Atrial natriuretic factor modifies bile flow and composition in the rat

Atrial natriuretic factor stimulates efflux of cAMP in rat exocrine pancreas via multidrug resistance-associated proteins

BACKGROUND & AIMS

Atrial natriuretic factor (ANF) prevents increases in intracellular levels of cAMP that are induced by secretin in the exocrine pancreas. We investigated the contribution of cyclic adenosine monophosphate (cAMP) efflux to ANF inhibition of secretin signaling.

METHODS

Intracellular and extracellular cAMP were measured by radio-binding assays in isolated pancreatic acini exposed to secretin and other secretagogues, alone or with ANF. Levels of messenger RNA for multidrug resistance-associated protein (MRP)4, MRP5, and MRP8 were measured by real-time polymerase chain reaction. MRP4 was knocked down in AR42J cells by small interfering RNA. In vivo studies were performed in rats.

RESULTS

Pancreatic secretagogues increased levels of intracellular cAMP, but only secretin and vasoactive intestinal peptide promoted cAMP efflux; efflux was increased by ANF, through signaling via natriuretic peptide receptor-C and phospholipase C-protein kinase C. In time-course studies with active phosphodiesterases, levels of intracellular and extracellular cAMP increased earlier after the addition of secretin and ANF (1 min) than after the addition of secretin alone (3 min). Similar kinetic patterns occurred with a phosphodiesterase inhibitor. A probenecid-sensitive transporter mediated cAMP egression. The main cAMP transporter, MRP4, was expressed in AR42J cells and pancreas. cAMP egression occurred in AR42J cells exposed to secretin, but this response was reduced in cells that expressed MRP4 small interfering RNA. In rats, levels of cAMP in plasma and pancreatic juice increased after infusion with secretin alone or secretin plus ANF.

CONCLUSIONS

ANF signals via natriuretic peptide receptor-C coupled to the phospholipase C-protein kinase C pathway to increase secretin-induced efflux of cAMP, probably through MPR-4. Cyclic AMP extrusion might be a mechanism, in addition to phosphodiesterase action, to regulate intracellular cAMP levels in pancreatic acinar cells.

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.

Atrial natriuretic factor intracellular signaling in the rat submandibular gland

We previously reported that intravenously administered atrial natriuretic factor (ANF) induced no salivation but enhanced agonist-evoked secretion in submandibular glands. The gene expression of ANF and natriuretic peptide receptors (NPR) was later reported in the glands. In the present study we sought to establish the intracellular signalling mechanisms underlying ANF modulation of salivary secretion. Fasted rats were prepared with submandibular duct and femoral cannulation. Dose-response curves to methacholine (MC) and norepinephrine (NE) were performed in the presence of cANP (4-23 amide) (selective NPR-C agonist) and ANF. Local injection of the agonist or ANF-induced no salivation, but enhanced MC and NE-evoked secretion. ANF and cANP (4-23 amide) enhanced phosphoinositide turnover being the effect abolished by U73122 (PLC inhibitor). Further ANF and cANP (4-23 amide) decreased basal cAMP content but failed to affect isoproterenol or forskolin-evoked cAMP. ANF response was inhibited by pertussis toxin and mimicked by cANP (4-23 amide) strongly supporting NPR-C activation. ANF-induced cAMP reduction was abolished by PLC and PKC inhibitors. The content of cGMP was dose dependently stimulated by ANF but not modified by cANP (4-23 amide). These findings support that ANF through NPR-C receptors coupled to PLC activation and adenylyl cyclase inhibition interacts with sialogogic agonists in the submandibular gland to potentiate salivation.

Atrial natriuretic factor negatively modulates secretin intracellular signaling in the exocrine pancreas

