Release of PYY from pig intestinal mucosa; luminal and neural regulation

Glucagon-like peptide-1: Are its roles as endogenous hormone and therapeutic wizard congruent?

Glucagon-like peptide-1 (GLP-1) is a peptide derived from differential processing of the precursor for the hormone glucagon. It is secreted predominantly by endocrine cells in the gut epithelium in response to nutrient stimulation. Studies from the last 35 years have given us an idea about its physiological functions. On the basis of some of its many actions, it has also been developed into a pharmaceutical agent for the treatment of obesity and type 2 diabetes (T2DM). It is currently positioned as the most effective anti-obesity agent available and is recommended in both national and international guidelines as an effective second-in line treatment for T2DM, in particular in patients with increased cardiovascular risk. In this review, I first discuss whether the processing of proglucagon may also result in GLP-1 formation in the pancreas and in glucagon in the gut. Next, I discuss the relationship between the physiological actions of GLP-1 and the therapeutic effects of the GLP-1 receptor agonists, which are far from being congruent and generally poorly understood. These relationships illustrate both the difficulties and the benefits of bridging results obtained in the laboratory with those emerging from the clinic.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Postprandial PYY increase by resistant starch supplementation is independent of net portal appearance of short-chain fatty acids in pigs

Increased dietary fiber (DF) fermentation and short-chain fatty acid (SCFA) production may stimulate peptide tyrosine-tyrosine (PYY) secretion. In this study, the effects of hindgut SCFA production on postprandial PYY plasma levels were assessed using different experimental diets in a porto-arterial catheterized pig model. The pigs were fed experimental diets varying in source and levels of DF for one week in 3×3 Latin square designs. The DF sources were whole-wheat grain, wheat aleurone, rye aleurone-rich flour, rye flakes, and resistant starch. Postprandial blood samples were collected from the catheters and analyzed for PYY levels and net portal appearance (NPA) of PYY was correlated to NPA of SCFA. No significant effects of diets on NPA of PYY were observed (P > 0.05), however, resistant starch supplementation increased postprandial NPA of PYY levels by 37 to 54% compared with rye-based and Western-style control diets (P = 0.19). This increase was caused by higher mesenteric artery and portal vein PYY plasma levels (P < 0.001) and was independent of SCFA absorption (P > 0.05). The PYY levels were higher in response to the second daily meal compared with the first daily meal (P < 0.001), but similar among diets (P > 0.10). In conclusion, the increased postprandial PYY responses in pigs fed with different levels and sources of DF are not caused by an increased SCFA absorption and suggest that other mechanisms such as neural reflexes and possibly an increased flow of digesta in the small intestine may be involved. The content of DF and SCFA production did not affect PYY levels.

The Role of the Autonomic Nervous System in the Pathophysiology of Obesity

Obesity is reaching epidemic proportions globally and represents a major cause of comorbidities, mostly related to cardiovascular disease. The autonomic nervous system (ANS) dysfunction has a two-way relationship with obesity. Indeed, alterations of the ANS might be involved in the pathogenesis of obesity, acting on different pathways. On the other hand, the excess weight induces ANS dysfunction, which may be involved in the haemodynamic and metabolic alterations that increase the cardiovascular risk of obese individuals, i.e., hypertension, insulin resistance and dyslipidemia. This article will review current evidence about the role of the ANS in short-term and long-term regulation of energy homeostasis. Furthermore, an increased sympathetic activity has been demonstrated in obese patients, particularly in the muscle vasculature and in the kidneys, possibily contributing to increased cardiovascular risk. Selective leptin resistance, obstructive sleep apnea syndrome, hyperinsulinemia and low ghrelin levels are possible mechanisms underlying sympathetic activation in obesity. Weight loss is able to reverse metabolic and autonomic alterations associated with obesity. Given the crucial role of autonomic dysfunction in the pathophysiology of obesity and its cardiovascular complications, vagal nerve modulation and sympathetic inhibition may serve as therapeutic targets in this condition.

