Increase In Atrial Natriuretic Peptide Secretion Research Articles

Effects of liraglutide on ANP secretion and cardiac dynamics.

To observe the effects of liraglutide (analog of glucagon-like peptide 1 (GLP-1)) on atrial natriuretic peptide (ANP) secretion and atrial dynamics, an ex vivo isolated rat atrial perfusion model was used to determine atrial ANP secretion and pulse pressure. DPP-4-/- mice were also established in vivo. ANP levels were determined by radioimmunoassay; GLP-1 content was determined by Elisa. The expression levels of GLP-1 receptor (GLP-1R), PI3K/AKT/mTOR, piezo 1, and cathepsin K were analyzed by Western blot. In the clinical study, patients with acute coronary syndrome (ACS) had low levels of plasma GLP-1 but relatively high levels of plasma ANP. In ex vivo (3.2 nmol/L) and in vivo (30 μg/kg) models, liraglutide significantly decreased ANP levels and atrial pulse pressure. Exendin9-39 alone (GLP-1R antagonist) reversibly significantly increased ANP secretion, and the reduction effect of liraglutide on the secretion of ANP was significantly alleviated by Exendin9-39. Exendin9-39 demonstrated slightly decreased atrial pulse pressure; however, combined liraglutide and Exendin9-39 significantly decreased atrial pulse pressure. Ly294002 (PI3K/AKT inhibitor) inhibited the increase of ANP secretion by liraglutide for a short time, while Ly294002 didn't counteract the decrease in pulse pressure by liraglutide in atrial dynamics studies. Liraglutide increased the expression of GLP-1R and PI3K/AKT/mTOR in isolated rat atria and the hearts of mice in vivo, whereas Exendin9-39 reversibly reduced the expression of GLP-1R and PI3K/AKT/mTOR. Piezo 1 was significantly decreased in wild type and DPP-4-/- mouse heart or isolated rat atria after being treated with liraglutide. Cathepsin K expression was only decreased in in vivo model hearts. Liraglutide can inhibit ANP secretion while decreasing atrial pulse pressure mediated by GLP-1R. Liraglutide probably plays a role in the reduction of ANP secretion via the PI3K/AKT/mTOR signaling pathway. Piezo 1 and cathepsin K may be involved in the liraglutide mechanism of reduction.

The protein tyrosine kinase inhibitor genistein suppresses hypoxia-induced atrial natriuretic peptide secretion mediated by the PI3K/Akt-HIF-1α pathway in isolated beating rat atria.

Genistein, an isoflavonoid that can inhibit protein tyrosine kinase (PTK) phosphorylation, has been shown to play pivotal roles in the signal transduction pathways of hypoxic disorders. In this study, we established a rat model of isolated beating atrium and investigated the regulator role of genistein and its downstream signaling pathways in acute hypoxia-induced atrial natriuretic peptide (ANP) secretion. Radioimmunoassay was used to detect the ANP content in the atrial perfusates. Western blot analysis was used to determine the protein level of hypoxia-inducible factor 1α (HIF-1α), and GATA4 in the atrial tissue. The results showed that acute hypoxia substantially promoted ANP secretion, whereas this effect was partly attenuated by the PTKs inhibitor genistein (3 μM). By Western blotting analysis, we found that hypoxia-induced increase in phosphorylation of Akt and transcriptional factors, including HIF-1α, were also reversed by genistein. The perfused HIF-1α inhibitors rotenone (0.5μM) or CAY10585 (10μM) plus genistein significantly abolished the enhanced ANP section induced by hypoxia. Additionally, the perfused PI3K/Akt agonist insulin-like growth factor 1 (30μM) also abolished ANP secretion induced by genistein and inhibited expression of HIF-1α. In summary, our data suggested that acute hypoxia markedly increased ANP secretion by PTKs through the phosphoinositide-3 kinase (PI3K)/HIF-1α dependent pathway.

Preeminent role of the cardiorenal axis in the antihypertensive response to an arteriovenous fistula: an in silico analysis.

