Effects of atrial natriuretic peptide in the gut

Guanylyl cyclase/natriuretic peptide receptor-A: Identification, molecular characterization, and physiological genomics

The natriuretic peptides (NPs) hormone family, which consists mainly of atrial, brain, and C-type NPs (ANP, BNP, and CNP), play diverse roles in mammalian species, ranging from renal, cardiac, endocrine, neural, and vascular hemodynamics to metabolic regulations, immune responsiveness, and energy distributions. Over the last four decades, new data has transpired regarding the biochemical and molecular compositions, signaling mechanisms, and physiological and pathophysiological functions of NPs and their receptors. NPs are incremented mainly in eliciting natriuretic, diuretic, endocrine, vasodilatory, and neurological activities, along with antiproliferative, antimitogenic, antiinflammatory, and antifibrotic responses. The main locus responsible in the biological and physiological regulatory actions of NPs (ANP and BNP) is the plasma membrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), a member of the growing multi-limbed GC family of receptors. Advances in this field have provided tremendous insights into the critical role of Npr1 (encoding GC-A/NPRA) in the reduction of fluid volume and blood pressure homeostasis, protection against renal and cardiac remodeling, and moderation and mediation of neurological disorders. The generation and use of genetically engineered animals, including gene-targeted (gene-knockout and gene-duplication) and transgenic mutant mouse models has revealed and clarified the varied roles and pleiotropic functions of GC-A/NPRA in vivo in intact animals. This review provides a chronological development of the biochemical, molecular, physiological, and pathophysiological functions of GC-A/NPRA, including signaling pathways, genomics, and gene regulation in both normal and disease states.

Input-output signal processing plasticity of vagal motor neurons in response to cardiac ischemic injury

Vagal stimulation is emerging as the next frontier in bioelectronic medicine to modulate peripheral organ health and treat disease. The neuronal molecular phenotypes in the dorsal motor nucleus of the vagus (DMV) remain largely unexplored, limiting the potential for harnessing the DMV plasticity for therapeutic interventions. We developed a mesoscale single-cell transcriptomics data from hundreds of DMV neurons under homeostasis and following physiological perturbations. Our results revealed that homeostatic DMV neuronal states can be organized into distinguishable input-output signal processing units. Remote ischemic preconditioning induced a distinctive shift in the neuronal states toward diminishing the role of inhibitory inputs, with concomitant changes in regulatory microRNAs miR-218a and miR-495. Chronic cardiac ischemic injury resulted in a dramatic shift in DMV neuronal states suggestive of enhanced neurosecretory function. We propose a DMV molecular network mechanism that integrates combinatorial neurotransmitter inputs from multiple brain regions and humoral signals to modulate cardiac health.

Expression, Localization, and Effect of High Salt Intake on Electroneutral Na+/HCO3 - Cotransporter NBCn2 in Rat Small Intestine: Implication in Intestinal NaCl Absorption

The electroneutral Na+/HCO3 - cotransporter NBCn2 (SLC4A10) of solute carrier family 4 (SLC4) plays important physiological and pathological roles in the body. Our previous study showed that NBCn2 is expressed on the protein level in the small intestine of rat. Here, by reverse-transcription polymerase chain reaction (PCR), we identified a novel full-length NBCn2 variant, i.e., NBCn2-K, from rat small intestine. By pHi measurement with Xenopus oocytes, the activity of NBCn2-K is not significantly different from NBCn2-G. By western blotting, NBCn2 and the Na+/H+ exchanger NHE3 (SLC9A3) are predominantly expressed in the jejunum of rat small intestine. By immunofluorescence, NBCn2 and NHE3 are localized at the apical domain of the jejunum. NaCl overload decreases the expression of NBCn2 by 56% and that of NHE3 by 40% in the small intestine. We propose that NBCn2 is involved in the transepithelial NaCl absorption in the small intestine, and that the down-regulation of NBCn2 by NaCl represents an adaptive response to high salt intake in rat.

