Characteristics of distension-induced release of immunoreactive atrial natriuretic peptide in isolated perfused rabbit atria

Role of atrial natriuretic peptide and oxytocin in gastric emptying delay induced by right atrial stretch in rats

Fluid volume and osmolality balance are maintained by complex neuroendocrine and liquid-salt intake behavior, cardiovascular and renal mechanisms, and gastrointestinal adjustments. Mechanical stretching of the right atrium [atrial stretch (AS)] enhances central venous pressure and heart rate while decreasing gastric emptying (GE) of liquid in rats. We evaluated the effect of AS on GE and plasma levels of atrial natriuretic peptide (ANP), oxytocin (OT), and corticosterone (CORT) to determine whether ANP contributes to the OT-mediated GE delay of liquids due to AS in awake rats. Initially, we performed thoracotomy followed by right appendectomy (AX) or sham thoracotomy. One week later, rats were randomly subjected to pretreatment with NaCl 0.15 M (control), atosiban (AT, OT-antagonist), anantin (ANT, ANP-antagonist), or dexamethasone (DEX). Afterward, 50 µL of AS was administered for 5 min or not (sham). Then, the rats were fed a test meal, and GE of liquids or solids was performed. The other animals were pretreated with NaCl 0.15 M, atosiban, anantin, or dexamethasone, followed by OT treatment for GE assessment. Compared with the sham group, 50 µL of AS decreased the GE of the liquid and solid test meals. This phenomenon was prevented by AT, ANT, DEX, and surgical procedures with AX. AS also increased plasma levels of ANP, OT, and CORT. In turn, oxytocin treatment decreased GE and increased plasma ANP, OT, and CORT levels, while AT, ANT, and DEX prevented OT-induced GE delay. Hence, AS delayed GE of liquid in rats, a phenomenon that involves oxytocinergic pathways and ANP activities.NEW & NOTEWORTHY We suggest a cardiogastric reflex with the participation of neuroendocrine mediators, which contributes to regulating liquid balance in the animal's body. Atrial natriuretic peptide and oxytocin are substances recognized for participating in diuresis and regulating the transit of liquids in the gastrointestinal tract in situations of cardiac volume overload, as was simulated with atrial stretching in the present experimental model.

Herbal medicine Oryeongsan (Wulingsan): Cardio-renal effects via modulation of renin-angiotensin system and atrial natriuretic peptide system

Background

Oryeongsan (Wulingsan, Goreisan) has long been used for the treatment of impaired body fluid metabolism. However, the action mechanisms have not been clearly defined. Recently, effects of Oryeongsan on the body fluid and Na+ metabolism and the action mechanisms have been shown more clearly. The present review focuses on the recent findings on the effects of Oryeongsan in the cardio-renal system in relation with body fluid metabolism and action mechanisms leading to a decrease in blood pressure in animal models of hypertension.

Methods

The new and recent findings were searched by using searching systems including PubMed-NCBI and Google-Scholar.

Results

Oryeongsan induced an increase in glomerular filtration rate, and natriuresis and diuresis with a decreased osmolality and resulted in a contraction of the body fluid and Na+ balance. These findings were associated with a suppression of abundance of Na+-H +-exchanger isoform 3 expression and V2 receptor/aquaporin2 water channel signaling pathway in the kidney. Further, treatment with Oryeongsan accentuated atrial natriuretic peptide secretion in the atria from spontaneously hypertensive rats in which the secretion was suppressed. In addition, Oryeongsan ameliorated impaired vasodilation in spontaneously hypertensive rats.

Conclusion

The effects of Oryeongsan in the kidney, atria, and vessel were accompanied by a suppression of AT1 receptor and concurrent accentuation of abundance of AT2/Mas receptors expression and modulation of the natriuretic peptide system in these organs from hypertensive rats. The review shows multiple sites of action of Oryeongsan and mechanisms involved in the regulation of volume and pressure homeostasis in the body.

Carbon monoxide releasing molecule-2 suppresses stretch-activated atrial natriuretic peptide secretion by activating large-conductance calcium-activated potassium channels

Carbon monoxide (CO) is a known gaseous bioactive substance found across a wide array of body systems. The administration of low concentrations of CO has been found to exert an anti-inflammatory, anti-apoptotic, anti-hypertensive, and vaso-dilatory effect. To date, however, it has remained unknown whether CO influences atrial natriuretic peptide (ANP) secretion. This study explores the effect of CO on ANP secretion and its associated signaling pathway using isolated beating rat atria. Atrial perfusate was collected for 10 min for use as a control, after which high atrial stretch was induced by increasing the height of the outflow catheter. Carbon monoxide releasing molecule-2 (CORM-2; 10, 50, 100 µM) and hemin (HO-1 inducer; 0.1, 1, 50 µM), but not CORM-3 (10, 50, 100 µM), decreased high stretch-induced ANP secretion. However, zinc porphyrin (HO-1 inhibitor) did not affect ANP secretion. The order of potency for the suppression of ANP secretion was found to be hemin > CORM-2 >> CORM-3. The suppression of ANP secretion by CORM-2 was attenuated by pretreatment with 5-hydroxydecanoic acid, paxilline, and 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one, but not by diltiazem, wortmannin, LY-294002, or NG-nitro-L-arginine methyl ester. Hypoxic conditions attenuated the suppressive effect of CORM-2 on ANP secretion. In sum, these results suggest that CORM-2 suppresses ANP secretion via mitochondrial K_ATP channels and large conductance Ca²⁺-activated K⁺ channels.

NOX4/Src regulates ANP secretion through activating ERK1/2 and Akt/GATA4 signaling in beating rat hypoxic atria

Nicotinamide adenine dinucleotide phosphate oxidases (NOXs) are the major enzymatic source of reactive oxygen species (ROS). NOX2 and NOX4 are expressed in the heart but its role in hypoxia-induced atrial natriuretic peptide (ANP) secretion is unclear. This study investigated the effect of NOX on ANP secretion induced by hypoxia in isolated beating rat atria. The results showed that hypoxia significantly upregulated NOX4 but not NOX2 expression, which was completely abolished by endothelin-1 (ET-1) type A and B receptor antagonists BQ123 (0.3 µM) and BQ788 (0.3 µM). ET-1-upregulated NOX4 expression was also blocked by antagonists of secreted phospholipase A2 (sPLA2; varespladib, 5.0 µM) and cytosolic PLA2 (cPLA2; CAY10650, 120.0 nM), and ET-1-induced cPLA2 expression was inhibited by varespladib under normoxia. Moreover, hypoxia-increased ANP secretion was evidently attenuated by the NOX4 antagonist GLX351322 (35.0 µM) and inhibitor of ROS N-Acetyl-D-cysteine (NAC, 15.0 mM), and hypoxia-increased production of ROS was blocked by GLX351322. In addition, hypoxia markedly upregulated Src expression, which was blocked by ET receptors, NOX4, and ROS antagonists. ET-1-increased Src expression was also inhibited by NAC under normoxia. Furthermore, hypoxia-activated extracellular signal-regulated kinase 1/2 (ERK1/2) and protein kinase B (Akt) were completely abolished by Src inhibitor 1 (1.0 µM), and hypoxia-increased GATA4 was inhibited by the ERK1/2 and Akt antagonists PD98059 (10.0 µM) and LY294002 (10.0 µM), respectively. However, hypoxia-induced ANP secretion was substantially inhibited by Src inhibitor. These results indicate that NOX4/Src modulated by ET-1 regulates ANP secretion by activating ERK1/2 and Akt/GATA4 signaling in isolated beating rat hypoxic atria.

Similarity and dissimilarity between angiotensin A and angiotensin II in cardiovascular functions in a rat model

Angiotensin (Ang) A differs from Ang II in a single N-terminal alanine residue. The aim of this study was to investigate whether the effects of Ang A on postischemic cardiac injury and hemodynamics differ from Ang II. After euthanizing Sprague-Dawley rats, hearts were perfused with Krebs-Henseleit buffer for a 20 min preischemic period with or without Ang A or Ang II, followed by 20 min global ischemia and 50 min reperfusion. The blood pressure was measured in anesthetized rats. Ang A (0.1, 1.0, 10 μg/kg) deteriorated the postischemic left ventricular hemodynamics in a dose-dependent manner, which was similar to that by Ang II. Ang A (10 μg/kg) increased the infarct size and the lactate dehydrogenase level, and decreased the coronary flow, which were attenuated by the pretreatment with Ang type 1 receptor (AT1R) antagonist (losartan) but not by AT2R antagonist (PD123319). Ang A increased the expression of apoptotic proteins and decreased the expression of antioxidative proteins. Interestingly, Ang A increased the atrial natriuretic peptide (ANP) level in coronary effluent and in atrial perfusate but Ang II did not increase it. Ang A increased mean arterial blood pressure, which was less potent than Ang II. These results suggest that Ang A has a similar effect on postischemic injury via AT1R and less potent vasopressor effect but opposite effect on ANP secretion as compared to Ang II.