We previously reported that atrial natriuretic factor (ANF) stimulates pancreatic secretion through NPR-C receptors coupled to PLC and potentiates secretin response without affecting cAMP levels. In the present study we sought to establish the intracellular signaling mechanism underlying the interaction between both peptides. In isolated pancreatic acini 100 nM ANF abolished cAMP accumulation evoked by any dose of secretin. Lower doses of ANF (1 fM, 1 pM, 1 and 10 nM) dose dependently reduced EC50 secretin-evoked cAMP. Although ANF failed to affect cAMP stimulated by amthamine (selective H2 agonist) or isoproterenol (beta-adrenergic agonist), it abolished VIP-induced cAMP formation. ANF inhibitory effect was prevented by U-73122 (PLC inhibitor) and GF-109203X (PKC inhibitor) but unaltered by PKG and nitric oxide synthase inhibition, supporting that the PLC/PKC pathway mediated the effect. ANF response was mimicked by cANP (4-23 amide) and abolished by pertussis toxin, strongly supporting NPR-C receptor activation. In vivo studies showed that ANF at 0.5 microg x kg(-1) x h(-1) enhanced secretion stimulated by 1 U x kg(-1) x h(-1) secretin but at 1 and 2 microg x kg(-1) x h(-1) it abolished secretin response. However, ANF at such doses failed to modify the secretion evoked by carbachol or CCK. Present results show that ANF negatively modulated secretin secretory response and intracellular signaling through the activation of NPR-C receptors coupled to the PLC/PKC pathway. Furthermore, the finding that ANF also inhibited VIP-evoked cAMP supports a selective modulation of class II G-protein coupled receptors by ANF. Present findings suggest that ANF may play a protective role by reducing secretin response to avoid overstimulation.

Atrial natriuretic factor stimulates exocrine pancreatic secretion in the rat through NPR-C receptors

Increasing evidence supports the role of atrial natriuretic factor (ANF) in the modulation of gastrointestinal physiology. The effect of ANF on exocrine pancreatic secretion and the possible receptors and pathways involved were studied in vivo. Anesthetized rats were prepared with pancreatic duct cannulation, pyloric ligation, and bile diversion into the duodenum. ANF dose-dependently increased pancreatic secretion of fluid and proteins and enhanced secretin and CCK-evoked response. ANF decreased chloride secretion and increased the pH of the pancreatic juice. Neither cholinergic nor adrenergic blockade affected ANF-stimulated pancreatic secretion. Furthermore, ANF response was not mediated by the release of nitric oxide. ANF-evoked protein secretion was not inhibited by truncal vagotomy, atropine, or Nomega-nitro-l-arginine methyl ester administration. The selective natriuretic peptide receptor-C (NPR-C) receptor agonist cANP-(4-23) mimicked ANF response in a dose-dependent fashion. When the intracellular signaling coupled to NPR-C receptors was investigated in isolated pancreatic acini, results showed that ANF did not modify basal or forskolin-evoked cAMP formation, but it dose-dependently enhanced phosphoinositide hydrolysis, which was blocked by the selective PLC inhibitor U-73122. ANF stimulated exocrine pancreatic secretion in the rat, and its effect was not mediated by nitric oxide or parasympathetic or sympathetic activity. Furthermore, CCK and secretin appear not to be involved in ANF response. Present findings support that ANF exerts a stimulatory effect on pancreatic exocrine secretion mediated by NPR-C receptors coupled to the phosphoinositide pathway.

NPR-C receptors are involved in C-type natriuretic peptide response on bile secretion

C-type natriuretic peptide (CNP) is a member of the natriuretic peptide family. Previous studies reported the presence of natriuretic peptide receptors and mRNA CNP in the liver. In the present work, we sought to establish the role of CNP in the regulation of bile secretion in the rat and the possible pathways involved.CNP diminished basal as well as bile salt-evoked bile flow and bile acid output in a dose-dependent manner. It also reduced the excretion of sodium, chloride, and potassium but did not modify bile pH or the excretion of phospholipids, total proteins, and glutathione. Neither parasympathetic nor sympathetic blockade abolished CNP inhibitory response on bile secretion. The selective NPR-C agonist, C-ANP-(4-23) amide, diminished bile flow and the co-administration of both peptides did not further decrease it. CNP did not alter mean arterial pressure or portal venous pressure at any given doses.CNP decreased bile acid-dependent flow without affecting bile acid-independent flow. The inhibitory effect of CNP did not involve the participation of the autonomic nervous system or hemodynamic changes. The participation of NPR-C receptors in CNP response is strongly supported by present findings. The present study shows that CNP modulates bile secretion in the rat, suggesting that CNP may be part of the large family of peptides involved in the regulation of gastrointestinal physiology.