Percutaneous creation of a small central arteriovenous (AV) fistula is currently being evaluated for the treatment of uncontrolled hypertension (HT). Although the mechanisms that contribute to the antihypertensive effects of the fistula are unclear, investigators have speculated that chronic blood pressure (BP) lowering may be due to 1) reduced total peripheral resistance (TPR), 2) increased secretion of atrial natriuretic peptide (ANP), and/or 3) suppression of renal sympathetic nerve activity (RSNA). We used an established integrative mathematical model of human physiology to investigate these possibilities from baseline conditions that mimic sympathetic overactivity and impaired renal function in patients with resistant HT. After a small fistula was stimulated, there were sustained increases in cardiac output, atrial pressures, and plasma ANP concentration (3-fold), without suppression of RSNA; at 8 wk, BP was reduced 14 mmHg along with a 32% fall in TPR. In contrast, when this simulation was repeated while clamping ANP at baseline BP decreased only 4 mmHg, despite a comparable fall in TPR. Furthermore, when chronic resetting of atrial mechanoreceptors was prevented during the fistula, RSNA decreased 7%, and along with the same threefold increase in ANP, BP fell 19 mmHg. This exaggerated fall in BP occurred with a similar decrease in TPR when compared with the above simulations. These findings suggest that ANP, but not TPR, is a key determinant of long-term BP lowering after the creation of an AV fistula and support a contribution of suppressed RSNA if resetting of the atrial-renal reflex is truly incomplete.NEW & NOTEWORTHY The mechanisms that contribute to the antihypertensive effects of a small arteriovenous (AV) fistula comparable to the size used by the ROX coupler currently in clinical trials are unclear and not readily testable in clinical or experimental studies. The integrative mathematical model of human physiology used in the current study provides a tool for understanding key causal relationships that account for blood pressure (BP) lowering and for testing competing hypotheses. The findings from the simulations suggest that after creation of a small AV fistula increased ANP secretion plays a critical role in mediating long-term reductions in BP. Measurement of natriuretic peptide levels in hypertensive patients implanted with the ROX coupler would provide one critical test of this hypothesis.

Atrial secretion of ANP is suppressed in renovascular hypertension: shifting of ANP secretion from atria to the left ventricle.

In the present study, the change in secretion of atrial natriuretic peptide (ANP) from the atria was defined in hypertension accompanied by ventricular hypertrophy and increased synthesis of ANP. To identify the change of the secretion and mechanisms involved, experiments were performed in isolated perfused beating atria from sham-operated normotensive and renovascular hypertensive rats. Expression of ANP, natriuretic peptide receptor (NPR)-C, components of the renin-angiotensin system, and muscarinic signaling pathway was measured in cardiac tissues. Basal levels of ANP secretion and acetylcholine (ACh)- and stretch-induced activation of ANP secretion were suppressed in the atria from hypertensive compared with normotensive rats. ACh increased ANP secretion via M2 muscarinic ACh receptor-ACh-sensitive K+ channel signaling. In hypertensive rats, ANP concentration increased in the left ventricle but decreased in the right ventricle. The atrial concentration of ANP was not changed in hypertensive compared with normotensive rats. ANP mRNA expression was accentuated in the left ventricle but suppressed in the other cardiac chambers in the hearts of hypertensive rats. NPR-C expression was inversely related to ANP mRNA levels. Angiotensin II type 1 receptor (AT1R) expression was accentuated in the cardiac chambers from hypertensive rats compared with normotensive rats, whereas angiotensin II type 2 receptor, M2 muscarinic receptor, and Kir3.4 channels were suppressed. AT1R blockade with losartan reversed the change observed in hypertensive rats. The present findings indicate that renovascular hypertension shifts the major site of ANP secretion and synthesis from the atria to the left ventricle through modulation of the expression of ANP, NPR-C, AT1R, and the M2 muscarinic signaling pathway. NEW & NOTEWORTHY Renovascular hypertension suppresses the atrial secretion of ANP and shifts the major site of the regulation of ANP secretion and synthesis from atria to the hypertrophied left ventricle possibly via modulation of the expression of ANP, natriuretic peptide receptor-C, angiotensin II subtype 1 receptor, and M2 muscarinic signaling pathway.