Hypoxia augments NaHS-induced ANP secretion via KATP channel, HIF-1α and PPAR-γ pathway

It has been reported that sodium hydrosulfide (NaHS) stimulated high stretch induced-atrial natriuretic peptide (ANP) secretion via ATP sensitive potassium (K_ATP) channel. K_ATP channel is activated during hypoxic condition as a compensatory mechanism. However, whether NaHS affects ANP secretion during hypoxia remains obscure. The purpose of the present study is to discover the impact of NaHS on ANP secretion during hypoxia and to unravel its signaling pathway. Isolated beating rat atria were perfused with buffer exposed to different O₂ tension (to 100% O₂, normoxia; to 20% O₂, hypoxia). The ANP secretion increased negatively correlated with O₂ tension. NaHS (50 μM) did not show any significant effect on low stretch induced-ANP secretion in normoxic condition but augmented low stretch induced-ANP secretion in hypoxic condition. The augmentation of NaHS-induced ANP secretion during hypoxia was blocked by the pretreatment with K_ATP channel blocker (glibenclamide) and was enhanced by the pretreatment with K_ATP channel activator (pinacidil). Hypoxia increased the expression of PPAR-γ protein but did not change the expression of HIF-1α protein and eNOS phosphorylation. The NaHS-induced ANP secretion during hypoxia was also blocked by the pretreatment with HIF-1α inhibitor (2-methoxy- estradiol), PPAR-γ inhibitor (GW9662) but not by NOS inhibitor (L-NAME) and endothelin receptor inhibitor (bosentan). The intravenous infusion of NaHS increased plasma ANP level in monocrotaline-treated rats but not in sham rats. These results suggest that hypoxia augmented NaHS-induced ANP secretion partly through K_ATP channel, HIF-1α, and PPAR-γ pathway.

Hydrogen sulfide donor, NaHS, stimulates ANP secretion via the KATP channel and the NOS/sGC pathway in rat atria

Hydrogen sulfide (H₂S) is normally produced from l-cysteine in mammalian tissues and related to the pathogenesis of cardiovascular diseases. The aim of this study is to investigate the effects of H₂S donor on atrial natriuretic peptide (ANP) secretion and define its mechanism using normal and isoproterenol (ISP)-treated rats. Several H₂S donors were perfused into isolated beating rat atria, and atrial pressure (AP) and ANP secretion were measured. NaHS augmented high stretch-induced ANP secretion and decreased AP in a dose-dependent manner. The high stretch-induced ANP secretion was stimulated by Na₂S but was not changed by GYY4137 and sodium thiosulfate. NaHS and Na₂S produced very high amount of H₂S rapidly whereas GYY4137 produced very low amount of H₂S slowly. NaHS-stimulated ANP secretion was blocked by the pretreatment with inhibitor for K_ATP channel, nitric oxide synthase (NOS), soluble guanylyl cyclase (sGC), phosphoinositol 3 kinase (PI3K) or protein kinase B. H₂S synthesis enzyme inhibitor (DL-propargylglycine) did not show any significant changes in atrial parameters. However, the response of ANP secretion to NaHS markedly attenuated and DL-propargylglycine suppressed ANP secretion in ISP-treated rat atria. The expression of eNOS protein was decreased but the expression of cardiomyocyte-specific H₂S producing enzyme, cystathione γ-lyase, was not changed in ISP-treated rat atria. The attenuation of NaHS-induced ANP secretion in ISP-treated rat atria may be due to the low expression of eNOS protein. These findings clarify that NaHS stimulates ANP secretion via the K_ATP channel and the PI3K/Akt/NOS/sGC pathway in rat atria.

Modification of levosimendan-induced suppression of atrial natriuretic peptide secretion in hypertrophied rat atria

This study aimed to determine the effects of levosimendan, a calcium sensitizer, on atrial contractility and atrial natriuretic peptide (ANP) secretion and its modification in hypertrophied atria. Isolated perfused beating rat atria were used from control and isoproterenol-treated rats. Levosimendan and its metabolite OR-1896 caused a positive inotropic effect and suppressed ANP secretion in rat atria. Similar to levosimendan, the selective phosphodiesterase 3 (PDE3) or PDE4 inhibitor also suppressed ANP secretion. Suppression of ANP secretion by 1 µM levosimendan was abolished by PDE3 inhibitor, but reversed by PDE4 inhibitor. Levosimendan-induced suppression of ANP secretion was potentiated by K_ATP channel blocker, but blocked by K_ATP channel opener. Levosimendan alone did not significantly change cyclic adenosine monophosphate (cAMP) efflux in the perfusate; however, levosimendan combined with PDE4 inhibitor markedly increased this efflux. The stimulation of ANP secretion induced by levosimendan combined with PDE4 inhibitor was blocked by the protein kinase A (PKA) inhibitor. In isoproterenol-treated atria, levosimendan augmented the positive inotropic effect and ANP secretion in response to an increased extracellular calcium concentration ([Ca⁺]_o). These results suggests that levosimendan suppresses ANP secretion by both inhibiting PDE3 and opening K_ATP channels and that levosimendan combined with PDE4 inhibitor stimulates ANP secretion by activating the cAMP-PKA pathway. Modification of the effects of levosimendan on [Ca⁺]_o-induced positive inotropic effects and ANP secretion in isoproterenol-treated rat atria might be related to a disturbance in calcium metabolism.

Comparative effects of angiotensin II and angiotensin-(4-8) on blood pressure and ANP secretion in rats

Angiotensin II (Ang II) is metabolized from N-terminal by aminopeptidases and from C-terminal by Ang converting enzyme (ACE) to generate several truncated angiotensin peptides (Angs). The truncated Angs have different biological effects but it remains unknown whether Ang-(4-8) is an active peptide. The present study was to investigate the effects of Ang-(4-8) on hemodynamics and atrial natriuretic peptide (ANP) secretion using isolated beating rat atria. Atrial stretch caused increases in atrial contractility by 60% and in ANP secretion by 70%. Ang-(4-8) (0.01, 0.1, and 1 µM) suppressed high stretch-induced ANP secretion in a dose-dependent manner. Ang-(4-8) (0.1 µM)-induced suppression of ANP secretion was attenuated by the pretreatment with an antagonist of Ang type 1 receptor (AT₁R) but not by an antagonist of AT₂R or AT₄R. Ang-(4-8)-induced suppression of ANP secretion was attenuated by the pretreatment with inhibitor of phospholipase (PLC), inositol triphosphate (IP₃) receptor, or nonspecific protein kinase C (PKC). The potency of Ang-(4-8) to inhibit ANP secretion was similar to Ang II. However, Ang-(4-8) 10 µM caused an increased mean arterial pressure which was similar to that by 1 nM Ang II. Therefore, we suggest that Ang-(4-8) suppresses high stretch-induced ANP secretion through the AT₁R and PLC/IP₃/PKC pathway. Ang-(4-8) is a biologically active peptide which functions as an inhibition mechanism of ANP secretion and an increment of blood pressure.

Effects of endothelin family on ANP secretion

The endothelins (ET) peptide family consists of ET-1, ET-2, ET-3, and sarafotoxin (s6C, a snake venom) and their actions appears to be different among isoforms. The aim of this study was to compare the secretagogue effect of ET-1 on atrial natriuretic peptide (ANP) secretion with ET-3 and evaluate its physiological meaning. Isolated nonbeating atria from male Sprague-Dawley rats were used to evaluate stretch-activated ANP secretion in response to ET-1, ET-2, ET-3, and s6C. Changes in mean blood pressure (MAP) were measured during acute injection of ET-1 and ET-3 with and without natriuretic peptide receptor-A antagonist (A71915) in anesthetized rats. Changes in atrial volume induced by increased atrial pressure from o to 1, 2, 4, or 6cm H2O caused proportional increases in mechanically-stimulated extracellular fluid (ECF) translocation and stretch-activated ANP secretion. ET-1 (10nM) augmented basal and stretch-activated ANP secretion in terms of ECF translocation, which was blocked by the pretreatment with ETA receptor antagonist (BQ123, 1μM) but not by ETB receptor antagonist (BQ788, 1μM). ETA receptor antagonist itself suppressed stretch-activated ANP secretion. As compared to ET-1- induced ANP secretion (3.2-fold by 10nM), the secretagogue effects of ANP secretion by ET-2 was similar (2.8-fold by 10nM) and ET-3 and s6C were less potent (1.7-fold and 1.5-fold by 100nM, respectively). Acute injection of ET-1 or ET-3 increased mean blood pressure (MAP), which was augmented in the presence of natriuretic peptide receptor-A antagonist. Therefore, we suggest that the order of secretagogue effect of ET family on ANP secretion was ET-1≥ET-2>>ET-3>s6C and ET-1-induced ANP secretion negatively regulates the pressor effect of ET-1.

Attenuation of renovascular hypertension by cyclooxygenase-2 inhibitor partly through ANP release

Angiotensin II (Ang II) is an important inflammatory mediator. Ang II induces cyclooxygenase-2 (COX-2) expression and prostaglandin F2α release followed by cardiac hypertrophy. Inhibition of COX-2 may modulate high blood pressure but controversy still exists. The aim of this study was to determine the role of COX-2 in the regulation of blood pressure and to define the mechanisms in two kidney one-clip hypertensive (2K1C) rats. Chronic treatment with nimesulide or NS-398 (5 mg/kg/day) for 3 weeks lowered high blood pressure and cardiac hypertrophy with decreased expression levels of cardiac hypertrophy markers [atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP)], Ang type 1 receptor, urotensin II, and urotensin II receptor in 2K1C rats. Plasma level of ANP was markedly increased and plasma levels of Ang II and aldosterone were decreased by treatment with nimesulide or NS-398. In both in vitro and in vivo experiments, nimesulide or NS-398 augmented ANP release in 2K1C rats. The inhibitory effect of NS-398 on blood pressure was attenuated by the pretreatment with natriuretic peptide receptor-A (NPR-A) antagonist (A71915, 30 μg/kg/day). These results suggest that chronic treatment with nimesulide or NS-398 attenuated hypertension and cardiac hypertrophy partly through ANP release in 2K1C rats.