Bile secretion is centrally regulated by C-type natriuretic peptide

1. Current evidence supports that C-type natriuretic peptide (CNP) is the brain natriuretic peptide. Natriuretic peptide receptors and mRNA CNP have been reported in the liver and in discrete areas and nucleus of the central nervous system involved in the regulation of gastrointestinal physiology. In the present work, we sought to establish the role of CNP in the central regulation of bile secretion in the rat and to delineate the possible pathways and mechanisms involved. 2. To examine the role of CNP on bile secretion, the peptide was applied in the brain lateral ventricle (1, 10, and 100 ng/microL) and bile samples were collected every 15 min for 60 min. The role of the autonomic nervous system in CNP response was assessed by atropine or combined phentolamine and propranolol administration. 3. Centrally applied CNP diminished basal as well as bile salt-evoked bile flow in a dose-dependent manner. CNP reduced bile acid output as well as sodium and potassium excretion, supporting CNP effect on bile acid-dependent flow. CNP also decreased chloride excretion and increased bile pH. The excretion of total glutathione was not affected by centrally applied CNP suggesting that this peptide does not alter bile acid-independent flow. Neither parasympathetic nor sympathetic blockade abolished CNP inhibitory response on bile secretion. Mean arterial pressure and portal venous pressure were not modified by CNP. 4. Present findings show that centrally applied CNP modulates bile secretion in a dose-dependent fashion. CNP alkalinized bile and reduced bile acid-dependent flow without affecting bile acid-independent flow. The inhibitory response of CNP on bile secretion was not mediated by the autonomic nervous system. Present findings give further support to the role of CNP as the brain natriuretic peptide.

Centrally applied atrial natriuretic factor diminishes bile secretion in the rat

Little is known about the role of centrally applied peptides in the regulation of bile secretion. We previously reported that the intravenous injection of atrial natriuretic factor (ANF) reduces bile acid dependent flow without affecting portal venous pressure in the rat. In the present work, we studied the effects of centrally applied ANF on bile secretion and the possible pathways involved. Rats were cannulated in the brain lateral ventricle for the administration of 1, 10 and 100 ng/microl ANF. After 1 week, the common bile duct was cannulated and bile samples were collected every 15 min for 60 min after the administration of ANF. The excretion rate of various biliary components was assessed. Bile secretion experiments were also performed after bilateral truncal vagotomy or atropine administration to evaluate the participation of a vagal pathway. In addition, the role of the sympathetic system was addressed by combined administration of propranolol and phentolamine. Centrally applied ANF did not modify blood pressure but diminished bile flow and bile acid output. It also reduced sodium and potassium secretion but did not modify protein or phospholipid excretion. Neither bilateral truncal vagotomy nor atropine administration abolished ANF response. Furthermore, combined administration of adrenergic antagonists did not alter ANF inhibitory effect on bile flow. In conclusion, centrally applied ANF reduced bile acid dependent flow not through a vagal or adrenergic pathway in the rat, suggesting the involvement of a peptidergic pathway.

Evidence for a novel natriuretic peptide receptor that prefers brain natriuretic peptide over atrial natriuretic peptide

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) exert their physiological actions by binding to natriuretic peptide receptor A (NPRA), a receptor guanylate cyclase (rGC) that synthesizes cGMP in response to both ligands. The family of rGCs is rapidly expanding, and it is plausible that there might be additional, as yet undiscovered, rGCs whose function is to provide alternative signalling pathways for one or both of these peptides, particularly given the low affinity of NPRA for BNP. We have investigated this hypothesis, using a genetically modified (knockout) mouse in which the gene encoding NPRA has been disrupted. Enzyme assays and NPRA-specific Western blots performed on tissues from wild-type mice demonstrate that ANP-activated cGMP synthesis provides a good index of NPRA protein expression, which ranges from maximal in adrenal gland, lung, kidney, and testis to minimal in heart and colon. In contrast, immunoreactive NPRA is not detectable in tissues isolated from NPRA knockout animals and ANP- and BNP-stimulatable GC activities are markedly reduced in all mutant tissues. However, testis and adrenal gland retain statistically significant, high-affinity responses to BNP. This residual response to BNP cannot be accounted for by natriuretic peptide receptor B, or any other known mammalian rGC, suggesting the presence of a novel receptor in these tissues that prefers BNP over ANP.

Atrial natriuretic peptide and guanylin-activated guanylate cyclase isoforms in human sweat glands

The ultracytochemical localization of membrane-bound guanylate cyclases A and C, stimulated by atrial natriuretic peptide and guanylin respectively, has been studied in human sweat glands. The results showed that the peptides stimulated guanylate cyclases A and C in both eccrine and apocrine glands. In the secretory cells, enzymatic activity was present on the plasma membranes and on intracellular membranes involved in the secretory mechanism. In eccrine glands, the cells of the excretory duct also presented enzymatic activity on the plasma membranes. In both glands, myoepithelial cells, surrounding the secretory cells, exhibited only guanylate cyclase A activity. These localizations of enzymatic activity suggest a role for both atrial natriuretic peptide and guanylin in regulating glandular secretion.