Feeding an Elemental Dietvsa Milk-Based Formula Does Not Decrease Intestinal Mucosal Growth in Infant Pigs

BACKGROUND

We previously showed that the level of enteral nutrient intake determines the rate of intestinal growth in piglets. Our objective was to determine whether providing enteral nutrition in the form of elemental nutrients (glucose, amino acids, lipid [ED]) rather than cow's milk formula (lactose, protein, lipid [FORM]) reduces small intestinal growth and lactase activity.

METHODS

Three-week-old piglets were fed either ED (n = 7) intragastrically or FORM (n = 6) orally for 6 days.

RESULTS

Intestinal protein and DNA masses, villus height, and crypt depth were not different in ED and FORM pigs. Crypt cell proliferation, measured by in vivo bromodeoxyuridine labeling, was significantly (p < .05) higher (+37%) in ED than in FORM pigs. Rates of mucosal protein synthesis (%/d), measured by in vivo 2H-leucine incorporation, were higher (p < .05) in ED than FORM (147 vs 89) pigs. Circulating concentrations (pmol/L) of the intestinotrophic peptide, glucagon-like peptide-2 (GLP-2), were also higher (p < .05) in ED than in FORM (148 vs 87) pigs. The mean lactase-specific activity (micromol/min/g) in proximal and distal segments was higher (p < .05) in FORM than in ED (124 vs 58) pigs.

CONCLUSIONS

We conclude that intestinal mucosal growth and villus morphology are similar in pigs fed ED and FORM, despite higher cell proliferation and protein synthesis rates and lower lactase activity with ED. This implies that elemental diets may be as trophic as polymeric formulas to simultaneously provide nutrition and a stimulus for intestinal growth during bowel rest.

Ghrelin, CCK, GLP-1, and PYY(3-36): Secretory Controls and Physiological Roles in Eating and Glycemia in Health, Obesity, and After RYGB

The efficacy of Roux-en-Y gastric-bypass (RYGB) and other bariatric surgeries in the management of obesity and type 2 diabetes mellitus and novel developments in gastrointestinal (GI) endocrinology have renewed interest in the roles of GI hormones in the control of eating, meal-related glycemia, and obesity. Here we review the nutrient-sensing mechanisms that control the secretion of four of these hormones, ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), and peptide tyrosine tyrosine [PYY(3-36)], and their contributions to the controls of GI motor function, food intake, and meal-related increases in glycemia in healthy-weight and obese persons, as well as in RYGB patients. Their physiological roles as classical endocrine and as locally acting signals are discussed. Gastric emptying, the detection of specific digestive products by small intestinal enteroendocrine cells, and synergistic interactions among different GI loci all contribute to the secretion of ghrelin, CCK, GLP-1, and PYY(3-36). While CCK has been fully established as an endogenous endocrine control of eating in healthy-weight persons, the roles of all four hormones in eating in obese persons and following RYGB are uncertain. Similarly, only GLP-1 clearly contributes to the endocrine control of meal-related glycemia. It is likely that local signaling is involved in these hormones' actions, but methods to determine the physiological status of local signaling effects are lacking. Further research and fresh approaches are required to better understand ghrelin, CCK, GLP-1, and PYY(3-36) physiology; their roles in obesity and bariatric surgery; and their therapeutic potentials.

A Large Animal Survival Model to Evaluate Bariatric Surgery Mechanisms

Background

The impact of Roux-en-Y gastric bypass (RYGB) on type 2 diabetes mellitus is thought to result from upper and/or lower gut hormone alterations. Evidence supporting these mechanisms is incomplete, in part because of limitations in relevant bariatric-surgery animal models, specifically the lack of naturally insulin-resistant large animals. With overfeeding, Ossabaw swine develop a robust metabolic syndrome, and may be suitable for studying post-surgical physiology. Whether bariatric surgery is feasible in these animals with acceptable survival is unknown.