Angiotensin IV stimulates high atrial stretch-induced ANP secretion via insulin regulated aminopeptidase

Angiotensin IV (Ang IV) is formed by aminopeptidase N (APN) from angiotensin III (Ang III) by removing the first N-terminal amino acid. Previously, we reported that angiotensin II (Ang II) inhibits atrial natriuretic peptide (ANP) secretion via angiotensin II type 1 receptor (AT1R). In contrast, angiotensin-(1-7) [Ang-(1-7)] and Ang III stimulate ANP secretion via Mas receptor (Mas R) and angiotensin II type 2 receptor (AT2R), respectively. However, it is not known whether there is any relationship between Ang IV and ANP secretion. Therefore, the aim of the present study was to determine the effect of Ang IV on ANP secretion and to find its downstream signaling pathway using in isolated perfused beating atria. Ang IV (0.1, 1 and 10μM) stimulated high atrial stretch-induced ANP secretion and ANP concentration in a dose-dependent manner. The augmented effect of Ang IV (1μM) on high atrial stretch-induced ANP secretion and concentration was attenuated by pretreatment with insulin-regulated aminopeptidase (IRAP) antagonist but not by AT1R or AT2R antagonist. Pretreatment with inhibitors of downstream signaling pathway including phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt) and mammalian target of rapamycin (mTOR) blocked Ang IV-induced ANP secretion and concentration. Therefore, these results suggest that Ang IV stimulates ANP secretion and concentration via IRAP and PI3K-Akt-mTOR pathway.

Comparision of secretagogue effects of rosiglitazone and telmisartan on ANP secretion in rats

Peroxisome proliferator-activated receptor-gamma (PPAR-γ), a nuclear transcription factor, is a key regulator of insulin signaling, and glucose and fat metabolism. In this study, we evaluated the direct effect of PPAR-γ ligand on the secretion of atrial natriuretic peptide (ANP). The isolated perfused beating atria were used and rosiglitazone (0.01, 0.3 and 1 μM) or telmisartan was perfused into atria with and without inhibitors. High frequency stimulation caused a decreased atrial contractility by 40% and an increased ANP secretion by 80%. Rosiglitazone augmented high frequency-induced ANP secretion and concentration in a dose-dependent manner. Rosiglitazone-induced ANP secretion was attenuated by the pretreatment with PPAR-γ antagonist (GW 9662), or inhibitor for phosphoinositol 3-kinase (PI3-kinase, wortmannin), Akt (API-2) or nitric oxide synthase (l-NAME). Telmisartan, a partial agonist of PPAR-γ with angiotensin II type 1 receptor (AT1R) blocker, also stimulated ANP secretion, which was more potent than rosiglitazone or losartan. Infusion of rosiglitazone or telmisartan in anesthetized rats tended to decrease mean arterial pressure and to increase pulse pressure without difference. A plasma ANP level was increased by telmisartan more than by rosiglitazone. In diabetic rats, an increased plasma ANP level was more prominent than sham rats. Therefore, we suggest that rosiglitazone stimulates high frequency-induced ANP secretion through the PPAR-γ receptor-PI3-kinase-Akt-eNOS pathway and telmisartan shows synergistic effect on ANP secretion.

Cardioprotective effects of angiotensin III against ischemic injury via the AT2 receptor and KATP channels

Angiotensin III (Ang III) has similar effects on blood pressure and aldosterone secretion as Ang II, but cardioprotective effects are also proposed. In this study, we investigated whether Ang III protects the heart against ischemia/reperfusion (I/R) injury. After sacrificing Sprague-Dawley rats, the hearts were perfused with Krebs–Henseleit buffer for a 20 min preischemic period with and without Ang III followed by 20-min global ischemia and 50-min reperfusion. Pretreatment with Ang III (1 μmol/L) improved an increased postischemic left ventricular end-diastolic pressure (LVEDP) and a decreased postischemic left ventricular developed pressure (LVDP) induced by reperfusion compared to untreated hearts. Ang III markedly decreased infarct size and lactate dehydrogenase levels in effluent during reperfusion. Ang III increased coronary flow and the concentrations of atrial natriuretic peptide in coronary effluent during reperfusion. Pretreatment with Ang II type 2 receptor (AT₂R) antagonist or ATP-sensitive K⁺ channel (K_ATP) blocker for 15 min before ischemia attenuated the improvement of LVEDP, LVDP, and ±dP/dt induced by Ang III. Ang III treatment increased Mn-superoxide dismutase, catalase, and heme oxygenase-1 protein levels, which was attenuated by pretreatment with AT₂R antagonist or K_ATP blocker. Ang III treatment also decreased Bax, caspase-3, and caspase-9 protein levels, and increased Bcl-2 protein level, which were attenuated by pretreatment with AT₂R antagonist or K_ATP blocker. These results suggest that the cardioprotective effects of Ang III against I/R injury may be partly related to activating antioxidant and antiapoptotic enzymes via AT₂R and K_ATP channels.

Stimulation of ANP by angiotensin-(1-9) via the angiotensin type 2 receptor

AIMS

Angiotensin-(1-9) [Ang-(1-9)] and Ang-(1-7) are cleaved by Ang converting enzyme 2 forming Ang I and Ang II, respectively, and the truncated Angs play a role in regulating atrial natriuretic peptide (ANP) secretion. Previously, we found that Ang-(1-7) stimulates ANP secretion via the Mas receptor. However, the effect of Ang-(1-9) on ANP secretion is still unknown. The aim of the present study is to determine whether Ang-(1-9) stimulates ANP secretion and to characterize the signaling pathway involved in stimulating secretion.

MAIN METHODS

We examined the effects of Ang-(1-9) on ANP secretion and atrial contractility with and without inhibitors in isolated perfused atria.

KEY FINDINGS

Ang-(1-9) stimulated ANP secretion and concentration without change in atrial contractility. Ang-(1-9)-induced-ANP secretion was increased from 5% to 60% by 3 μM Ang-(1-9) during the low-stretch state of the atrium. This stimulatory effect of Ang-(1-9) on ANP secretion was attenuated by pretreatment with an Ang II type 2 receptor (AT2R) antagonist but not by AT1R or Mas receptor antagonist. In addition, pretreatment with inhibitors of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) blocked Ang-(1-9)-induced ANP secretion. In the high-stretch atrial state, Ang-(1-9)-induced ANP secretion was increased more than in the low-stretch state following addition of 1 μM Ang-(1-9) (from 108% to 170%). In an in vivo experiment, acute infusion of Ang-(1-9) increased plasma ANP level without altering arterial blood pressure. This effect was attenuated by pretreatment with AT2R antagonist but not by Mas receptor antagonist.

SIGNIFICANCE

These results suggest that Ang-(1-9) stimulates ANP secretion via the AT2R-PI3K-Akt-NO-cGMP pathway.

Angiotensin AT2 receptor agonist stimulates high stretch induced- ANP secretion via PI3K/NO/sGC/PKG/pathway

Angiotensin II (Ang II) type 1 receptor (AT1R) mediates the major cardiovascular effects of Ang II. However, the effects mediated via AT2R are still controversial. The aim of the present study is to define the effect of AT2R agonist CGP42112A (CGP) on high stretch-induced ANP secretion and its mechanism using in vitro and in vivo experiments. CGP (0.01, 0.1 and 1μM) stimulated high stretch-induced ANP secretion and concentration from isolated perfused rat atria. However, atrial contractility and the translocation of extracellular fluid did not change. The augmented effect of CGP (0.1μM) on high stretch-induced ANP secretion was attenuated by the pretreatment with AT2R antagonist or inhibitor for phosphoinositol 3-kinase (PI3K), nitric oxide (NO), soluble guanylyl cyclase (sGC), or protein kinase G (PKG). However, antagonist for AT1R or Mas receptor did not influence CGP-induced ANP secretion. In vivo study, acute infusion of CGP for 10min increased plasma ANP level without blood pressure change. In renal hypertensive rat atria, AT2R mRNA and protein levels were up-regulated and the response of plasma ANP level to CGP infusion in renal hypertensive rats augmented. The pretreatment with AT2R antagonist for 10min followed by CGP infusion attenuated an increased plasma ANP level induced by CGP. However, pretreatment with AT1R or Mas receptor antagonist unaffected CGP-induced increase in plasma ANP level. Therefore, we suggest that AT2R agonist CGP stimulates high stretch-induced ANP secretion through PI3K/NO/sGC/PKG pathway and these effects are augmented in renal hypertensive rats.

Angiotensin III stimulates high stretch-induced ANP secretion via angiotensin type 2 receptor

Angiotensin III (Ang III) is metabolized from Ang II by aminopeptidase (AP) A and in turn, Ang III is metabolized to Ang IV by APN. Ang III is known to have a similar effect to Ang II on aldosterone secretion, but the effect of Ang III on atrial natriuretic peptide (ANP) secretion from cardiac atria is not known. The aim of the present study is to define the effect of Ang III on ANP secretion and its receptor subtype using isolated perfused beating atria. The volume load was achieved by elevating the height of outflow catheter connected with isolated atria from 5 cmH2O to 7.5 cmH2O. Atrial stretch by volume load increased atrial contractility and ANP secretion. Ang III stimulated stretch-induced ANP secretion in a dose-dependent manner without change in atrial contractility. The stimulated effect of Ang III (1 μM) on stretch-induced ANP secretion was blocked by the pretreatment of Ang II type 2 (AT2) receptor antagonist but not by AT1 or Mas receptor antagonist. Pretreatment with inhibitor of phosphoinositide 3-kinase (PI3K), Akt, nitric oxide synthase, soluble guanylyl cyclase, or protein kinase G (PKG) attenuated Ang III-stimulated ANP secretion. When Ang III (40 nM) or Ang II (4nM) was infused for 10 min into anesthetized rats, mean arterial pressure was increased about 10%. However, Ang III increased plasma ANP level by 35.81±10.19% but Ang II decreased plasma ANP level by 30.41±7.27%. Therefore, we suggest that Ang III, opposite to Ang II, stimulated stretch-induced ANP secretion through AT2 receptor/PI3K/Akt/nitric oxide/PKG pathway.