Endogenous Endothelin-1 Regulates Hypoxia-Induced Atrial Natriuretic Peptide Secretion by Activating the MAPK/ERK and PI3K/Akt Signaling Pathways in Isolated Beating Rabbit Atria

The present study investigated a possible mechanism for endogenous endothelin-1 (ET-1) regulation of atrial natriuretic peptide (ANP) secretion in isolated perfused acute hypoxic rabbit atria. Acute hypoxia significantly enhanced the release of ET-1 and the expression of the ET receptor (ETR) type A and B (ETRA and ETRB) in atrial tissues, with a concomitant increase in ANP secretion. The ETRA or ETRB antagonist, BQ123 (0.3 μmol/L) or BQ788 (0.3 μmol/L), respectively attenuated hypoxia-induced ANP secretion. Both antagonists significantly attenuated the levels of hypoxiainduced atrial phosphorylated (p)-extracellular signal-regulated kinase (ERK) and p-protein kinase B (Akt). The ERK and Akt inhibitors, PD098059 (30 μmol/L) and LY294002 (30 μmol/L), respectively mimicked the effect of the ETR antagonists. These results demonstrated that acute hypoxia- mediated atrial ET-1 regulated ANP secretion through ETR and the subsequent mitogenactivated protein kinase (MAPK)/ERK and ETR-phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathways. These pathways may mediate atrial endocrine functions under hypoxic conditions.

Sibjotang Increases Atrial Natriuretic Peptide Secretion in Beating Rabbit Atria.

Sibjotang (Shizaotang), traditional herbal medicine formula, which was first documented in the Shanghanlun, has long been prescribed for the treatment of impairment of the body fluid homeostasis. The purpose of the present study was to identify the effects of Sibjotang on the secretion of a cardiac hormone, atrial natriuretic peptide (ANP), one of the main hormones involved in the regulation of the body fluid and blood pressure homeostasis. Water extract of Sibjotang increased ANP secretion concomitantly with an increase in atrial dynamics in a concentration-dependent manner. Sibjotang-induced increase in ANP secretion and positive inotropic effect were attenuated by GO6976 and LY333531, selective inhibitors of conventional protein kinase C, but not Rottlerin, an inhibitor of novel PKCδ. Similarly to the effect of Sibjotang, extracts of components of Sibjotang, Euphorbia kansui, and Daphne genkwa, but not Euphorbia pekinensis and Ziziphus jujuba, increased ANP secretion and atrial dynamics. Ingredients of Sibjotang, apigenin, rosmarinic acid, and salvianolic acid B decreased ANP secretion and atrial dynamics. These findings suggest that Sibjotang increases ANP secretion and atrial dynamics via activation of conventional protein kinase C signaling. This finding provides experimental evidence for the rationale in the use of Sibjotang in the treatment of impairment of the regulation of body fluid and blood pressure homeostasis.

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 1μM) 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.

Effect of berberine on the atrial natriuretic peptide secretion in cardiac atria (1150.9)

Effect of Berberine on the Atrial Natriuretic Peptide Secretion in Cardiac AtriaHye Rim Yoo 1,2, Hye Yoom Kim 1,2, Oh Jeong Kwon 1,2, Kyung Woo Cho 2, Dae Gill Kang 1,2, and Ho Sub Lee 1,2*1College of Oriental Medicine and Professional Graduate School of Oriental Medicine, Wonkwang University, Iksan, Jeonbuk 570‐749, Republic of Korea; 2Hanbang Body‐fluid Research Center, Wonkwang University, Shinyong‐dong, Iksan, Jeonbuk, 570‐749, Republic of KoreaThe aim of the present study is to identify the effects of berberine on the secretion of atrial natriuretic peptide (ANP) from the atrium. Experiments were performed in perfused beating rat left atria paced electrically. Atrial dynamics, atrial pulse pressure, was measured with physiograph. ANP was measured using radioimmunoassay. As a result, berberine increased ANP secretion and atrial dynamics in a concentration‐ dependent manner. Atropine‐non‐selective muscarinic receptor antagonist, methoctramine‐selective M2 muscarinic receptor antagonist, and hemicholinium 3‐inhibitor of choline transporter which inhibits acetylcholine synthesis had no effects on the berberine‐induced changes in ANP secretion and pulse pressure. Berberine is known to activate AMP‐activated protein kinase (AMPK), experiments were performed to identify involvements of this possibility on the berberine‐induced changes in ANP secretion and pulse pressure. Compound C, an inhibitor of AMPK activation, slightly inhibited the berberine‐induced increase in ANP secretion and pulse pressure. These findings suggest that berberine increases ANP secretion and atrial pulse pressure via, at least in part, activation of AMPK signaling.