Transcriptional networks in rodent models support a role for gut-brain communication in neurogenic hypertension: a review of the evidence

Hypertension (HTN) is the most prevalent condition observed in primary health care. Hypertension shows complex etiology, and neuroinflammation, overactive sympathetic drive, and the microbiome are each associated with the disease. To obtain mechanistic perspective into neurogenic HTN, we first constructed a framework for transcriptional regulators of the disease using the Comparative Toxicogenomics Database. This approach yielded a core group of 178 transcripts that are prevalent in studies of HTN, including leptin and neuropeptide Y. We then conducted a meta-analysis for transcriptome data generated in brain tissue from HTN studies. Eight expression studies were reanalyzed, in which transcriptomics was conducted in hypertensive animal models [spontaneously hypertensive rats (SHR) and high blood pressure (BPH/2J) Schlager mice] (140 microarrays). Most strikingly, a gut-brain connection was a dominant theme in both rodent models of HTN. The transcriptomic data in the rat CNS converged on processes that included gastrointestinal motility and appetite, among others. In the mouse model, pathways converged on gastrointestinal transit. Thus, our data provide a powerful review of current molecular evidence of the interplay between gut and brain in HTN. Analyses of meta-genome data also suggested that transcriptome networks related to natriuresis, thermoregulation, reproduction (lactation and pregnancy), and vasoconstriction were associated to HTN, supporting physiological observations in independent studies by others. Lastly, we present novel transcriptome networks that may contribute to a neurogenic origin of HTN. Using this framework, new therapeutic targets can be proposed and investigated in treatment strategies.

Modulation in Natriuretic Peptides System in Experimental Colitis in Rats

BACKGROUND

Renin-angiotensin system is involved in the pathophysiology of colonic inflammation. However, there are a few reports about modulation of natriuretic peptide system.

AIMS

This study investigates whether a local atrial natriuretic peptide (ANP) system exists in rat colon and whether ANP plays a role in the regulation of colonic motility in experimental colitis rat model.

METHODS

Experimental colitis was induced by an intake of 5 % dextran sulfate sodium (DSS) dissolved in tap water for 7 days. After rats were killed, plasma hormone concentrations and mRNAs for natriuretic peptide system were measured. Functional analysis of colonic motility in response to ANP was performed using taenia coli.

RESULTS

DSS-treated colon showed an increased necrosis with massive infiltration of inflammatory cells. The colonic natriuretic peptide receptor-A mRNA level and particulate guanylyl cyclase activity in response to ANP from colonic tissue membranes were higher, and the mRNA levels of ANP and natriuretic peptide receptor-B were lower in DSS-treated rats than in control rats. ANP decreased the frequency of basal motility in a dose-dependent manner but did not change the amplitude. The inhibitory responses of frequency of basal motility to ANP and 8-bromo-cGMP were enhanced in DSS-treated rat colon.

CONCLUSION

In conclusion, augmentation of inhibitory effect on basal motility by ANP in experimental colitis may be due an increased expression of colonic natriuretic peptide receptor-A mRNA. These data suggest that local natriuretic peptide system is partly involved in the pathophysiology of experimental colitis.

Ideal target arterial pressure after control of bleeding in a rabbit model of severe traumatic hemorrhagic shock: results from volume loading-based fluid resuscitation

BACKGROUND

Previously reported ideal target mean arterial pressure (MAP) after control of bleeding in traumatic hemorrhagic shock (THS) requires further verification in more clinically related models. The authors explored this issue via gradient volume loading without vasopressor therapy. As certain volume loading can induce secretion of atrial natriuretic peptide (ANP), which has been shown to be protective, the authors also observed its potential role.

MATERIALS AND METHODS

Fifty male New Zealand rabbits were submitted to 1.5 h of uncontrolled THS (with another eight rabbits assigned to the sham group). After bleeding control, treated rabbits were randomly (n = 10, respectively) resuscitated with blood and Ringer lactate (1:2) to achieve target MAP of 50, 60, 70, 80, and 90 mm Hg within 1 h. During the following 2 h, they were resuscitated toward baseline MAP. Rabbits were observed until 7 h.