Methods

Thirty-two Ossabaws were fed a high-fat, high-cholesterol diet to induce obesity and insulin resistance. These animals were assigned to RYGB (n = 8), RYGB with vagotomy (RYGB-V, n = 5), gastrojejunostomy (GJ, n = 10), GJ with duodenal exclusion (GJD, n = 7), or sham operation (n = 2) and were euthanized 60 days post-operatively. Post-operative changes in weight and food intake are reported.

Results

Survival to scheduled necropsy among surgical groups was 77%, living an average of 57 days post-operatively. Cardiac arrest under anesthesia occurred in 4 pigs. Greatest weight loss (18.0% ± 6%) and food intake decrease (57.0% ± 20%) occurred following RYGB while animals undergoing RYGB-V showed only 6.6% ± 3% weight loss despite 50.8% ± 25% food intake decrease. GJ (12.7% ± 4%) and GJD (1.2% ± 1%) pigs gained weight, but less than sham controls (13.4% ± 10%).

Conclusions

A survival model of metabolic surgical procedures is feasible, leads to significant weight loss, and provides the opportunity to evaluate new interventions and subtle variations in surgical technique (e.g. vagus nerve sparing) that may provide new mechanistic insights.

Critical review evaluating the pig as a model for human nutritional physiology

The present review examines the pig as a model for physiological studies in human subjects related to nutrient sensing, appetite regulation, gut barrier function, intestinal microbiota and nutritional neuroscience. The nutrient-sensing mechanisms regarding acids (sour), carbohydrates (sweet), glutamic acid (umami) and fatty acids are conserved between humans and pigs. In contrast, pigs show limited perception of high-intensity sweeteners and NaCl and sense a wider array of amino acids than humans. Differences on bitter taste may reflect the adaptation to ecosystems. In relation to appetite regulation, plasma concentrations of cholecystokinin and glucagon-like peptide-1 are similar in pigs and humans, while peptide YY in pigs is ten to twenty times higher and ghrelin two to five times lower than in humans. Pigs are an excellent model for human studies for vagal nerve function related to the hormonal regulation of food intake. Similarly, the study of gut barrier functions reveals conserved defence mechanisms between the two species particularly in functional permeability. However, human data are scant for some of the defence systems and nutritional programming. The pig model has been valuable for studying the changes in human microbiota following nutritional interventions. In particular, the use of human flora-associated pigs is a useful model for infants, but the long-term stability of the implanted human microbiota in pigs remains to be investigated. The similarity of the pig and human brain anatomy and development is paradigmatic. Brain explorations and therapies described in pig, when compared with available human data, highlight their value in nutritional neuroscience, particularly regarding functional neuroimaging techniques.

The vagus nerve, food intake and obesity

Food interacts with sensors all along the alimentary canal to provide the brain with information regarding its composition, energy content, and beneficial effect. Vagal afferents innervating the gastrointestinal tract, pancreas, and liver provide a rapid and discrete account of digestible food in the alimentary canal, as well as circulating and stored fuels, while vagal efferents, together with the sympathetic nervous system and hormonal mechanisms, codetermine the rate of nutrient absorption, partitioning, storage, and mobilization. Although vagal sensory mechanisms play a crucial role in the neural mechanism of satiation, there is little evidence suggesting a significant role in long-term energy homeostasis. However, increasing recognition of vagal involvement in the putative mechanisms making bariatric surgeries the most effective treatment for obesity should greatly stimulate future research to uncover the many details regarding the specific transduction mechanisms in the periphery and the inter- and intra-neuronal signaling cascades disseminating vagal information across the neuraxis.

Development of a porcine Roux-en-Y gastric bypass survival model for the study of post-surgical physiology

BACKGROUND

Rodents have been used to examine physiologic changes after bariatric surgery, but differences in gastric/vagal anatomy may limit their utility. Swine may be a more appropriate animal model because of anatomic and physiologic similarities to humans. The aim of this study was to establish a survival model of Roux-en-Y gastric bypass (RYGBP) in swine and to evaluate its potential in studies of physiology.