Urotensin II protects ischemic reperfusion injury of hearts through ROS and antioxidant pathway

Urotensin II (UII) is a vasoactive peptide which is bound to a G protein-coupled receptor. UII and its receptor are upregulated in ischemic and chronic hypoxic myocardium, but the effect of UII on ischemic reperfusion (I/R) injury is still controversial. The aim of the present study was to investigate whether UII protects heart function against I/R injury. Global ischemia was performed using isolated perfused Langendorff hearts of Sprague-Dawley rats. Hearts were perfused with Krebs-Henseleit buffer for 20min pre-ischemic period followed by a 20min global ischemia and 50min reperfusion. Pretreatment with UII (10nM) for 10min increased recovery percentage of the post-ischemic left ventricular developed pressure and ±dp/dt, and decreased post-ischemic left ventricular end-diastolic pressure as compared with I/R group. UII decreased infarct size and an increased lactate dehydrogenase level during reperfusion. Cardioprotective effects of UII were attenuated by pretreatment with UII receptor antagonist. The hydrogen peroxide activity was increased in UII-treated heart before ischemia. The Mn-SOD, catalase, heme oxygenase-1 and Bcl-2 levels were increased, and the Bax and caspase-9 levels were decreased in UII-treated hearts. These results suggest that UII has cardioprotective effects against I/R injury partly through activating antioxidant enzymes and reactive oxygen species.

Synthesis of atrial natriuretic peptide in the rabbit inner ear

OBJECTIVES/HYPOTHESIS

Atrial natriuretic peptide (ANP) exhibits natriuretic, diuretic, and vasorelaxant activity, maintaining fluid and electrolyte homeostasis. Although ANP is mainly synthesized and secreted in/from atrium, ANP has been found in various tissues including the rat inner ear. The aim of this study was to identify the synthesis of ANP in lateral cochlear wall tissues and the presence of ANP in perilymph using rabbits.

STUDY DESIGN

In vivo study using a rabbit model.

METHODS

Expression of ANP in the rabbit inner ear tissue and the presence of perilymph were examined by radioimmunoassay and polymerase chain reaction using an ANP polyclonal antibody and rat ANP primers, respectively. Characteristics of ANP and pro-ANP present in the inner ear were also evaluated by high-performance liquid chromatography (HPLC), comparing to rat cardiac ANP and pro-ANP.

RESULTS

Immunoreactive ANP (ir-ANP) was found in the perilymph and lateral cochlear wall tissue of rabbits. The levels of ir-ANP in the perilymph were 5 to 16 ng/mL. Elusion profiles of HPLC showed two main peaks that were exactly matched with rat cardiac ANP and pro-ANP. Expression of ANP mRNA was also detected.

CONCLUSIONS

Our findings suggest that ANP and its precursor protein are synthesized in the ear tissues and secreted to perilymph. This hormone may play a role in control of water and/or ion homeostasis of the fluids in the ear that are responsible for normal hearing.

Suppression of high pacing-induced ANP secretion by antioxidants in isolated rat atria

Reactive oxygen species (ROS) are formed as a natural by-product of the normal metabolism of oxygen and have important roles in cell signaling. The aim of this study was to investigate direct effects of ROS on atrial hemodynamics and ANP secretion in isolated perfused beating rat atria with antioxidants. When atria were paced at 1.2 Hz, N-acetyl cystein (antioxidant, NAC), α-lipoic acid (antioxidant), tempol (superoxide dismutase mimic), and apocynin (NADPH oxidase inhibitor; NOX inhibitor) did not affect ANP secretion and atrial contractility. When pacing frequency was increased from 1.2 Hz to 4 Hz, the ANP secretion increased and atrial contractility decreased. H(2)O(2) level was increased in perfusate obtained from atria stimulated by high pacing frequency. NAC, α-lipoic acid and tempol attenuated high pacing frequency-induced ANP secretion but apocynin did not. In contrast, pyrogallol (a superoxide generator) augmented high pacing frequency-induced ANP secretion. NOX-4 protein was increased by high pacing stimulation and in diabetic rat atria. In diabetic rat atria, high pacing frequency caused an increased ANP secretion and a decreased atrial contractility, that were markedly attenuated as compared to control rats. NAC and apocynin reduced high pacing frequency-induced ANP secretion in diabetic rat atria. These results suggest that intracellular ROS formation partly through an increasing NOX activity in response to high pacing frequency is associated with an increased ANP secretion in rat atria.

Caveolae are essential for angiotensin II type 1 receptor-mediated ANP secretion

Caveolae may act as mechanosensors and function as binding sites for calcium ions. The intracaveolar localization of atrial natriuretic peptide (ANP) derived from the direct interaction of atrial granules with caveolae has been demonstrated. The aim of this study was to define the effect of caveolae on ANP secretion induced by stretch and angiotensin II. The isolated perfused beating atria from Sprague-Dawley rats were used. To disrupt caveolae, 10mM methyl-β-cyclodextrin (MbCD) was applied for 1h and the number of caveoli were markedly decreased. MbCD increased basal ANP secretion and atrial diastolic pressure. The molecular profile of ANP in perfusate from control atria showed mainly one major peak corresponded to synthetic ANP whereas that from MbCD-treated atria showed two major immunoreactive peaks corresponded to synthetic rat ANP and proANP. High atrial stretch induced by elevating the height of outflow catheter from 5 cm H₂O to 7.5 cm H₂O increased atrial contractility and ANP secretion. The response of ANP secretion to high stretch was attenuated in MbCD-pretreated atria. Pretreatment with MbCD abolished angiotensin II-induced suppression and losartan-induced stimulation of ANP secretion. However, the effect of angiotenisin (1-7) on ANP secretion was not altered by MbCD treatment. The expression of angiotensin II type 1 receptor protein was reduced by MbCD treatment. These data suggest that caveolae are essential for angiotensin II type 1 receptor-mediated ANP secretion and relate to the processing of proANP.

Oxidative stress augments the secretion of atrial natriuretic peptide in isolated rat atria

Reactive oxygen species (ROS) play a role in cardiovascular diseases such as hypertension and heart failure. The objective of the present study was to investigate the role of endogenous ROS in atrial hemodynamics and ANP secretion in isolated perfused beating rat atria. Pyrogallol (a generator of superoxide anion, 0.1, 1mM) or hydrogen peroxide (0.1, 1, 10, 30mM) was perfused into atria paced at 1.2Hz. Pyrogallol and hydrogen peroxide stimulated ANP secretion and concentration in a dose-dependent manner and dramatically decreased atrial contractility and translocation of extracellular fluid. The stimulatory effect of pyrogallol and hydrogen peroxide on ANP secretion was attenuated by the pretreatment with ascorbic acid (an antioxidant; 1mM) and cariporide (an inhibitor of the Na(+)/H(+) exchanger; 1μM) but negative inotropic effect was not changed. U120 (a MAPK(erk) pathway inhibitor; 10μM) attenuated the stimulatory effect of hydrogen peroxide on ANP secretion. However, U120 augmented negative inotropic effect and stimulatory effect of ANP concentration induced by pyrogallol. Antioxidant such as N-acetyl cystein, gallate, propyl gallate, or ellagic acid did not cause any significant changes in atrial parameters. These results suggest that intracellular - formed ROS stimulates ANP secretion partly through activation of MAPK(erk) pathway and Na(+)/H(+) exchanger.

Suppression of ANP secretion by somatostatin through somatostatin receptor type 2

Somatostatin is a cyclic-14 amino acid peptide which mainly distributed in digestive system and brain. Somatostatin receptor (SSTR) is a G-protein coupled receptor and all five SSTR subtypes are expressed in cardiomyocytes. The aim of this study was to investigate the effect of somatostatin on atrial natriuretic peptide (ANP) secretion and its signaling pathway. Somatostatin (0.01 and 0.1nM) decreased ANP secretion in isolated beating rat atrium in a dose-dependent manner. But atrial contractility and translocation of extracellular fluid were not changed. Somatostatin-induced decrease in ANP secretion was significantly attenuated by the pretreatment with CYN 154806 (SSTR type 2 antagonist; 0.1μM), but not by BIM 23056 (SSTR type 5 antagonist; 0.1μM) and urantide (urotensin II receptor antagonist; 0.1μM). When pretreated with an agonist for SSTR type 2 (Seglitide, 0.1nM) and SSTR type 5 (L 817818, 0.1nM), only Seglitide reduced ANP secretion similar to that of somatostatin. The suppressive effect of somatostatin on ANP secretion was attenuated by the pretreatment with an inhibitor for adenylyl cyclase (MDL-12330A, 5μM) or protein kinase A (KT 5720, 0.1μM). In diabetic rat atria, the suppressive effect of somatostatin on ANP secretion and concentration was attenuated. Real time-PCR and western blot shows the decreased level of SSTR type 2 mRNA and protein in diabetic rat atria. These data suggest that somatostatin decreased ANP secretion through SSTR type 2 and an attenuation of suppressive effect of somatostatin on ANP secretion in diabetic rat atria is due to a down-regulation of SSTR type 2.