RESULTS

After resuscitation, infused fluid was lower and oxidative stress injury was milder in the 70 mm Hg group. Fluid volume loaded during the initial hour after hemostasis was negatively correlated with pH, oxygen saturation, and base excess at the end of resuscitation. It also correlated positively with proinflammatory responses in bronchoalveolar lavage fluid at 7 h and 7-h mortality. Moreover, after volume loading, the 80 mm Hg group showed significantly increased serum ANP level, which correlated with the expression of Akt protein in the jejunum at 7 h.

CONCLUSIONS

In rabbits the ideal target MAP during the initial resuscitation of severe THS after hemostasis was 70 mm Hg. ANP may have a critical role in gut protection.

Regulation of electroneutral NaCl absorption by the small intestine

Na(+) and Cl(-) movement across the intestinal epithelium occurs by several interconnected mechanisms: (a) nutrient-coupled Na(+) absorption, (b) electroneutral NaCl absorption, (c) electrogenic Cl(-) secretion by CFTR, and (d) electrogenic Na(+) absorption by ENaC. All these transport modes require a favorable electrochemical gradient maintained by the basolateral Na(+)/K(+)-ATPase, a Cl(-) channel, and K(+) channels. Electroneutral NaCl absorption is observed from the small intestine to the distal colon. This transport is mediated by apical Na(+)/H(+) (NHE2/3) and Cl(-)/HCO(3)(-) (Slc26a3/a6 and others) exchangers that provide the major route of NaCl absorption. Electroneutral NaCl absorption and Cl(-) secretion by CFTR are oppositely regulated by the autonomic nerve system, the immune system, and the endocrine system via PKAα, PKCα, cGKII, and/or SGK1. This integrated regulation requires the formation of macromolecular complexes, which are mediated by the NHERF family of scaffold proteins and involve internalization of NHE3. Through use of knockout mice and human mutations, a more detailed understanding of the integrated as well as subtle regulation of electroneutral NaCl absorption by the mammalian intestine has emerged.

Atrial natriuretic peptide signal pathway upregulated in stomach of streptozotocin-induced diabetic mice

AIM

To investigate atrial natriuretic peptide (ANP) secretion from gastric mucosa and the relationship between the ANP/natriuretic peptide receptor type A (NPR-A) pathway and diabetic gastroparesis.

METHODS

Male imprinting control region (ICR) mice (4 wk old) were divided into two groups: control mice, and streptozotocin-induced diabetic mice. Eight weeks after injection, spontaneous gastric contraction was recorded by using physiography in control and streptozotocin-induced diabetic mice. The ANP-positive cells in gastric mucosa and among dispersed gastric epithelial cells were detected by using immunohistochemistry and flow cytometry, respectively. ANP and natriuretic peptide receptor type A (NPR-A) gene expression in gastric tissue was observed by using the reverse transcriptase polymerase chain reaction.

RESULTS

The frequency of spontaneous gastric contraction was reduced from 12.9 +/- 0.8 cycles/min in the control group to 8.4 +/- 0.6 cycles/min in the diabetic mice (n = 8, P < 0.05). However, the amplitude of contraction was not significantly affected in the diabetic group. The depletion of interstitial cells of Cajal in the gastric muscle layer was observed in the diabetic mice. ANP-positive cells were distributed in the gastric mucosal layer and the density index of ANP-positive cells was increased from 20.9 +/- 2.2 cells/field in control mice to 51.8 +/- 2.9 cells/field in diabetic mice (n = 8, P < 0.05). The percentage of ANP-positive cells among the dispersed gastric epithelial cells was increased from 10.0% +/- 0.9% in the control mice to 41.2% +/- 1.0% in the diabetic mice (n = 3, P < 0.05). ANP and NPR-A genes were both expressed in mouse stomach, and the expression was significantly increased in the diabetic mice.

CONCLUSION

These results suggest that the ANP/NPR-A signaling pathway is upregulated in streptozotocin-induced diabetic mice, and contributes to the development of diabetic gastroparesis.