METHODS

13 miniature swine, 5 Yucatan [26.4 +/- 1.6 kg], 4 Hanford [28.3 +/- 0.6 kg] and 4 other breed [54.9 +/- 6.2 kg] underwent open RYGBP, and were kept alive to 30 (n=4), 60 (n=1) or 90 (n=2) postoperative days.

RESULTS

4 early animals had staple-line leakage within 7 days from surgery and 1 animal experienced unmanageable pain at 42 days after surgery. One animal experienced immediate cardiopulmonary collapse. 58% of animals survived to their projected endpoint. Necropsy of 1 animal at its 90-day endpoint revealed a gastro-gastric fistula. Anatomic features in swine that differ from humans, such as thick perigastric membranes, required adjustment to the standard RYGBP technique used in humans to achieve satisfactory results. Caloric intake decreased in some but not all animals, and was linked to feeding regimen. By postoperative day 30, animals weighed 5.7-29.1% less than their projected, non-operative weight. Serum assays of ghrelin and PYY were conducted, with results consistent with the procedure.

CONCLUSIONS

The use of swine as a model for bariatric surgery has promise, but also has associated pitfalls that must be addressed for this to be an effective model.

Peptide YY: a neuroendocrine neighbor of note

Endocrine cells, enteric neurons and enterocytes provide an integrated functional defense against luminal factors, including nutrients, microbes and toxins. Prominent among intrinsic mediators is peptide YY (PYY) which is present in approximately 50% of colorectal endocrine cells and neuropeptide Y (NPY), a neurotransmitter expressed in submucous and myenteric nerves. Both peptides and their long fragments (PYY(3-36) and NPY(3-36)) are potent, long-lasting anti-secretory agents in vitro and in vivo and, they provide significant Y receptor-mediated absorptive tone in human and mouse colon mucosa. The main function of the colon is to absorb 90% of approximately 2l of daily ileal effluent (in adult humans) and Y-absorptive tone can contribute significantly to this electrolyte absorption. Blockade or loss of this mucosal Y-absorptive tone (i.e. with Y(1) or Y(2) antagonists) leads to hypersecretion and potentially to diarrhea, so Y agonists are predicted to rescue absorption by mimicking endogenous neuroendocrine PYY or neuronal NPY.

Glucagon-like peptide-1 secretion is influenced by perfusate glucose concentration and by a feedback mechanism involving somatostatin in isolated perfused porcine ileum

Glucagon-like peptide-1 (GLP-1) is released from intestinal L-cells in response to ingestion of meals. The mechanisms regulating its secretion are not clear, but local somatostatin (SS) restrains GLP-1 secretion. We investigated feedback and substrate regulation of GLP-1 and SS secretion, using isolated perfused porcine ileum (n=17). Effluents were measured for GLP-1 and SS. Perfusion pressure and motility were recorded. Investigated parameters included spontaneous fluctuations, changes in perfusate glucose concentrations (3.5, 5, 11 mM) and addition of insulin (1 nM). We also investigated the effect of proglucagon products, glucagon (10 nM), GLP-1 and GLP-2 (0.1, 1, and 10 nM) on GLP-1 and SS secretion, as well as on glucagon-like peptide-2 (GLP-2), peptide YY (PYY) and GIP secretion, all possible product of L-cells or neighbour cells. Perfusate glucose concentration dose-dependently stimulated GLP-1 secretion (p=0.011). Insulin had no effect. Glucagon weakly stimulated GIP secretion. GLP-1 stimulated SS secretion and motor activity, but inhibited GLP-2, GIP and PYY secretion and perfusion pressure. GLP-2 weakly stimulated SS secretion. We conclude (a) that GLP-1 secretion is influenced by perfusate glucose concentration and (b) that L-cell secretion is feedback regulated by GLP-1 itself, probably via paracrine SS activity.