Nitric oxide and guanosine 3',5'-cyclic monophosphate stimulate bile secretion in isolated rat hepatocyte couplets, but not in isolated bile duct units

Nitric oxide (NO) and guanosine 3',5'-cyclic monophosphate (cGMP) have recently been shown to stimulate bile acid-independent bile flow in the isolated perfused rat liver (IPRL). However, the cellular origin and mechanisms of this choleresis have not yet been determined. To address these questions, we examined the effects of NO and cGMP on bile secretion in isolated rat hepatocyte couplets (IRHC) and in isolated bile duct units (IBDU), both of which are isolated cell systems in which cell polarity is maintained and secretion can be measured directly. Changes in the area of the canalicular and ductular lumens were determined in IRHC and IBDU, respectively, as indicators of the rate of fluid secretion using video microscopy. In addition, Cl-/HCO3- exchanger activity in IBDU was evaluated by measuring changes in intracellular pH (pHi) after Cl- removal/readmission by microfluorometric methods. In the presence of HCO3-, both the NO donor, S-nitroso-acetyl-penicillamine (SNAP), and the cell-permeant cGMP analogue, dibutyryl cGMP (DBcGMP), stimulated canalicular bile secretion (P <.05), as did the cell-permeant cAMP analogue, dibutyryl cAMP (DBcAMP) (P <.05). Removal of HCO3- from the buffer completely abolished the choleretic effects of DBcGMP, but had no effect on NO-induced choleresis. In contrast, secretion in IBDU was not stimulated following incubations with SNAP or DBcGMP over 30 minutes, whereas DBcAMP and secretin, a cholangiocyte secretagogue and cAMP agonist, both had a marked effect on ductular secretion over this same time interval (P <.05). SNAP also had no effect on Cl-/HCO3- exchanger activity in IBDU, and inhibition of endogenous NO synthesis by NG-monomethyl-L-arginine (L-NMMA) did not alter secretin-induced stimulation of ductular bile secretion and Cl-/HCO3- exchanger activity. In summary, NO and cGMP stimulate bile secretion exclusively at the the level of hepatocytes, whereas cAMP mediates choleresis at both hepatocyte and bile duct levels. These findings may have important implications for the regulation of ductular bile secretion by hormones and neuropeptides, as well as under pathological conditions with increased hepatic NO synthesis.

Atrial natriuretic factor-induced amylase output in the rat parotid gland appears to be mediated by the inositol phosphate pathway

In previous in vivo studies we have reported that atrial natriuretic factor enhanced induced salivary secretion and increased isoproterenol-induced amylase release in the rat suggesting that, ANF effect could be mediated by phosphatidylinositol hydrolysis. In the present work, the effect of ANF on rat parotid tissue incubated in vitro was investigated with the aim to assess whether the phosphoinositol pathway was involved in ANF intracellular signaling in the parotid gland. Results showed that ANF induced a dose dependent increase in amylase fractional release, which was lower than that evoked by any concentration of isoproterenol. Furthermore 100 nM ANF enhanced isoproterenol-evoked amylase release. The effect of ANF was not affected in the presence of propranolol suggesting the noninvolvement of the beta adrenergic receptor, which is the main stimulus for the output of the enzyme in the parotid gland. However, ANF increased phosphatidylinositol hydrolysis, which implies an increase in intracellular calcium, which is necessary for the achievement of maximal response in amylase release. This effect was abolished in the presence of neomycin supporting ANF direct stimulation of phospholipase C. These results suggest the involvement of the C type natriuretic peptide receptor coupled to phospholipase C in ANF evoked amylase release and ANF enhancement of the isoproterenol-induced output of the enzyme.