Atrial natriuretic Peptide in young and elderly children with mild gastroenteritis

Objective. Atrial Natriuretic Peptide (ANP) has natriuretic and diuretic effects, synthesized and stored in the atrial cells, released in response to stretch of the atrial muscle during increase venous return. Acute gastroenteritis (AGE) causes dehydration. We intend to determine whether the decrease in venous return due to dehydration would lead to a decrease in ANP levels. Patients and Methods. This is a prospective observational controlled study. Blood collected from 30 children with AGE and ANP's levels were compared with 25 controls. ANP levels were determined by radioimmunoassay. Results. The study group was in mild dehydration. As a significant difference was found in ANP levels between children in the 3mo–3y group and older children 3y–14y. We analyzed the results according to age. No difference was found between children with AGE and control, in the 3mo–3y, ANP was 12.1 ± 11 pg/ml versus 13.4 ± 12 pg/ml respectively, and 3 ± 2 versus 3.8 ± 3 pg/ml in the 3y–14y groups, respectively. Conclusion. Dehydration due to AGE does not change the ANP's plasma levels. A weak positive correlation between sodium levels and ANP was found r = 0.29. The significant finding of our study is the difference in ANP levels related to age, in the control as well as the GE group.

Natriuretic peptides as regulatory mediators of secretory activity in the digestive system

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are members of the natriuretic peptide family best known for their role in blood pressure regulation. However, in recent years all the natriuretic peptides and their receptors have been described in the gastrointestinal tract, digestive glands and central nervous system, as well as implicated in the regulation of digestive gland functions. The current review highlights the regulatory role of ANP and CNP in pancreatic and other digestive secretions. ANP and CNP stimulate basal as well as induced pancreatic secretion and modify bicarbonate and chloride secretions. Whereas ANP and CNP exert effects directly on pancreatic cells, CNP also acts through a vago-vagal reflex. At high doses both peptides attenuate pancreatic secretion induced by high doses of secretin through the PLC/PKC pathway. With regards to other digestive secretions, ANP and CNP decrease bile secretion in the rat. ANP does not induce salivation by itself but enhances stimulated salivary secretion and modifies salivary composition in rat parotid as well as submandibular glands. In rat pancreatic, hepatic, parotid and submandibular tissues, the NPR-C receptor mediates mostly peripheral responses whereas NPR-A and NPR-B receptors, which are coupled to guanylate cyclase, likely mediate the central response. In addition, ANP modulates gastric acid secretion via a vagal-dependent mechanism. In the intestine, ANP and CNP decrease water and sodium chloride absorption through an increase in cGMP levels. Overall, these findings indicate that ANP and CNP are members of the large group of regulatory peptides affecting digestive secretions.

The intestinal guanylin system and seawater adaptation in eels

Guanylin and uroguanylin are principal intestinal hormones secreted into the lumen to regulate ion and water absorption via a specific receptor, guanylyl cyclase-C (GC-C). As the intestine is an essential organ for seawater (SW) adaptation in teleost fishes, the intestinal guanylin system may play a critical role in SW adaptation. Molecular biological studies identified multiple guanylins (guanylin, uroguanylin and renoguanylin) and their receptors (GC-C1 and GC-C2) in eels. The relative potency of the three ligands on cGMP production in transiently expressed receptors was uroguanylin > guanylin >or= renoguanylin for CG-C1 and guanylin >or= renoguanylin > uroguanylin for GC-C2. Eel guanylin and GC-C genes are expressed exclusively in the intestine and kidney, and the level of expression is greater in SW eels than in freshwater (FW) eels except for renoguanylin. Physiological studies using Ussing chambers showed that the middle and posterior intestine are major sites of action of guanylins, where they act on the mucosal side to decrease short circuit current (I(sc)) in a dose-dependent manner. The ID(50) of guanylins for transport inhibition was 50-fold greater than that of atrial natriuretic peptide that acts from the serosal side as an endocrine hormone. However, only guanylins reversed I(sc) to levels below zero. Pharmacological analyses using various blockers showed that among transporters and channels localized on the intestinal cells of SW teleost fish, the cystic fibrosis transmembrane conductance regulator Cl(-) channel (CFTR) on the apical membrane is the major target of guanylins. Collectively, guanylins are synthesized locally in the intestine and secreted into the lumen to act on the GC-Cs in the apical membrane of eel intestinal cells. Then, intracellular cGMP production after ligand-receptor interaction activates CFTR and probably induces Cl(-) and/or HCO3- secretion into the lumen as suggested in mammals. The physiological significance of the anion secretion induced by the luminal guanylin/GC-C system on SW adaptation may rival or exceed that of the serosally derived natriuretic peptides in the euryhaline eel.