Endogenous angiotensin II suppresses stretch-induced ANP secretion via AT1 receptor pathway

Angiotensin II (Ang II) is released by stretch of cardiac myocytes and has paracrine and autocrine effects on cardiac myocytes and fibroblasts. However, the direct effect of Ang II on the secretion of atrial natriuretic peptide (ANP) is unclear. The aim of the present study is to test whether Ang II affects stretch-induced ANP secretion. The isolated perfused beating atria were used from control and two-kidney one-clip hypertensive (2K1C) rats. The volume load was achieved by elevating the height of outflow catheter connected with isolated atria from 5cmH(2)O to 7.5cmH(2)O. Atrial stretch by volume load caused increases in atrial contractility by 60% and in ANP secretion by 100%. Ang II suppressed stretch-induced ANP secretion and tended to increase atrial contractility whereas losartan stimulated stretch-induced ANP secretion. Neither PD123319 nor A779 had direct effect on stretch-induced ANP secretion. The suppressive effect of Ang II on stretch-induced ANP secretion was blocked by the pretreatment of losartan but not by the pretreatment of PD123319 or A779. In hypertrophied atria from 2K1C rats, stretch-induced ANP concentration attenuated and atrial contractility augmented. The response of stretch-induced ANP secretion to Ang II and losartan augmented. The expression of AT1 receptor protein and mRNA increased but AT2 and Mas receptor mRNA did not change in 2K1C rat atria. Therefore, we suggest that Ang II generated endogenously by atrial stretch suppresses stretch-induced ANP secretion through the AT1 receptor and alteration of Ang II effect in 2K1C rat may be due to upregulation of AT1 receptor.

Urotensin II receptor antagonist attenuates monocrotaline-induced cardiac hypertrophy in rats

Urotensin II (UII) is a vasoactive peptide with potent cardiovascular effects through a G protein-coupled receptor. Hypoxia stimulates the secretion of UII and atrial natriuretic peptide (ANP). However, the effect of UII on hypoxia-induced cardiac hypertrophy is still controversial. The present study was conducted to determine whether human UII (hUII)-mediated ANP secretion influences hypoxia-induced cardiac hypertrophy using in vitro and in vivo models. Hypoxia caused an increase in ANP secretion and a decrease in atrial contractility in isolated perfused beating rat atria. hUII (0.01 and 0.1 nM) attenuated hypoxia-induced ANP secretion without changing the atrial contractility, and the hUII effect was mediated by the UII receptor signaling involving phospholipase C, inositol 1,3,4 trisphosphate receptor, and protein kinase C. Rats treated with monocrotaline (MCT, 60 mg/kg) showed right ventricular hypertrophy with increases in pulmonary arterial pressure and its diameter and plasma levels of UII and ANP that were attenuated by the pretreatment with an UII receptor antagonist, urantide. An acute administration of hUII (5 μM injection plus 2.5 μM infusion for 15 min) decreased the plasma ANP level in MCT-treated rats but increased the plasma ANP level in MCT plus urantide-treated and sham-operated rats. These results suggest that hUII may deteriorate MCT-induced cardiac hypertrophy mainly through a vasoconstriction of the pulmonary artery and partly through the suppression of ANP secretion.

Angiotensin-(1-7) attenuates hyposmolarity-induced ANP secretion via the Na+-K+ pump

The alteration in osmolarity challenges cell volume regulation, a vital element for cell survival. Hyposmolarity causes an increase in cell volume. Recently, it has been reported that the renin-angiotensin system (RAS) plays a role in cell volume regulation. We investigated the effect of angiotensin-(1-7) [Ang-(1-7)] on hyposmolarity-induced atrial natriuretic peptide (ANP) secretion in normal and diabetic (DM) rat atria and modulation of the effect of Ang-(1-7) by the Na(+)-K(+) pump. Using isolated control rat atria, we observed that perfusion of hyposmotic solution into the atria increased ANP secretion. When Ang-(1-7) [0.1 microM or 1 microM] was perfused in a hyposmolar solution, it decreased the hyposmolarity-induced ANP secretion in a dose-dependent manner. This effect of Ang-(1-7) could be mediated by the Na(+)-K(+) pump, since ouabain, an Na(+)-K(+) pump inhibitor, significantly decreased the effect of Ang-(1-7) on hyposmolarity-induced ANP secretion. In contrast, N(omega) Nitro-l-arginine methyl ester hydrochloride (l-NAME) did not modify the effect of Ang-(1-7) on the hyposmolarity-induced ANP secretion. Interestingly, the ANP secretion was increased robustly by the perfusion of the hyposmolar solution in the DM atria, as compared to the control atria. However, the inhibitory effect of Ang-(1-7) on the hyposmolarity-induced ANP secretion was not observed in the DM atria. In the DM atria, atrial contractility was significantly increased. Taken together, we concluded that Ang-(1-7) attenuated hyposmolarity-induced ANP secretion via the Na(+)-K(+) pump and a lack of Ang-(1-7) response in DM atria may partly relate to change in Na(+)-K(+) pump activity.

Leptin reduces plasma ANP level via nitric oxide-dependent mechanism

Leptin is a circulating adipocyte-derived hormone that influences blood pressure (BP) and metabolism. This study was designed to define the possible role of leptin in regulation of the atrial natriuretic peptide (ANP) system using acute and chronic experiments. Intravenous infusion of rat leptin (250 μg/kg injection plus 2 μg·kg−1·min−1 for 20 min) into Sprague-Dawley rats increased BP by 25 mmHg and decreased plasma level of ANP from 80.3 ± 3.45 to 51.8 ± 3.3 pg/ml. Reserpinization attenuated the rise in BP, but not the reduction of plasma ANP during leptin infusion. Nω-nitro-l-arginine methyl ester prevented the effects of leptin on the reduction of ANP level. In hyperleptinemic rats that received adenovirus containing rat leptin cDNA (AdCMV-leptin), BP increased during first 2 days and then recovered to control value. Plasma concentration of ANP and expression of ANP mRNA, but not of atrial ANP, in hyperleptinemic rats were lower than in the control groups on the first and second week after administration of AdCMV-leptin. These effects were not observed by the pretreatment with Nω-nitro-l-arginine methyl ester. No differences in renal function and ANP receptor density in the kidney were found between hyperleptinemic and control rats. Basal ANP secretion and isoproterenol-induced suppression of ANP secretion from isolated, perfused atria of hyperleptinemic rats were not different from those of other control groups. These data suggest that leptin inhibits ANP secretion indirectly through nitric oxide without changing basal or isoproterenol-induced ANP secretion.

Angiotensin-(1-7) stimulates high atrial pacing-induced ANP secretion via Mas/PI3-kinase/Akt axis and Na+/H+ exchanger

Angiotensin-(1-7) [ANG-(1-7)], one of the bioactive peptides produced in the renin-angiotensin system, plays a pivotal role in cardiovascular physiology by providing a counterbalance to the function of ANG II. Recently, it has been considered as a potential candidate for therapeutic use in the treatment of various types of cardiovascular diseases. The aim of the present study is to explain the modulatory role of ANG-(1-7) in atrial natriuretic peptide (ANP) secretion and investigate the functional relationship between two peptides to induce cardiovascular effects using isolated perfused beating rat atria and a cardiac hypertrophied rat model. ANG-(1-7) (0.01, 0.1, and 1 muM) increased ANP secretion and ANP concentration in a dose-dependent manner at high atrial pacing (6.0 Hz) with increased cGMP production. However, at low atrial pacing (1.2 Hz), ANG-(1-7) did not cause changes in atrial parameters. Pretreatment with an antagonist of the Mas receptor or with inhibitors of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), or nitric oxide synthase blocked the augmentation of high atrial pacing-induced ANP secretion by ANG-(1-7). A similar result was observed with the inhibition of the Na(+)/H(+) exchanger-1 and Ca(2+)/calmodulin-dependent kinase II (CaMKII). ANG-(1-7) did not show basal intracellular Ca(2+) signaling in quiescent atrial myocytes. In an in vivo study using an isoproterenol-induced cardiac hypertrophy animal model, an acute infusion of ANG-(1-7) increased the plasma concentration of ANP by twofold without changes in blood pressure and heart rate. A chronic administration of ANG-(1-7) increased the plasma ANP level and attenuated isoproterenol-induced cardiac hypertrophy. The antihypertrophic effect was abrogated by a cotreatment with the natriuretic peptide receptor-A antagonist. These results suggest that 1) ANG-(1-7) increased ANP secretion at high atrial pacing via the Mas/PI3K/Akt pathway and the activation of Na(+)/H(+) exchanger-1 and CaMKII and 2) ANG-(1-7) decreased cardiac hypertrophy which might be mediated by ANP.

Urotensin II stimulates high frequency-induced ANP secretion via PLC-PI 3K-PKC pathway

Urotensin II (U-II) and its receptor are coexpressed in the heart and show various cardiovascular functions. However, the relationship between U-II and cardiac hormone atrial natriuretic peptide (ANP) is still unknown. The aim of the present study is to test whether U-II affects ANP secretion using in vitro perfusion experiments and in vivo studies. Human U-II (hU-II) (10(-11), 5x10(-11), 10(-10), 5x10(-10)M) stimulated ANP secretion from isolated perfused rat atria paced with high frequency (6.0Hz). However, atrial contractility and translocation of extracellular fluid (ECF) did not change. An increase in ANP secretion by rat U-II was similar to that by hU-II; however, urotensin-related peptide showed no significant effect on ANP secretion. Pretreatment with urotensin receptor antagonist and inhibitor for phospholipase C (PLC), phosphoinositide 3-kinase (PI3K), or protein kinase C (PKC) attenuated hU-II-induced ANP secretion from atria paced with high frequency, but an inhibitor for inositol triphosphate did not. Intravenous infusion of hU-II at a dose of 2.5microM for 20min increased plasma ANP level, along with increased heart rate and pulse pressure in anesthetized rats. Therefore, we suggest that U-II stimulates high stimulation frequency-induced ANP secretion partly through the urotensin receptor and the PLC/PI3K/PKC pathway.