GLP-1 and GIP are colocalized in a subset of endocrine cells in the small intestine

BACKGROUND

The incretin hormones GIP and GLP-1 are thought to be produced in separate endocrine cells located in the proximal and distal ends of the mammalian small intestine, respectively.

METHODS AND RESULTS

Using double immunohistochemistry and in situ hybridization, we found that GLP-1 was colocalized with either GIP or PYY in endocrine cells of the porcine, rat, and human small intestines, whereas GIP and PYY were rarely colocalized. Thus, of all the cells staining positively for either GLP-1, GIP, or both, 55-75% were GLP-1 and GIP double-stained in the mid-small intestine. Concentrations of extractable GIP and PYY were highest in the midjejunum [154 (95-167) and 141 (67-158) pmol/g, median and range, respectively], whereas GLP-1 concentrations were highest in the ileum [92 (80-207) pmol/l], but GLP-1, GIP, and PYY immunoreactive cells were found throughout the porcine small intestine.

CONCLUSIONS

Our results provide a morphological basis to suggest simultaneous, rather than sequential, secretion of these hormones by postprandial luminal stimulation.

Central and peripheral regulation of gastric acid secretion by peptide YY

Peptide YY (PYY) released postprandially from the ileum and colon displays a potent inhibition of cephalic and gastric phases of gastric acid secretion through both central and peripheral mechanisms. To modulate vagal regulation of gastric functions, circulating PYY enters the brain through the area postrema and the nucleus of the solitary tract, where it exerts a stimulatory action through PYY-preferring Y1-like receptors, and an inhibitory action through Y2 receptors. In the gastric mucosa, PYY binds to Y1 receptors in the enterochromaffin-like cells to inhibit gastrin-stimulated histamine release and calcium signaling via a pertussis toxin-sensitive pathway.

Multiple regulation of peptide YY secretion in the digestive tract

In the last two decades, multiple aspects of the peptide YY (PYY) secretion have been investigated. Besides fat and fatty acids, many luminal nutrients in the distal intestine appear to induce PYY release. Some studies have shown that bile acid, but not nutrients, plays a crucial role in the regulation of PYY secretion. Moreover, chyme in the proximal intestine also regulates the peptide release by indirect action through humoral and neuronal factors. Gastrin, cholecystokinin, and the vagus nerve are major candidates for mediators of these indirect actions. Several growth factors have been shown to regulate PYY synthesis in mucosa of the distal intestine. This review is aimed at presenting an overview of these recent studies on PYY secretion in the distal intestine.

Secretion of trophic gut peptides is not different in bolus- and continuously fed piglets

In neonates, bolus feeding is associated with greater rates of intestinal growth than is continuous feeding. We tested whether the concentrations and secretion rates of trophic gut peptides are higher in bolus-fed than in continuously fed piglets. Five 21-d-old piglets were surgically implanted with gastric, arterial and portal catheters and a portal blood flow probe. At postnatal d 30 and 31, pigs received an equal amount of primed continuous or bolus feeding of a cow's milk formula in a randomized, crossover design. During a 6-h period, portal blood flow and arterial and portal concentrations of glucagon-like peptide-2 (GLP-2), peptide YY (PYY) and gastric inhibitory polypeptide (GIP) were measured. All hormone levels were significantly increased within 1 h of the start of the experiment, independent of the feeding modality. There were no differences between bolus and continuous feeding in either the arterial concentrations or secretion rates of GLP-2, PYY and GIP. In both treatment groups, the increases in the plasma concentrations of GLP-2 and GIP after feeding were substantially greater than those for PYY. We conclude that the production or circulating concentrations of GLP-2, PYY and GIP are not significantly different in bolus- and primed continuously fed piglets.

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.