Localization of ANP-synthesizing cells in rat stomach

AIM: To study the morphological positive expression of antrial natriuretic peptide (ANP)-synthesizing cells and ultrastructural localization and the relationship between ANP-synthesizing cells and microvessel density in the stomach of rats and to analyze the distribution of the three histologically distinct regions of ANP-synthesizing cells.

METHODS: Using immunohistochemical techniques, we studied positive expression of ANP-synthesizing cells in rat stomach. A postembedding immunogold microscopy technique was used for ultrastructural localization of ANP-synthesizing cells. Microvessel density in the rat stomach was estimated using tannic acid-ferric chloride (TAFC) method staining. Distribution of ANP-synthesizing cells were studied in different regions of rat stomach histochemically.

RESULTS: Positive expression of ANP-synthesizing cells were localized in the gastric mucosa of rats. Localization of ANP-synthesizing cells identified them to be enterochrochromaffin cells (EC) by using a postembedding immunogold electron microscopy technique. EC cells were in the basal third of the cardiac mucosa region. ANP-synthesizing cells existed in different regions of rat stomach and its density was largest in the gastric cardiac region, and the distribution order of ANP-synthesizing cells in density was cardiac region, pyloric region and fundic region in mucosa layer. We have also found a close relationship between ANP-synthesizing cells and microvessel density in gastric mucosa of rats using TAFC staining.

CONCLUSION: ANP-synthesizing cells are expressed in the gastric mucosa. EC synthesize ANP. There is a close relationship between ANP-synthesizing cells and microvessel density in gastric mucosa of rats.The distribution density of ANP-synthesizing cells is largest in the gastric cardiac region.

Integrative approach to osmoregulatory action of atrial natriuretic peptide in seawater eels

Atrial natriuretic peptide (ANP) reduces plasma Na+ concentration and promotes seawater (SW) adaptation in SW eels. However, little is known about the mechanisms for the hyponatremic effect of ANP. In order to evaluate the role of ANP in the whole-body Na+ homeostasis of marine teleost, we reviewed previous in vivo experiments using exogenously administered ANP and present additional experiments to assess the role of endogenous ANP in Na+ homeostasis in conscious SW eels. The Na+ influx and efflux rate across the body surfaces including the gills measured with isotopic 22Na were not altered by the hyponatremic dose (5 pmol kg-1 min-1) of ANP infusion in SW eels. ANP infusion also had no effect on renal Na+ excretion in SW eels. In contrast, ANP strongly inhibited drinking, and the inhibition was quantitatively correlated with the hyponatremic effect of ANP. Further, intestinal absorption of Na+ was inhibited by ANP as examined in situ using intestinal sac in conscious SW eels. The combined inhibitory actions of ANP on drinking and intestinal absorption were sufficient to explain the decrease in plasma Na+ concentration. In addition, removal of endogenous circulating ANP by immunoneutralization increased plasma Na+ concentration with a concomitant increase in drinking rate in SW eels. These results strongly suggest that endogenous ANP is involved in the hyponatremic regulation through actions on drinking, and probably on intestine, in SW eels.

Involvement of drinking and intestinal sodium absorption in hyponatremic effect of atrial natriuretic peptide in seawater eels