Regulation of ANP secretion from isolated atria by prostaglandins and cyclooxygenase-2

Cyclooxygenase (COX) is a key enzyme regulating the production of various prostaglandins (PGs) from arachidonic acid. Angiotensin II has been reported to be an important inflammatory mediator, which increases COX-2. The aim of this study was to determine the role of various PGs and COX-2 in the regulation of atrial natriuretic peptide (ANP) secretion. PGF2alpha and PGD2 caused dose-dependent increases in ANP release and intra-atrial pressure. The potency for the stimulation of ANP secretion by PGF2alpha was higher than that by PGD2. In contrast, PGE2, PGI2, PGJ2, and thromboxane A2 did not show any significant effects. The increases in intra-atrial pressure and ANP secretion induced by PGF2alpha and PGD2 were significantly attenuated by the pretreatment with an inhibitor of PGF2alpha receptor. By the pretreatment with an inhibitor for phospholipase C (PLC), inositol 3-phosphate (IP3) receptor, protein kinase C (PKC), or myosin light chain kinase (MLCK), PGF2alpha-mediated increase in ANP secretion and positive inotropy were attenuated. Inhibitor for COX-1 or COX-2 did not cause any significant effects on atrial parameters. In hypertrophied rat atria, PGF2alpha-induced positive inotropy and ANP secretion were markedly attenuated whereas COX-2 inhibitor stimulated ANP secretion. The expression of COX-2 increased and the expression of PGF2alpha receptor mRNA decreased in hypertrophied rat atria. These results suggest that PGF2alpha increased the ANP secretion and positive inotropy through PLC-IP3-PKC-MLCK pathway, and the modulation of ANP secretion by COX-2 inhibitor and PGF2alpha may partly relate to the development of renal hypertension.

Osmoregulation of natriuretic peptide receptors in bromoethylamine-treated rat kidney

Extracellular osmolarity is known as an important factor for the regulation of natriuretic peptide receptors (NPRs). We investigated the intra-renal osmoregulation of NPRs using renal medullectomized rats with bromoethylamine hydrobromide (BEA, 200mg/kg). The administration of BEA caused the decreased food intake and body weight. Water intake was decreased on the first day and then increased from the second day. Urine volume was persistently increased from the first day and free water clearance was also increased from the second day. Urinary excretions of sodium and potassium were decreased on the second day and then recovered to control level. Plasma levels of atrial natriuretic peptide (ANP) and Dendroaspis natriuretic peptide (DNP) in BEA-treated rats were not different from control rats. The inactive renin was increased. The maximum binding capacities of (125)I-ANP as well as (125)I-DNP decreased in glomeruli and medulla of BEA-treated rat kidneys but the binding affinity was not changed. In renal cortex, the gene expressions of ANP, NPR-A, and NPR-B were not changed but that of NPR-C decreased. In renal medulla, the gene expressions of NPR-A, -B, and -C decreased without change in ANP mRNA. Both renal medullary osmolarity and sodium concentration by BEA treatment were lower than those in control kidney. The cGMP concentrations in renal medulla and urine in BEA-treated rats were higher than those in control rats. These results suggest that the increased cGMP production may be partly involved in the decrease in NPRs mRNA expression and their binding capacities by BEA-induced medullectomy.

Modification of atrial natriuretic peptide system in cold-induced hypertensive rats

Cold exposure induces hypertension and cardiac hypertrophy via sympathetic activation. The sympathetic nervous system is fundamentally important for the regulation of cardiac atrial natriuretic peptide (ANP) secretion. The present study aimed to define changes in ANP level with renal functions during cold exposure of rats. We also measured the direct effects of adrenergic stimulation on ANP secretion in cold-induced hypertensive rat atria. Sustained elevation of blood pressure and tachycardia were observed by 2-wk cold exposure. Cold exposure increased urine volume, UNaV, UKV and positive water balance. Atrial ANP content, its mRNA level, and plasma ANP concentration increased. Plasma norepinephrine level was increased but both alpha(1A)- and beta(1)-adrenoceptor (AR) mRNA levels in atrium were decreased. In isolated perfused atria from cold-exposed rats, basal ANP secretion increased and pulse pressure decreased. Phenylephrine (alpha(1)-AR agonist)-induced stimulation of ANP secretion, and isoproterenol (beta-AR agonist)-induced suppression of ANP secretion were significantly attenuated. These results suggest that an increased plasma and atrial ANP level by cold exposure may be a compensatory response to changes in hemodynamics and body fluid balance. The phenylephrine- and isoproterenol-induced attenuation of ANP secretion in cold-exposed rat atria may be due to the downregulation of alpha(1A)- and beta(1)-adrenoceptors mRNA levels.

Stimulation of ANP secretion by 2-Cl-IB-MECA through A(3) receptor and CaMKII

Adenosine is a potent mediator of myocardial protection against hypertrophy via A(1) or A(3) receptors that may be partly related to atrial natriuretic peptide (ANP) release. However, little is known about the possible involvement of the A(3) receptor on ANP release. We studied the effects of the A(3) receptor on atrial functions and its modification in hypertrophied atria. A selective A(3) receptor agonist, 2-chloro-N(6)-(3-iodobenzyl) adenosine-5'-N-methyluronamide (2-CI-IB-MECA), was perfused into isolated, beating rat atria with and without receptor modifiers. 2-CI-IB-MECA dose-dependently increased the ANP secretion, which was blocked by the A(3) receptor antagonist, but the increased atrial contractility and decreased cAMP levels induced by 30muM 2-CI-IB-MECA were not affected. The 100muM 2-(1-hexylnyl)-N-methyladenosine (HEMADO) and N(6)-(3-iodobenzyl) adenosine-5'-N-methyluronamide (IB-MECA), A(3) receptor agonist, also stimulated the ANP secretion without positive inotropy. The potency for the stimulation of ANP secretion was 2-CI-IB-MECA>>IB-MECA=HEMADO. The inhibition of the ryanodine receptor or calcium/calmodulin-dependent kinase II (CaMKII) attenuated 2-CI-IB-MECA-induced ANP release, positive inotropy, and translocation of extracellular fluid. However, the inhibition of L-type Ca(2+) channels, sarcoplasmic reticulum Ca(2+)-reuptake, phospholipase C or inositol 1,4,5-triphosphate receptors did not affect these parameters. 2-CI-IB-MECA decreased cAMP level, which was blocked only with an inhibitor of CaMKII or adenylyl cyclase. These results suggest that 2-CI-IB-MECA increases the ANP secretion mainly via A(3) receptor activation and positive inotropy by intracellular Ca(2+) regulation via the ryanodine receptor and CaMKII.

Attenuation of hypoosmotic stress-induced ANP secretion via I(Cl,swell) in renal hypertensive rat atria

Cardiac hypertrophy, an adaptive process to an increased hemodynamic overload, includes not only an increase in cell size but also qualitative changes in constituent proteins. Although swelling-activated chloride channels (I(Cl,swell)) chronically activate in hypertrophied atrial myocytes, the role of I(Cl,swell) in regulation of atrial natriuretic peptide (ANP) release is poorly understood. We investigated the effects of I(Cl,swell) on ANP release and contractility and its modification in hypertrophied rat atria. To stimulate I(Cl,swell), hypoosmotic HEPES buffered solution (0.8T, 0.7T and 0.6T) was perfused into isolated perfused beating atria. The hypoosmotic HEPES buffered solution increased ANP release as compared to isoosmotic buffered solution (1T) in an osmolarity-reduction dependent manner. Atrial contractility and extracellular fluid translocation did not change. Exposure to hypoosmotic buffer (0.8T) containing low chloride (8mM), tamoxifen or diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) significantly attenuated hypoosmolarity-induced ANP release. The pretreatment with genistein, okdaic acid, U73122, GF109203x, and staurosporine attenuated hypoosmolarity-induced ANP release whereas orthovanadate augmented it significantly. In hypertrophied atria from renal hypertensive rats, hypoosmolarity-induced ANP release was markedly attenuated and DIDS-induced decrease in ANP release and negative inotropy were augmented as compared to sham-operated rat atria. Therefore, we suggest that I(Cl,swell) may partly participate hypoosmolarity-induced ANP release through protein tyrosine kinase and phospholipase C-protein kinase C pathway. The modification of responses of ANP release to hypoosmolarity and DIDS in hypertrophied atria may relate to changes in I(Cl,swell) activity by persistent high blood pressure.

Different response of ANP secretion to adrenoceptor stimulation in renal hypertensive rat atria

Sympathetic nervous system and atrial natriuretic peptide (ANP) system play fundamental roles in the regulation of cardiovascular functions. Overactivity of sympathetic nervous system can lead into cardiovascular diseases such as heart failure and hypertension. The present study aimed to define which adrenergic receptors (ARs) affect atrial contractility and ANP release and to determine their modification in renal hypertensive rat atria. An alpha(1)-AR agonist, cirazoline increased ANP release with positive inotropism. These alpha(1)-AR agonist-mediated responses were attenuated by the alpha(1A)-AR antagonist, but not by the alpha(1B)- or alpha(1D)-AR antagonist. An alpha(2)-AR agonist, guanabenz and clonidine increased ANP release with negative inotropism and decreased cAMP level. The order of potency for the increased ANP release was cirazoline>>phenylephrine=guanabenz>>clonidine. In contrast, a beta-AR agonist, isoproterenol decreased ANP release with positive inotropism and these responses were blocked by the beta(1)-AR antagonist but not by the beta(2)-AR antagonist. The increased cAMP level by isoproterenol was suppressed by pretreatment with both beta(1)- and beta(2)-AR antagonists. In renal hypertensive rat atria, the effects of isoproterenol on atrial contractility, ANP release, and cAMP level were attenuated whereas the effect of cirazoline on ANP release was unaltered. Atrial beta(1)-AR mRNA level but not alpha(1A)-AR mRNA level was decreased in renal hypertensive rats. These findings suggest that alpha(1A)- and beta(1)-AR oppositely regulate atrial ANP release and that atrial beta(1)-AR expression/function is impaired in renal hypertensive rats.