Peptide YY selectively stimulates expression of the colonocytic phenotype

Peptide YY (PYY) is produced by colonic mucosal endocrine cells and modulates gastrointestinal endocrine activity through specific Y-receptors. The direct effects of PYY on intestinal mucosal growth and differentiation remain uncharacterized. The abundance of PYY in colonic mucosa suggests that PYY acts locally to maintain colonocytic differentiation. We tested this hypothesis in human Caco-2 intestinal epithelial cells, which express alkaline phosphatase (AP) and dipeptidyl dipeptidase (DP), brush-border enzymes differentially concentrated in large and small intestinal mucosa, respectively. The effects of PYY on enzyme specific activity were compared with those of pancreatic polypeptide, neuropeptide-Y, vasoactive intestinal peptide, pentagastrin, bombesin, and selective Y1- and Y2-receptor agonists. Brush-border enzyme activity was assessed by AP and DP specific activity in cell lysates quantitated spectrophotometrically following synthetic substrate digestion. PYY, neuropeptide-Y, pancreatic polypeptide, and vasoactive intestinal peptide (10(-7) mol/L) stimulated AP activity. PYY brought about the greatest increase (38.0%+/-11.0%, n=48). Only PYY decreased DP specific activity (7.9%+/-2.2%, n=48). The Y2-agonist but not the Y1-agonist mimicked these PYY effects (increasing AP 28.3%+/-3.5% and decreasing DP 10.4%+/-3.6%). These data suggest that PYY promotes differentiation toward a colonocytic phenotype in Caco-2 intestinal epithelial cells and that this effect may be mediated through the Y2-receptor subtype.

Release of peptide YY by neurotransmitters and gut hormones in the isolated, vascularly perfused rat colon

BACKGROUND

Peptide YY (PYY) is promptly released from endocrine cells of the distal part of the gut after food intake. To test the possibility that hormones produced by the proximal small intestine or transmitters of the enteric nervous system may take part in the early phase of meal-induced PYY release, various regulatory peptides and neurotransmitters of the gut were administered intra-arterially in the isolated, vascularly perfused rat colon.

METHODS

A colonic loop was perfused with a Krebs-Henseleit buffer containing 20% washed bovine erythrocytes via the superior mesenteric artery. The release of PYY in portal effluent was measured by radioimmunoassay.

RESULTS

Cholecystokinin and secretin produced a small release of PYY. In contrast, infusion of gastric inhibitory polypeptide (GIP) over the concentration range 0.25-1 nM for 30 min produced a dose-dependent secretion of PYY with a maximal response at 800% above basal. Tetrodotoxin (TTX) did not modify the GIP-induced PYY release. Bethanechol (10(-5) M, 10(-4) M) produced a PYY release that was maximal at the end of the 30-min infusion period. The beta-adrenergic agonist isoproterenol (10(-7) M, 10(-6) M) caused a prompt release of PYY, followed by a sustained release at a lower value. Calcitonin gene-related peptide (CGRP) (5.10(-9) M and 5.10(-8) M) induced a PYY release with kinetics similar to that found for isoproterenol. Finally, bombesin (10(-9)-10(-7) M) provoked a dose-dependent release of PYY, consisting of an early peak followed by a sustained response. TTX did not modify the bethanechol-, isoproterenol-, CGRP-, and bombesin-induced PYY secretion.

CONCLUSION

The hormonal peptide GIP and several transmitters of the nervous enteric system may mediate the release of PYY through the occupation of receptors possibly located at the surface of the colonic L-cells.

Effect of carbachol on the release of peptide YY from isolated vascularly and luminally perfused rat ileum

Possible cholinergic control on the release of PYY from intestine into the lumen or blood vessel was studied by radioimmunoassay in the isolated perfused rat ileum. The basal release of PYY into the lumen was 43.1 +/- 8.9 pg/min, which was comparable with that into the vasculature (35.2 +/- 2.6 pg/min). The administration of 1 microM carbachol into the vascular perfusate resulted in a more than 40-fold increase of the luminal release but only a twofold increase of the vascular release. Carbachol-induced release of PYY into both lumen and vasculature was completely blocked by atropine, but not by hexamethonium. Tetrodotoxin abolished carbachol-induced release of PYY into lumen and vasculature. These data suggest that the ileal PYY release, into either lumen or vasculature, is under the control of postganglionic cholinergic neurons via muscarinic receptors.