Accentuation of ursolic acid on muscarinic receptor-induced ANP secretion in beating rabbit atria

AimsUrsolic acid has recently been reported to increase both atrial natriuretic peptide (ANP) secretion and mechanical dynamics in rabbit atria. Main methodsThe present study was designed to clarify the regulatory effects of ursolic acid on the β-adrenergic or muscarinic receptor-mediated changes in ANP secretory and contractile function allowing measurement of atrial dynamics such as pulse pressure, stroke volume, and cAMP efflux in isolated perfused beating rabbit atria. Key findingsPretreatment with ursolic acid significantly attenuated the isoproterenol (β-adrenergic agonist)-induced decrease in ANP secretion and increases in cAMP levels and atrial dynamics. Interestingly, ursolic acid concentration-dependently accentuated the acetylcholine-induced increase in ANP secretion and decrease in pulse pressure in the presence of isoproterenol (p<0.001). These findings indicate that acetylcholine-induced increase in ANP secretion is potentiated by ursolic acid; furthermore, acetylcholine-induced decrease in atrial dynamics is also potentiated by ursolic acid, suggesting that ursolic acid regulates muscarinic receptor-mediated secretory and contractile responses in perfused beating rabbit atria. SignificanceThis implicates for the beneficial effects of ursolic acid in the regulation of cardiovascular and body fluid homeostasis.

Endogenous ACh tonically stimulates ANP secretion in rat atria

Exogenous acetylcholine (ACh) is known to stimulate atrial natriuretic peptide (ANP) secretion concomitantly with a decrease in atrial pulse pressure. However, the role of intrinsic ACh in the regulation of ANP secretion remains unknown. Recently, it was shown that nonneuronal and neuronal ACh is present in the cardiac atria. From this finding we hypothesize that endogenously released ACh is involved in the regulation of ANP secretion in an autocrine or paracrine manner in the atria. Experiments were performed in isolated beating rat atria. ANP was measured using radioimmunoassay. To increase the availability of the ACh in the extracellular space of the atrium, its degradation was inhibited with an inhibitor of acetylcholinesterase. Acetylcholinesterase inhibition with physostigmine increased ANP secretion concomitantly with a decrease in atrial dynamics in a concentration-dependent manner. Inhibitors of M2 muscarinic ACh receptor (mAChR), methoctramine, and ACh-activated K(+) (KACh(+)) channels, tertiapin-Q, abolished the physostigmine-induced changes. The effects were not observed in the atria from rats treated with pertussis toxin. Furthermore, the physostigmine-induced effects were attenuated by an inhibitor of high-affinity choline transporter, hemicholinium-3, which is a rate-limiting step of ACh synthesis. Inhibitors of the mAChR signaling pathway and ACh synthesis also attenuated the basal levels of ANP secretion and accentuated atrial dynamics. These findings suggest that endogenously released ACh tonically stimulates ANP secretion from atrial cardiomyocytes via activation of M2 mAChR-Gi/o-KACh(+) channel signaling. It is also suggested that the ACh-ANP signaling is implicated in cardiac physiology and pathophysiology.