Regulation of stretch-activated ANP secretion by chloride channels

This study was aimed to define roles of stretch-activated ion channels (SACs), especially Cl(-) channels, in regulation of atrial natriuretic peptide (ANP) secretion using isolated perfused beating atria. The volume load was achieved by elevating height of outflow catheter connected to isolated rat atria and the pressure load was achieved by decreasing diameter of outflow catheter. Both methods increased atrial contractility similarly although volume load was different (736microl for volume load vs. 129microl for pressure load). Atrial stretch by volume load markedly increased ECF translocation and ANP secretion but the pressure load slightly increased. The ANP secretion was positively correlated to workload generated by volume or pressure load. Treatment of atria with gadolinium, a blocker for SACs, attenuated the ECF translocation and the ANP secretion induced by volume load. A blocker for Ca2+-activated Cl(-) channel, niflumic acid (NFA), accentuated the ANP secretion induced by volume load whereas a blocker for swelling-activated Cl(-) channel, diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), attenuated the ANP secretion. The ANP secretion of hypertrophied atria by volume load was markedly reduced and the augmented effect of NFA on volume load-induced ANP secretion was not observed. These results indicate that Cl(-) channels may differently regulate stretch-activated ANP secretion.

Diadenosine tetraphosphate stimulates atrial ANP release via A(1) receptor: involvement of K(ATP) channel and PKC

Diadenosine polyphosphates (APnAs) are endogenous compounds and exert diverse cardiovascular functions. However, the effects of APnAs on atrial ANP release and contractility have not been studied. In this study, the effects of diadenosine tetraphosphate (AP4A) on atrial ANP release and contractility, and their mechanisms were studied using isolated perfused rat atria. Treatment of atria with AP4A resulted in decreases in atrial contractility and extracellular fluid (ECF) translocation whereas ANP secretion and cAMP levels in perfusate were increased in a dose-dependent manner. These effects of AP4A were attenuated by A(1) receptor antagonist but not by A(2A) or A(3) receptor antagonist. Other purinoceptor antagonists also did not show any effects on AP4A-induced ANF release and contractility. The increment of ANP release and negative inotropy induced by AP4A was similar to those induced by AP3A, AP5A, and AP6A. Protein kinase A inhibitors accentuated AP4A-induced ANP secretion. In contrast, an inhibitor of phospholipase C, protein kinase C or sarcolemma K(ATP) channel completely blocked AP4A-induced ANP secretion. However, an inhibitor of adenylyl cyclase or mitochondria K(ATP) channel had no significant modification of AP4A effects. These results suggest that AP4A regulates atrial inotropy and ANP release mainly through A(1) receptor signaling involving phospholipase C-protein kinase C and sarcolemmal K(ATP) channel and that protein kinase A negatively modulates the effects of AP4A.

Adenosine-stimulated atrial natriuretic peptide release through A1 receptor subtype

Adenosine acts as an important protector of ischemic myocardium through coronary vasodilation and the depression of cardiac contractility. The protective effect of adenosine may partly relate to the cardiac hormone atrial natriuretic peptide (ANP). The aim of the present study was to investigate the effects of adenosine and the adenosine receptor subtype on atrial hemodynamics and ANP release using isolated perfused beating rat atria. Adenosine, a nonselective adenosine receptor agonist, increased the ANP release with negative inotropism in a dose-dependent manner. Adenosine-stimulated ANP release was attenuated by a selective A1 antagonist but not A(2A) antagonist or A3 antagonist. The order of potency of the various agonists for the ANP release was A1 agonists>>A3 agonist=adenosine>A(2A) agonist. The order of potency for the negative inotropy was A1 agonists>adenosine=A(2A) agonist>A3 agonist. The negative inotropism and ANP release by a specific A1 agonist (N6-cyclopentyl-adenosine) were also attenuated by A1 antagonist but not A(2A) antagonist or A3 antagonist. Treatment with A1 agonist resulted in a decrease of cAMP contents in atria and perfusates. The agonist-stimulated ANP release was significantly attenuated in the presence of forskolin, isoproterenol 8-Br-cAMP, or an adenylyl cyclase inhibitor. These results suggest that the A1 receptor subtype is responsible for the adenosine-induced ANP release and negative inotropism through adenylyl cyclase-cAMP pathway.

ATP-stimulated ANP release through P1 receptor subtype

Extracellular ATP acts as a local regulator of physiological functions in the cardiovascular system via P1 and P2 receptors. However, little is known about the effect of ATP on the release of atrial natriuretic peptide (ANP) secretion. The purpose of this study was to investigate the effects of extracellular ATP on atrial hemodynamics and ANP release and to identify their receptor-mediated mechanism. ATP was infused into isolated perfused beating rat atria in the absence and presence of various receptor antagonists. ATP (from 0.1 to 30 microM) increased the ANP release with negative inotropism in a dose-dependent manner. ADP (30 microM) also caused an increase in ANP release with similarity to ATP, but alpha,beta-methylene ATP (alpha,beta-MeATP, P2X1 receptor agonist) and 2-methylthioADP (2-MesADP, P2Y1 receptor agonist) did not. The rank order of potency for the increment of ANP release was adenosine>ATP=ADP>2-MesADP>alpha,beta-MeATP. In contrast, UTP, an agonist for P2Y2,4,6 receptor, caused a decrease in ANP release without changes in contractility. Extracellular ATP-induced increase in ANP release and negative inotropism were completely blocked by the pretreatment of 8-cyclopentyl-1,3-dipropylxanthine (P1 receptor antagonist), but not by pyridoxal phosphate-6-azobenzene-2,4-disulfonic acid (P2X1 receptor antagonist) and suramin (P2XY receptor antagonist). Reactive Blue 2 (P2Y receptor antagonist) caused an augmentation of ATP-induced increase in ANP release without affecting negative inotropism. Adenosine 5'-(alpha,beta-methylene) diphosphate, an ectonucleotidase inhibitor, did not affect ATP-induced augmentation of ANP release with negative inotropy. These results suggest that extracellular ATP-induced increase in ANP release and negative inotropism are mediated mainly by P1 receptor, and UTP decreases ANP release. Therefore, we suggest that extracellular ATP and UTP may have opposite actions on the regulation of ANP secretion.

Dendroaspis natriuretic peptide and its functions in pig ovarian granulosa cells

Dendroaspis natriuretic peptide (DNP), a 38-amino acid peptide, was isolated from the venom of green mamba. It has structural and functional similarities to the other members of the natriuretic peptide family. The purpose of this study was to determine whether DNP is present in pig ovarian granulosa cells and to define its biological functions. The serial dilution curves of extracts of granulosa cells and follicular fluid were parallel to the standard curve of DNP, and a major peak of molecular profile of both extracts by HPLC was synthetic DNP. The concentration of DNP was 7.51+/-1.46 pg/10(7) cells and 24.81+/-2.38 pg/ml in granulosa cells and follicular fluid, respectively. Natriuretic peptides increased cGMP production in the purified membrane of granulosa cells with a rank order of potency of C-type natriuretic peptide (CNP)>atrial natriuretic peptide (ANP)=DNP. mRNAs for natriuretic peptide receptor-A (NPR-A), NPR-B and NPR-C were detected by RT-PCR. The binding site of (125)I-DNP was also observed in granulosa cell layer by in vitro autoradiography. Synthetic DNP inhibited the secretion of ANP from granulosa cells in a concentration-dependent manner and the potency was similar to CNP. The concentration of DNP and CNP, which inhibited the secretion of ANP by 50%, was about 1 nM. Increases in production of cGMP in granulosa cells were observed by DNP or CNP. Therefore, these results show the existence of DNP system and the cross-talk between natriuretic peptides in pig ovarian granulosa cells.

Osmoregulation of atrial myocytic ANP release: osmotransduction via cross-talk between L-type Ca2+ channel and SR Ca2+ release

Hyperosmolality has been known to increase ANP release. However, its physiological role in the regulation of atrial myocytic ANP release and the mechanism by which hyperosmolality increases ANP release are to be defined. The purpose of the present study was to define these questions. Experiments were performed in perfused beating rabbit atria. Hyperosmolality increased atrial ANP release, cAMP efflux, and atrial dynamics in a concentration-dependent manner. The osmolality threshold for the increase in ANP release was as low as 10 mosmol/kgH2O (approximately 3%) above the basal levels (1.55 +/- 1.71, 17.19 +/- 3.11, 23.15 +/- 5.49, 54.04 +/- 11.98, and 62.00 +/- 13.48% for 10, 20, 30, 60, and 100 mM mannitol, respectively; all P < 0.01). Blockade of sarcolemmal L-type Ca2+ channel activity, which increased ANP release, attenuated hyperosmolality-induced increases in ANP release (-13.58 +/- 4.68% vs. 62.00 +/- 13.48%, P < 0.001) and cAMP efflux but not atrial dynamics. Blockade of the Ca2+ release from the sarcoplasmic reticulum, which increased ANP release, attenuated hyperosmolality-induced increases in ANP release (13.44 +/- 7.47% vs. 62.00 +/- 13.48%, P < 0.01) and dynamics but not cAMP efflux. Blockades of Na+-K+-2Cl- cotransporter, Na+/H+ exchanger, and Na+/Ca2+ exchanger had no effect on hyperosmolality-induced increase in ANP release. The present study suggests that hyperosmolality regulates atrial myocytic ANP release and that the mechanism by which hyperosmolality activates ANP release is closely related to the cross-talk between the sarcolemmal L-type Ca2+ channel activity and sarcoplasmic reticulum Ca2+ release, possibly inactivation of the L-type Ca2+ channels.