[Pro34]peptide YY is a Y1-selective agonist at peptide YY/neuropeptide Y receptors

We have investigated binding and functional effects of a new peptide YY analogue, [Pro34]peptide YY, at Y1 and Y2-like subtypes of receptors for peptide YY and neuropeptide Y. In binding studies [Pro34]peptide YY had a similarly high affinity as peptide YY to human Y1-like receptors in SK-N-MC cells, a human neuroblastoma cell line of presumed neurogenic origin, and HEL cells, a human cell line derived from a patient with Hodgkin's disease. In functional studies [Pro34]peptide YY stimulated Ca2+ elevations in both Y1-like receptor cell lines with similar potency and efficacy as peptide YY. In contrast to peptide YY [Pro34]peptide YY was 1000-fold less potent in binding to Y2-like receptors in porcine splenic membranes and lacked agonistic effects in another Y2-like receptor-mediated model system, i.e. inhibition of [3H]serotonin release from rat cerebral cortical slices. Thus, [Pro34]peptide YY is a highly Y1-selective full agonist of peptide YY/neuropeptide Y receptors. [Pro34]peptide YY could be useful for studying the importance of Y receptor subtypes in mediating peptide YY physiological actions.

Peripheral peptide YY induces c-fos-like immunoreactivity in the rat brain

The influence of peripheral injection of peptide YY (PYY) on neuronal activity in the rat brain was examined by immunohistochemical detection of c-fos protein. Numerous c-fos-immunoreactive nuclei were found in the area postrema, nucleus tractus solitarius (commissural and medial subnuclei), central amygdala and thalamus (periventricular and medial) of rats injected i.p. with PYY at a dose of 300 micrograms/kg. c-fos-like immunoreactivity was found to be less when lower doses of PYY (50-200 micrograms/kg, i.p.) were injected. Either no or few cells were detected after i.p. injection of the vehicle alone. These data provide anatomical support for the centrally mediated actions of peripheral PYY on gut function.

Enteroendocrine peptides in a canine model of orthotopic jejunoileal autotransplantation

The enteroendocrine cells of the small bowel provide a rich source of regulatory peptides involved in the modulation of gastrointestinal function. Recent work from our laboratory showed that in situ neural isolation (autotransplantation) of the jejunoileum produced marked changes in tissue expression of several neuropeptides. In the present study, we examined the influence of extrinsic innervation on the tissue expression of endocrine peptides localized to various regions of the gastrointestinal tract. Concentrations of immunoreactive gastric inhibitory polypeptide (GIP), neurotensin (NT) and peptide tyrosine tyrosine (PYY) in fasting plasma and regional tissue biopsies were determined before and at varying time points (2, 6, 12 weeks) after a model of canine orthotopic jejunoileal autotransplantation. GIP was not altered in plasma or tissue at any time point. Plasma concentrations of NT and PYY increased after autotransplantation. Following a decrease in tissue concentrations two weeks after autotransplantation, NT increased progressively from 2 to 6 to 12 weeks, reaching a maximal increase of 895% over baseline in proximal ileum. Tissue concentrations of PYY followed much the same pattern as NT, but these trends never achieved statistical significance. Chromatographic characterization of tissue biopsy extracts revealed molecular heterogeneity of NT-like immunoreactivity, while GIP and PYY immunoreactivity coeluted as single species with authentic standards. Taken together with our earlier observations, it appears that disruption of extrinsic and intrinsic neural continuity to the jejunoileum (autotransplantation) does not affect gut endocrine peptides such as GIP and PYY to the same extent as enteric neuropeptides. NT has been localized to neural as well as endocrine cells and is involved in the temporal adaptive response to autotransplantation.