Atrial natriuretic peptide (ANP) decreases plasma Na+ concentration and promtes seawater (SW) adaptation in eels. The hyponatremia may most probably be caused by increased branchial extrusion of Na+, but the mechanism has not been determined yet. The present study examined initially the effects of ANP on branchial Na+ efflux in vivo using isotopic 22Na. However, the efflux rate was not altered by infusion of a hyponatremic dose of ANP (5 pmol.kg(-1).min(-1)). Therefore, we sought to examine whether the ANP-mediated hyponatremia is caused by a decrease in the uptake of Na+ from the environment. Since a decrease in drinking was highly correlated with a degree of hyponatremia, conscious SW eels were infused with dilute SW into the stomach at a normal drinking rate to offset the antidipsogenic effect of ANP. Under this regimen, the hyponatremic effect of ANP was abolished. Then, we examined the site of Na+ absorption in the alimentary tract by measuring the changes in ion composition of intraluminal fluid along the tract. Since Na+ was absorbed at the esophagus and anterior/middle intestine, a sac was prepared at each site and the effects of ANP were examined in situ in conscious SW eels. ANP infusion did not alter Na+ absorption at the esophagus, but it profoundly reduced the absorption at the intestine. Together with our previous finding that ANP does not alter renal Na+ excretion, we propose that ANP reduces plasma Na+ concentration in SW eels by inhibiting drinking and subsequent absorption of Na+ by the intestine.

Inhibitory effect of sildenafil on rat duodenal contractility In vitro: Putative cGMP involvement

1. Sildenafil citrate (Viagratrade mark; Pfizer, Sandwich, Kent, UK), a phosphodiesterase 5 inhibitor, rises cGMP levels in smooth muscle cells. It relaxes both vascular and visceral smooth muscle. In order to assess the intestinal effects of sildenafil, we decided to investigate its actions on rat duodenal motor activity in vitro. 2. In isolated duodenal segments maintained in Tyrode's solution, sildenafil exhibited a concentration-dependent antispasmodic effect on acetylcholine (ACh)-induced phasic contractions, with an IC50 value of 26.7 micromol/L (95% confidence interval (CI) 2.0-55.3 micromol/L). 3. Sildenafil also relaxed the carbamylcholine (CCh)-induced sustained contraction with an IC(50) of 16.2 micromol/L (95% CI 9.5-27.6 micromol/L). Sildenafil produced significant additional relaxation of 25.2 +/- 8.1% of the CCh-induced contraction, beyond basal tone. 4. Sildenafil reduced the amplitude of spontaneous duodenal contractions with an EC50 of 9.6 micromol/L (95% CI 5.7-16.2 micromol/L). This effect was significantly more potent than the effects of zaprinast and papaverine, which also reduced duodenal contractions with EC50 values of 91.6 micromol/L (95% CI 46.0-182.2 micromol/L) and 78.5 micromol/L (95% CI 37.1-166.3 micromol/L), respectively. 5. In preparations treated previously with methylene blue (10 micromol/L) or 1H-[1,2,4]oxadiazolo(4,3-a)quinoxalin-1-one (ODQ; 10 micromol/L), the EC50 values for the sildenafil effect were significantly increased to 39.0 micromol/L (95% CI 23.9-63.4 micromol/L) and 43.8 micromol/L (95% CI 24.5-78.3 micromol/L), respectively. These values were significantly greater than those obtained with sildenafil alone. 6. In conclusion, sildenafil has myorelaxant and antispasmodic effects on rat duodenal segments in vitro. The contractile inhibitory effect of sildenafil on rat isolated duodenum is probably due to intracellular cGMP accumulation as a result of its decreased degradation.

Dendroaspis natriuretic peptide system and its paracrine function in rat colon

Dendroaspis natriuretic peptide (DNP), a 38-amino-acid peptide, was isolated from the venom of Green Mamba. It has structural and functional similarities to other members of the natriuretic peptide family. The purpose of this study was to determine whether DNP system is present in the rat colon and to define its biological functions. The serial dilution curve of extracts of colonic tissues was parallel to the standard curve of DNP and a major peak of molecular profile by HPLC was synthetic DNP. The concentration of DNP was 0.5 +/- 0.04 ng/g of colonic tissues. DNP as well as atrial natriuretic peptide and C-type natriuretic peptide caused dose-dependent increases in cGMP production in the purified membrane of colonic tissues. Three types of natriuretic peptide receptor mRNAs were detected using semi-quantitative RT-PCR. Functionally, synthetic DNP inhibited the spontaneous contraction of rat colonic circular muscle in a concentration-dependent manner. The potency appeared to be at least 10 times greater than that of CNP. Furthermore, DNP inhibited carbachol-induced muscle contraction, suggesting that it also can modulate the nerve regulation of colonic motility. This study demonstrates the presence of DNP system in rat colon and its function as a local regulator of colonic motility.