Stretch-activated atrial natriuretic peptide secretion in atria with heat shock protein 70 overexpression

The mechanical overload of the heart is known to induce the expression of atrial natriuretic peptide (ANP) and heat-shock protein 70 (HSP70) in the heart. However, the relationship between these two substances remains unknown. In the present study, we characterized ANP secretion from mouse atria and examined a possible role that HSP70 may play in the regulation of ANP synthesis and secretion by using atria in transgenic mice in which HSP70 was overexpressed. We generated transgenic mice harboring the human HSP70gene under the transcriptional control of human myosin heavy-chain promoter. In these mice, the transgene was overexpressed in the heart. Both atrial ANP messenger RNA and its concentration in the HSP70 transgenic mice were measured; these were not significantly different from those in wild-type mice. In isolated perfused nonbeating atria, basal secretion of ANP was similar in both groups. When atrial volume was increased by changing atrial pressure, extracellular fluid (ECF) translocation and ANP secretion proportionately increased. Changes in atrial volume and ECF translocation and ANP secretion were positively correlated. However, these parameters did not significantly differ between the two groups. Endothelin-1 (ET-1), the strongest paracrine stimulus of ANP secretion, accentuated stretch-activated ANP secretion without significantly changing mechanically stimulated ECF translocation, as compared with that in the wild-type mice. The increased ANP secretion due to ET-1 in the transgenic mice was similar to that in the wild-type mice. The results suggest that both atrial stretching and ET-1 are important stimuli to ANP secretion from mouse atria, and the responsiveness of the ANP system to those stimuli are unlikely coupled to the pathway involving HSP70.

Regulation of ANP secretion by cardiac Na+/Ca2+ exchanger using a new controlled atrial model

The myocardial interstitium is important in regulating cardiac function. Between the atrial lumen and the pericardial space are transmural pathways, and movement of interstitial fluid (ISF) through these pathways is one of the main driving forces regulating translocation of substances from the interstitium into the blood. To define how ISF translocation from the interstitial space into the luminal space is regulated by each component of atrial hemodynamics, we devised a new rabbit atrial model in which each physical parameter could be controlled independently. Using this system, we also defined the physiological role of the cardiac Na(+)/Ca(2+) exchanger on secretion of atrial natriuretic peptide (ANP) by depletion of extracellular Na(+) ([Na(+)](o)). Increases in stroke volume and atrial end-systolic volume increased ISF translocation and ANP secretion. However, an increase in atrial rate did not influence ISF translocation but, rather, increased ANP secretion. Gradual depletion of [Na(+)](o) caused gradual increases in ANP secretion and intracellular Ca(2+) ([Ca(2+)](i)), which were blocked in the presence of Ca(2+)-free buffer and Ni(2+), but not in the presence of KB-R7943, diltiazem, mibefradil, caffeine, or monensin. Amiloride and its analog blocked an increase in ANP secretion but not an increase in [Ca(2+)](i) by [Na(+)](o) depletion. Therefore, we suggest that ANP secretion and ISF translocation may be differently controlled by each physical factor. These results also suggest that the increase in ANP secretion in response to [Na(+)](o) depletion may involve inhibition of Na(+)/Ca(2+) and Na(+)/H(+) exchangers but not an increase in [Ca(2+)](i).

Postnatal changes in inhibitory effect of C-type natriuretic peptide on secretion of ANP

To define developmental changes in atrial natriuretic peptide (ANP) secretion and in the cross talk between C-type natriuretic peptide (CNP) and ANP, we performed experiments in isolated perfused nonbeating cardiac atria isolated from rabbits between 1 and 8 wk of age. Changes in atrial pressure resulted in increases in atrial volume that rose with age and reached the peak value at 4 wk. A rise in volume change increased ANP secretion with concomitant translocation of extracellular fluid (ECF) into the atrial lumen, which increased with age and reached the peak value at 4 wk. The positive relationship between stretch-induced ANP secretion and ECF translocation shifted upward and leftward with age. CNP suppressed stretch-induced ANP secretion in the 8-wk-old group but not in the 2- and 4-wk-old groups without differences in ECF translocation and atrial volume. Therefore, the ANP secretion in terms of ECF translocation was markedly suppressed by CNP in the 8-wk-old group but not in the 2- and 4-wk-old groups. The production of cGMP by CNP in atrial tissue membranes was markedly attenuated in young rabbits. However, 8-bromo-cGMP suppressed stretch-induced ANP secretion in 2- and 8-wk-old groups. Natriuretic peptide receptor-B mRNA was similar in both groups. Therefore, we conclude that the inhibitory effect of CNP on atrial ANP secretion is developmentally regulated, being absent during normal cardiac development in young animals and intact in adult animals.

Attenuation of inhibitory effect of CNP on the secretion of ANP from hypertrophied atria

It has been shown that atrial natriuretic peptide (ANP) influences proliferation of cardiac cells. To define the possible role of C-type natriuretic peptide (CNP) in cardiac hypertrophy, the influence of CNP on the secretion of ANP was studied with the use of perfused nonbeating atria from monocrotaline-treated rats. Increases in atrial volume caused proportional increases in ANP secretion that were markedly suppressed by CNP (10(-6) M) in nonhypertrophied left atria and control right atria but not in hypertrophied right atria. However, increases in atrial volume and mechanically stimulated extracellular fluid (ECF) translocation by CNP were similar to those in the control group. Therefore, the secretion of ANP in terms of ECF translocation was decreased by CNP in nonhypertrophied left and control right atria but not in hypertrophied atria. However, the inhibitory effect of 8-bromo-cGMP on the secretion of ANP was observed in both atria. The cGMP productions from perfused hypertrophied atria and their membranes exposed to CNP were significantly lower than those from nonhypertrophied atria. No significant difference in natriuretic peptide receptor-B transcript was found. Therefore, attenuation of the inhibitory effect of CNP on the ANP secretion in hypertrophied atria may be due to lack of cGMP production. The results showing the relief of CNP-induced negative inhibition of ANP secretion by atrial hypertrophy suggest that CNP may be a contributing factor to delay the development of cardiac hypertrophy.

Accentuation of ANP secretion to endothelin-1 in hypertrophied atria

To investigate modulation of ANP secretion by atrial hypertrophy, the secretion of ANP in response to stretch and endothelin-1 was studied using isolated perfused quiescent atria from rats treated with monocrotaline (MCT). Male Sprague-Dawley rats were given a single subcutaneous injection of 50 mg/kg MCT or saline and were sacrificed at 6 weeks. Rats with right heart hypertrophy showed an increase in ANP mRNA and decrease in tissue concentration of ANP in hypertrophied atria and a marked increase in plasma concentration of ANP. In isolated perfused hypertrophied right atria from MCT rats, changes in atrial volume induced by increased atrial pressure caused proportional increases in mechanically stimulated extracellular fluid (ECF) translocation and stretch-activated ANP secretion. Changes in atrial volume and mechanically stimulated ECF translocation in hypertrophied right atria were not different from those in control right atria. The stretch-activated ANP secretion was suppressed without significant difference in basal ANP secretion, as compared to control right atria. Therefore, the stretch-activated ANP secretion from hypertrophied right atria into the atrial lumen in relation to the ECF translocation (ANP concentration in the interstitium) was lower than that from control atria. A positive correlation between the stretch-activated ANP secretion in relation to the ECF translocation and tissue ANP content was found in control atria but not in hypertrophied atria. Endothelin-1 caused increases in stretch-activated ANP secretion in a dose-dependent manner, which were accentuated in hypertrophied right atria. Therefore, we suggest that atrial hypertrophy causes an attenuated response to stretch and accentuated response to endothelin-1 of ANP secretion.

Effect of extracellular calcium depletion on the two-step ANP secretion in perfused rabbit atria

Employing isolated perfused rabbit atrial model we have found that stretch-activated atrial natriuretic peptide (ANP) secretion takes place in two steps: release of ANP from myocytes into surrounding intercellular space, and then translocation of the released ANP into atrial lumen along with the extracellular fluid (ECF) translocated upon releasing the stretch. Ca2+, one of the most important factors regulating secretory processes, has been reported by many workers to have influence on the ANP secretion, but at large variance. In the present study, therefore, we undertook to clarify the influence of Ca2+ depletion on ANP secretion and further to define which of the two steps is affected by Ca2+ removal. Extracellular Ca2+ depletion resulted in increased basal secretion and in accentuated secretion of immunoreactive (ir) ANP in response to mechanical stimuli. The translocation of ECF increased in response to atrial stretch-and-release, but it was not affected by Ca2+ depletion. The irANP concentration in the extracellular space calculated as the amount of irANP secreted in relation to the ECF translocated significantly increased when extracellular Ca2+ was depleted. These results indicate that extracellular Ca2+ depletion accentuates the stretch-induced ANP secretion through the augmentation of ANP release into the interstitium without changes in the ECF translocation.