Characterization of peptide YY receptors in rabbit colonic mucosa

Quantitative receptor autoradiography localized a high-affinity binding site for 125I-peptide YY to the mucosa in rabbit distal colon. Scatchard binding analysis revealed a single-affinity binding site (KD = 0.29 nM) with binding specificity similar to other peptide YY-preferring receptors (peptide YY > or = neuropeptide Y >> pancreatic polypeptide). Radioligand binding studies using colonic mucosal membranes confirmed high-affinity peptide YY binding sites (KD = 0.26 nM) with time, temperature, and protein dependence, as well as saturability characteristic of receptor-ligand binding. Selective peptide analogues showed a subpopulation of these 125I-peptide YY binding sites to resemble the Y1-type neuropeptide Y receptor. Peptide YY may exert local antisecretory effects on the colonic epithelium via these binding sites.

Extrinsic neural contribution to ileal peptide YY (PYY) release

Peptide YY (PYY) release into the ileal lumen is stimulated by cholecystokinin (CCK) and glucose ingestion. Previous data have implicated vagal activity in the mediation of PYY release into both the systemic circulation and the ileal lumen. The present study was designed to evaluate extrinsic neural involvement in CCK and glucose-stimulated circulating and ileal intraluminal PYY release. Distal ileal Thiry-Vella loops (TVL) of 25 cm were created in seven mongrel dogs. On separate days fasted dogs were given continuous infusions of CCK at 500 ng/kg/hr during the first hour of the study or an oral glucose (1.5 g/kg) tolerance test (OGTT) was performed. Peripheral blood samples and ileal effusates were collected before tests and following either CCK or glucose stimulation for 120 min at 20-min intervals. Ileal PYY recoveries were measured by the instillation and collection of 20 cc of normal saline from the TVL for each 20-min period. The dogs were again tested after surgical denervation of the TVL. OGTT resulted in a significant rise of PYY recovery from the TVL (P less than 0.05), while not affecting circulating PYY. Intravenous CCK resulted in significant increases in both plasma and ileal PYY concentrations (P less than 0.05). Denervation of the TVL decreased PYY recovery from the TVL after both CCK and OGTT, whereas this procedure did not affect circulating PYY levels or basal luminal levels. These data demonstrate the inhibition of CCK- and glucose-stimulated ileal PYY recovery from denervated ileal loops. The extrinsic neural pathways are involved in the mediation of glucose- and CCK-stimulated mechanisms for ileal PYY release.

Vagal cooling blocks circulating neuropeptide Y (NPY), peptide YY (PYY), and pancreatic polypeptide (PP) release

Neuropeptide Y (NPY), peptide YY (PYY), and pancreatic polypeptide (PP) are regulatory peptides that constitute a new family of gastrointestinal and neural peptides. The influence of vagal integrity on NPY, PYY, and PP basal and postprandial release was evaluated using a new technique of reversible cryogenic cervical vagal blockade in an awake canine model. Cooling coils were placed around bilateral cervical vagal trunks in five dogs along with omocervical arterial catheters. Vagal transmission was monitored by pulse and arterial pressure monitoring. Cryogenic blockade of vagal nerves was performed by circulating a mixture of 0 degrees C ethanol and water through the cooling coils. NPY, PYY, and PP were measured using standard radioimmunoassays. Vagal cooling decreased basal NPY and PYY levels (P less than 0.05) but not PP. After a standard meal, vagal cooling blocked the postprandial rise seen in circulating NPY and PP (P less than 0.05). These data demonstrate a technique of reversible vagal blockade to evaluate the role of cervical vagal integrity in gastrointestinal endocrinology. Cryogenic vagal blockade inhibits the postprandial rise of circulating PP into the circulation. Vagal pathways appear to contribute to fasting activity of PYY and NPY releasing cells. Inhibited meal-stimulated release of NPY supports a role for vagal modulation of postprandial NPY release into the circulation.