Atrial natriuretic factor modifies the composition of induced-salivary secretion in the rat

We have previously reported that although the atrial natriuretic factor (ANF) was not a sialogogic agonist, it enhanced cholinergic, alpha-adrenergic and peptidergic (substance P) stimulated salivation in the submaxillary and parotid gland of the rat. The purpose of the present work was to study whether ANF modified the composition of agonist-induced saliva in the rat. Results showed that in the submaxillary gland, ANF increased sodium and decreased potassium excretion when salivation was stimulated by methacholine (MC) or substance P (SP). However, when salivation was induced by methoxamine (MX), ANF only increased sodium excretion. On the other hand, in the parotid gland, ANF increased both sodium and potassium excretion when salivation was induced either by MC or SP but did not modify electrolyte output in MX induced salivary secretion. Protein output and amylase activity were not modified by the presence of ANF when the aforementioned sialogogic agonists were used to elicit salivation in either gland. Although ANF did not modify the volume of isoproterenol (IP) induced saliva, it increased protein output in both glands and it increased amylase activity in the parotid gland. The present results suggest that ANF may play a role in the modulation of salivary secretion in the parotid and submaxillary glands of the rat. ANF effect is likely to be mediated by modifications in the calcium level linked to phosphoinositide metabolism within the acinar and/or the ductal cells of the salivary glands.

Atrial natriuretic factor increases peritoneal dialysis efficiency in nephrectomized rats

The effect of atrial natriuretic factor (ANF) on peritoneal dialysis was studied in bilaterally nephrectomized rats. ANF was injected prior to every dialysis exchange and blood samples were obtained before the instillation of the dialysis solution and during the collection of dialysates. Urea, creatinine, potassium, and sodium were determined in both plasma and dialysates. Results showed that ANF increased the plasma clearance of all studied solutes, probably through vasodilation. Solute clearances showed a gradual increase with each dialysis exchange in both control and experimental animals. Therefore, ANF plasma levels were assayed before, during, and after peritoneal dialysis in a control group of nephrectomized rats to determine whether ANF plasma levels were modified during dialysis. Plasma ANF values were higher during and after peritoneal dialysis, though basal levels were similar to those of non-nephrectomized rats. These results suggest the release of endogenous ANF from the cardiac atria during peritoneal dialysis. The present results suggest that ANF may be of potential interest in the clinical field to increase the efficiency of peritoneal dialysis in man.

Quantitative modifications induced by angiotensin II on rat bile secretion

Angiotensin II (ANG II) effects on bile secretion were studied in the rat. ANG II (0.1 microgram/kg) was injected into the jugular vein every 30 min and bile samples of 30 min were collected for 120 min. Blood arterial and portal venous pressures were simultaneously recorded before and after the administration of ANG II. Results showed that ANG II decreased bile flow and the excretion of sodium, potassium, chloride and bile acids whereas it increased pH, bile osmolality and the excretion rate of bicarbonate and calcium. ANG II also led to a rapid increase in mean arterial pressure as well as portal venous pressure which reverted to control values within 1 min. The present results suggest that ANG II may modulate bile flow and the excretion rate of the different biliary constituents. We have previously investigated atrial natriuretic factor effects on bile secretion and although the atrial factor antagonizes most of ANG II biological actions, unexpectedly, ANG II effects on bile secretion were not opposite to those of atrial natriuretic peptide. The modifications induced by ANG II on bile secretion may play an important role in pathophysiological conditions such as hypertensive states with increased ANG II circulating levels.

Atrial natriuretic factor enhances induced salivary secretion in the rat

As atrial natriuretic factor (ANF) is intimately involved in water and electrolyte homeostasis, dose-response studies were performed in the parotid as well as submaxillary glands of the rat with increasing doses of the atrial peptide to investigate its possible role as a sialogogic agent. Dose-response studies were also performed in both salivary glands with different pharmacological agonists known to cause salivation in the rat (methacholine, noradrenaline, isoproterenol, methoxamine and substance P) in the absence and in the presence of ANF. The atrial factor did not induce salivation 'per se' at least in the investigated doses. However, it enhanced the salivary response to methacholine, methoxamine and substance P but it did not modify the salivation induced either by noradrenaline or isoproterenol. The present results showed that ANF enhanced the salivation induced by pharmacological agents which stimulate phosphatidylinositol hydrolysis. These effects of ANF may be probably related to the activation of the non-guanylate cyclase coupled receptor which has been associated with phosphatidylinositol turnover. Nevertheless, although the atrial factor induces vasorelaxation, its enhancement of blood flow may not be the major event underlying the present results. The present work suggests a potential physiological role of ANF on the modulation of salivary secretion and provides further evidence on the rol of ANF in the regulation of body fluid homeostasis.