus, mesencephalic trigeminal n, dorsal motor n of the vagus, the n solitary tract, and the area postrema. We conclude that PP receptors are distributed widely throughout the rat brain. The distribution of many of these PP binding sites corresponds to brain regions regulating digestion and autonomic function. We speculate, based on the patterns of binding in the olfactory and limbic systems, that PP receptors might be involved in positive reinforcement of ingestion behavioral as well as modulation of gastrointestinal function.

Dose-responses for the slowing of gastric emptying in a rodent model by glucagon-like peptide (7-36) NH2, amylin, cholecystokinin, and other possible regulators of nutrient uptake

Several peptides have been proposed as regulators of nutrient release from the stomach and subsequent uptake from the gut. Using a phenol red gavage method, we compared the potencies of subcutaneously preinjected amylin, glucagon-like peptide-1 (7-36)amide (GLP-1), cholecystokinin octapeptide (CCK-8), gastric inhibitory peptide (GIP), glucagon, and pancreatic peptide on slowing the release of an acaloric gel from rat stomach. The latter three peptides did not fully inhibit gastric emptying at subcutaneous doses up to 100 micrograms. Amylin, GLP-1, and CCK-8 fully inhibited gastric emptying, with ED50s of 0.42 +/- 0.07, 6.1 +/- 0.12, and 8.5 +/- 0.20 nmol/kg +/- SE of log, respectively.

Pancreatic polypeptide, microinjected into the dorsal vagal complex, potentiates glucose-stimulated insulin secretion in the rat

Specific binding sites for circulating pancreatic polypeptide (PP) have been found within the dorsal vagal complex (DVC) in the caudal medulla oblongata. Therefore, the effects of rat PP on pancreatic hormone secretion upon its microinjection into the DVC in halothane-anesthetized rats at doses of 0.4-40 pmol were investigated. At this range of doses, the changes in plasma concentrations of insulin, glucagon and glucose over basal levels did not differ from those after vehicle microinjection. In a separate series of experiments, vehicle and PP at doses of 0.4 and 4 pmol were microinjected into the right DVC 40 min after the continuous infusion of D-glucose had been started. In animals receiving continuous infusion of D-glucose, PP microinjected into the DVC (4 pmol), resulted in markedly higher insulin levels at corresponding time points compared to those with vehicle microinjected into the DVC. These data indicate, for the first time, that microinjection of PP into the DVC may potentiate glucose-stimulated insulin secretion in halothane-anesthetized rats.

Gastric emptying is accelerated in diabetic BB rats and is slowed by subcutaneous injections of amylin

Gastric emptying was measured in normal and insulin-treated spontaneously diabetic BB rats using the retention of an acaloric methylcellulose gel containing phenol red delivered by gavage. Dye content in stomachs removed after killing 20 min later was determined spectroscopically, and was compared to that in rats killed immediately after gavage to assess emptying. Diabetic rats had a markedly greater gastric emptying (90.3 +/- 1.7% passed) compared to normal Harlan Sprague Dawley rats (49.1 +/- 4.7% passed; p < 0.001) and non-diabetic BB rats (61.1 +/- 9.2% passed; p < 0.001). The pancreatic beta-cell peptide, amylin, which is deficient in insulin-dependent diabetes mellitus, dose-dependently inhibited gastric emptying in both normal and diabetic rats. The ED50 of the response in normal rats measured by phenol red and novel [3-3H]glucose gavage techniques was approximately 0.4 microgram. This dose was estimated to increase plasma amylin concentration by a mean of approximately 20 pmol/l to concentrations within the range observed in vivo. It is proposed that amylin could participate in the physiological control of nutrient entry into the duodenum and that the accelerated gastric emptying seen in BB rats could be related to their lack of amylin secretion.

Pancreatic polypeptide stimulates gastric motility through a vagal-dependent mechanism in rats

The present study examined the influence of peripherally administered pancreatic polypeptide (PP) on vagal control of gastric motility. The jugular vein was cannulated in urethane-anesthetized rats and a strain gauge was sewn onto the antrum to monitor motility. Intravenous infusion of rat PP (2-200 pmol over 45 min) resulted in a dose-dependent increase in antral contraction amplitude. The motility response to i.v. PP was eliminated by pretreatment with atropine or bilateral vagotomy. In contrast to i.v. infusion, close intra-arterial infusion of PP into the gastric circulation had no effect on motility suggesting that PP does not act upon peripheral afferent terminals or directly within the stomach. These results support the hypothesis that circulating PP indirectly enhances gastric motility through a vagal cholinergic mechanism.

Intracisternal injection of TRH antibody blocks gastric emptying stimulated by 2-deoxy-D-glucose in rats

We evaluated the effect of 2-deoxy-D-glucose (2-DG) on gastric emptying of a non nutrient solution in conscious rats using a Phenol red method. Intravenous injection of 2-deoxy-D-glucose dose-dependently increased the rate of gastric emptying. This stimulatory action of 2-DG was abolished by subdiaphragmatic vagotomy. Intracisternal injection of thyrotropin-releasing hormone (TRH) antibody blocked intracisternal TRH and intravenous 2-DG-induced enhancement of gastric emptying but not the stimulation of gastric emptying induced by intracisternal pancreatic polypeptide. The TRH antibody injected intraperitoneally had no effect. These results suggest that endogenous TRH in the brain is involved in vagal-dependent stimulation of gastric emptying by 2-DG.