MAPK and PI3K pathways regulate hypoxia-induced atrial natriuretic peptide secretion by controlling HIF-1 alpha expression in beating rabbit atria

Mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) signaling pathways are pivotal and intensively studied signaling pathways in hypoxic conditions. However, the roles of MAPK and PI3K in the regulation of hypoxia-induced atrial natriuretic peptide (ANP) secretion are not well understood. The purpose of the present study was to investigate the mechanism by which the MAPK/ERK (extracellular signal-regulated kinase) and PI3K signaling pathways regulate the acute hypoxia-induced ANP secretion in isolated beating rabbit atria. An acute hypoxic perfused beating rabbit atrial model was used. The ANP levels in the atrial perfusates were measured by radioimmunoassay, and the hypoxia-inducible factor-1α (HIF-1α) mRNA and protein levels in the atrial tissue were determined by RT-PCR and Western blot. Acute hypoxia significantly increased ANP secretion and HIF-1α mRNA and protein levels. Hypoxia-induced ANP secretion was markedly attenuated by the HIF-1α inhibitors, rotenone (0.5μmol/L) and CAY10585 (10μmol/L), concomitantly with downregulation of the hypoxia-induced HIF-1α mRNA and protein levels. PD098059 (30μmol/L) and LY294002 (30μmol/L), inhibitors of MAPK and PI3K, markedly abolished the hypoxia-induced ANP secretion and atrial HIF-1α mRNA and protein levels. The hypoxia-suppressed atrial dynamics were significantly attenuated by PD098059 and LY294002. Acute hypoxia in isolated perfused beating rabbit atria, markedly increased ANP secretion through HIF-1α upregulation, which was regulated by the MAPK/ERK and PI3K pathways. ANP appears to be part of the protective program regulated by HIF-1α in the response to acute hypoxic conditions.

Oleanolic acid increases plasma ANP levels via an accentuation of cardiac ANP synthesis and secretion in rats

Oleanolic acid is known to have beneficial effects on the regulation of cardiovascular homeostasis. The present study was designed to identify the effects of oleanolic acid on plasma levels and atrial synthesis and secretion of atrial natriuretic peptide (ANP). Experiments were performed in rats and isolated perfused beating rat atria. ANP was measured using a selective radioimmunoassay. ANP mRNA expression was measured using real-time quantitative polymerase chain reaction. Administration of oleanolic acid increased plasma ANP levels in a dose-related manner. Similarly, oleanolic acid increased atrial ANP content and ANP mRNA expression. To evaluate the effects of oleanolic acid on ANP secretion, atrial stretch and muscarinic acetylcholine receptor activation were applied to the atria from rats chronically treated with oleanolic acid. Baseline levels of ANP secretion were higher in the atria from rats treated with oleanolic acid compared to rats treated with vehicle. Furthermore, oleanolic acid treatment enhanced the stretch-induced increase in ANP secretion. Acetylcholine in the presence of isoproterenol increased ANP secretion. The acetylcholine-induced increase in ANP secretion was also enhanced in the atria from rats treated with oleanolic acid compared to atria from rats treated with vehicle. The present findings indicate that oleanolic acid increases plasma ANP levels via increased ANP synthesis and secretion in rats. It is proposed that an accentuation of the ANP system is involved in the beneficial effects of oleanolic acid on the regulation of cardiovascular homeostasis.

A herbal medicine decoction, Sibjotang, increases ANP secretion

Sibjotang (SJT), which was first documented in the Shanghanlun, has been prescribed for the remedy of body fluid disorders. The present study was designed to investigate the effect of SJT on Atrial natriuretic peptide (ANP) secretion, atrial dynamics and the possible mechanisms in beating rabbit left atria paced electrically. SJT significantly increased ANP secretion and atrial dynamics such as stroke volume and pulse pressure in a concentration‐dependent manner in perfused atria. EK or DG, components of SJT, also significantly increased ANP secretion and atrial dynamics, whereas, there were no significance on ANP secretion and atrial dynamics by treatment of EP or ZJ, respectively. Interestingly, the SJT‐induced increase of ANP secretion was significantly higher than the sum of each EK‐induced increase and DG‐induced increase of ANP secretion (p&lt;0.01). SJT‐induced increase in ANP secretion was attenuated by GO6976, PKC ¥á/¥â inhibitor. In addition, LY333531, PKC ¥â inhibitor, also decreased SJT‐induced ANP secretion.In conclusion, SJT‐induced ANP secretion was potentiated by combination with 4 components in beating rabbit atria. It provides an evidence for synergy effects of a herbal medicine decoction consisting of two or more components. Also, the data suggest that SJT‐induced ANP secretion was related with the activation of PKC ¥á/¥â signaling in perfused beating rabbit atria.