Clinical endocrinology and metabolism. Receptors for gut peptides

Most gut peptides exert their effects through G protein-coupled receptors, a family of about 700 membrane proteins, 87 of which are presently known to have peptide ligands. Three additional gut peptide receptors are not G protein-coupled receptors but regulate intracellular cyclic GMP accumulation. The aim of this review is to illustrate how the sequencing of the human genome and other recent advances in genomics has contributed to our understanding of the role of peptides and their receptors in gastrointestinal function. Recent discoveries include the identification of receptors for the peptides motilin and neuromedin U, and new physiological ligands for the PTH2 receptor, the CRF(2) receptor and the growth hormone secretagogue receptor. Knockout mice lacking specific peptide receptors or their ligands provide informative animal models in which to determine the functions of the numerous peptide-receptor systems in the gut and to predict which of them may be the most fruitful for drug development. Some peptide-receptor signalling systems may be more important in disease states than they are in normal physiology. For example, substance P, galanin, bradykinin and opioids play important roles in visceral pain and inflammation. Other peptides may have developmental roles: for example, disruption of endothelin-3 signalling prevents the normal development of the enteric nervous system and contributes to the pathogenesis of Hirschsprung disease.

Atrial natriuretic peptides elevate cyclic GMP levels in primary cultures of rat ependymal cells

The aim of this study was to examine the effect of atrial natriuretic peptides on primary cultures of ependymal cells, as measured by changes in intracellular levels of cyclic GMP. Incubation of ependymal cells with rat atrial natriuretic peptide-(1-28) (rANP) elicited a 30-fold increase in ependymal cGMP content within 1 min and more than a 100-fold increase within 10 min to a plateau value of approximately 30 pmol/mg protein. The C-type natriuretic peptide (CNP) elicited a similar increase in cGMP levels; however the maximal effect was observed within 1 min and the levels subsequently dropped by 90% to a low plateau within 10 min. A comparison of the concentration-response curves for rANP, human ANP-(1-28) (hANP) and CNP showed that rANP, hANP and CNP had similar effects, with regards to elevation of cGMP levels at high concentrations, but with differing EC50 values. These results demonstrate the presence of a heterogenous population of functional ANP receptors i n cultured ependymalcells suggesting that ANP may regulate specific ependymal cell activity.

Regulation of atrial natriuretic peptide secretion by cholinergic and PACAP neurons of the gastric antrum

Atrial natriuretic peptide (ANP) released from enterochromaffin cells helps regulate antral somatostatin secretion, but the mechanisms regulating ANP secretion are not known. We superfused rat antral segments with selective neural agonists/antagonists to identify the neural pathways regulating ANP secretion. The nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) stimulated ANP secretion; the effect was abolished by hexamethonium but doubled by atropine. Atropine's effect implied that DMPP activated concomitantly cholinergic neurons that inhibit and noncholinergic neurons that stimulate ANP secretion, the latter effect predominating. Methacholine inhibited ANP secretion. Neither bombesin nor vasoactive intestinal polypeptide stimulated ANP secretion, whereas pituitary adenylate cyclase-activating polypeptide (PACAP)-27, PACAP-38, and maxadilan [PACAP type 1 (PAC1) agonist] each stimulated ANP secretion. The PAC1 antagonist M65 1) abolished PACAP-27/38-stimulated ANP secretion; 2) inhibited basal ANP secretion by 28 +/- 5%, implying that endogenous PACAP stimulates ANP secretion; and 3) converted the ANP response to DMPP from 109 +/- 21% above to 40 +/- 5% below basal, unmasking the cholinergic component and indicating that DMPP activated PACAP neurons that stimulate ANP secretion. Combined atropine and M65 restored DMPP-stimulated ANP secretion to basal levels. ANP secretion in the antrum is thus regulated by intramural cholinergic and PACAP neurons; cholinergic neurons inhibit and PACAP neurons stimulate ANP secretion.