Ursolic acid increases the secretion of atrial natriuretic peptide in isolated perfused beating rabbit atria

Ursolic acid is reported to have beneficial effects on the regulation of cardiovascular homeostasis. However, the effects of ursolic acid on cardiac hormone secretion are yet to be defined. The present study was designed to test the effects of ursolic acid on the secretory and contractile functions of the atria. Experiments were conducted in isolated perfused beating rabbit atria. We measured the changes in atrial dynamics, pulse pressure, stroke volume, cAMP efflux, as well as the secretion of atrial natriuretic peptide (ANP). Ursolic acid increased ANP secretion and mechanical dynamics in a concentration-dependent manner. The inhibition of L-type Ca2+ channels with nifedipine attenuated the ursolic acid-induced increase in ANP secretion but not mechanical dynamics. The inhibition of K+ATP channels with glibenclamide attenuated the ursolic acid-induced increase in ANP secretion—but not atrial dynamics—in a concentration-dependent manner. The selective Na+–K+-ATPase inhibitor ouabain blocked the ursolic acid-induced increase in atrial dynamics but not ANP secretion. These findings show that ursolic acid increases ANP secretion via its activation of K+ATP channels and subsequent inhibition of Ca2+ entry through L-type Ca2+ channels in rabbit atria. These data also suggest that ursolic acid increases atrial dynamics via its inhibition of Na+–K+-ATPase activity.

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 μM or 1 μM] 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 ω 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.

Effects of urotensin II on hypoxia‐induced ANP secretion in rats

Urotensin II (UII) is a vasoactive peptide with many potent effects in the cardiovascular system through a G‐protein‐coupled receptor termed UT. The synthesis and secretion of atrial natriuretic peptide (ANP) are increased during hypoxic condition and plays an important role in adaptation to hypoxia and in the pathogenesis of cardiopulmonary diseases. The present study was to investigate the effects of human UII (hUII) on ANP secretion in hypoxia condition in vitro and in vivo experiments. Hypoxia caused an increase in ANP secretion compared to normoxia in isolated perfused beating rat atria. hUII (10‐10 M) decreased hypoxia‐induced ANP secretion whereas hUII receptor antagonist, urantide (10‐6 M), increased it. However, there were no significant changes in atrial contractility, stroke volume, and extracellular fluid (ECF) translocation by hUII in hypoxia condition. The pretreatment with urantide attenuated the effect of hUII on hypoxia‐induced ANP secretion. Monocrotaline‐treated (MCT, 60mg/kg, sc., 4weeks) rats increased UII level in plasma. In vivo experiment, hUII (5*10‐6 M injection plus 2.5*10‐6 M infusion for 15 min) was infused into MCT‐treated and control rats anesthetized with ketamine. hUII increased blood pressure and heart rate in both groups without significant difference. Interestingly, intravenous infusion of hUII decreased plasma concentration of ANP in MCT‐treated rats whereas hUII increased plasma concentration of ANP in control rats. The ANP generation may protect against the hypoxia and pulmonary hypertension. These results suggest that hUII may play a pathophysiological role in the regulation of ANP secretion in hypoxic condition. Supported by the KOSEF (R13‐2008‐005‐00000‐0).

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, 5×10−11, 10−10, 5×10−10M) 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 2.5μM 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. PGF2α and PGD2 caused dose-dependent increases in ANP release and intra-atrial pressure. The potency for the stimulation of ANP secretion by PGF2α 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 PGF2α and PGD2 were significantly attenuated by the pretreatment with an inhibitor of PGF2α 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), PGF2α-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, PGF2α-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 PGF2α receptor mRNA decreased in hypertrophied rat atria. These results suggest that PGF2α increased the ANP secretion and positive inotropy through PLC–IP3–PKC–MLCK pathway, and the modulation of ANP secretion by COX-2 inhibitor and PGF2α may partly relate to the development of renal hypertension.