Rat Atria Research Articles

Abstract 18999: The Effects of the Connexin40 Mutations P88S and G38A on Cardiac Physiology in the Intact Animal

Introduction: Prior patch clamping studies have revealed that the somatic human connexin40 mutations P88S and G38A, identified within the atria of patients with atrial fibrillation (AF), are associated with reduced gap junction conductances when compared to wild type connexin40 (wtCX40). Our aim was to phenotype the rat heart following expression of these mutations by somatic gene transfer. Methods: A mix of pluronic F127, trypsin, and 10 7 vector genomes of lentivirus was painted onto the left atrium of adult Sprague-Dawley rats via a left thoracotomy. Rats were randomised to receive lentivirus containing a transgene encoding either wtCX40, P88S, G38A or eGFP. Electrophysiology studies (EPS) consisting of surface and trans-oesophageal electrocardiograms (ECG) were performed under anaesthesia just prior to and 7 days after surgery. Following day 7 EPS, atria were excised, sectioned, and examined histologically with Sirius Red and Haematoxylin and Eosin. Data are expressed as means ± standard deviations. Results: At day 7, eGFP expression revealed approximately 5% heterogeneous atrial transduction. Rats transduced to express either P88S or G38A possessed significantly longer mean P wave durations and AF when compared to eGFP controls as shown in the table. There were no significant differences in all other characteristics. Conclusion: The human somatic connexin40 mutations P88S and G38A slow atrial conduction as evidenced by prolonged P wave durations at day 7. In addition, heterogonous expression of P88S and G38A within the atrial myocardium increases the propensity for inducible atrial fibrillation.

The role of adenosine receptors on amitriptyline-induced electrophysiological changes on rat atrium

We investigated the role of adenosine receptors in amitriptyline-induced cardiac action potential (AP) changes in isolated rat atria. In the first group, APs were recorded after cumulative addition of amitriptyline (1 μM, 10 μM and 50 μM). In other groups, each atrium was incubated with selective adenosine A(1) antagonist (8-cyclopentyl-1,3-dipropylxanthine (DPCPX), 10(-4) M) or selective adenosine A(2a) receptor antagonist (8-(3-chlorostyryl) caffeine, 10(-5) M) before amitriptyline administration. Resting membrane potential, AP amplitude (APA), AP duration at 50% and 80% of repolarization (APD(50) and APD(80), respectively), and the maximum rise and decay slopes of AP were recorded. Amitriptyline (50 μM) prolonged the APD(50) and APD(80) (p < 0.001) and the maximum rise slope of AP was reduced by amitriptyline (p < 0.0001). Amitriptyline reduced maximum decay slope of AP only at 50 μM (p < 0.01). DPCPX significantly decreased the 50-μM amitriptyline-induced APD(50) and APD(80) prolongation (p < 0.001). DPCPX significantly prevented the effects of amitriptyline (1 μM and 50 μM) on maximum rise slope of AP (p < 0.05). DPCPX significantly prevented the amitriptyline-induced (50 μM) reduction in maximum decay slope of AP (p < 0.001). The selective adenosine A(1) receptor antagonist prevented the electrophysiological effects of amitriptyline on atrial AP. A(1) receptor stimulation may be responsible for the cardiovascular toxic effects produced by amitriptyline.

Pro-apoptotic effect of anti-β1-adrenergic receptor antibodies in periodontitis patients

An anti-β1-adrenergic antibody from the sera of periodontitis patients (anti-β1-AR IgG) against the second extracellular loop of the human β1-adrenoceptor (β1-AR) has been shown to cause rat atria apoptosis. The anti-β1-AR IgG binds and activates atria β1-AR, increasing the intracellular calcium concentration, which, in turn, activates caspases-3, -8, and -9. The β1-AR and the post-receptor activation of calcium/calmodulin (CaM) lead to increased inducible nitric oxide synthase (iNOS) activity, with an increase in cyclic GMP (cGMP) accumulation as well as increased JNK phosphorylation and cyclic AMP (cAMP) production. We also observed an apoptotic effect of anti-β1-AR IgG, with increased generation of PGE2. Comparatively, xamoterol, an authentic β1-AR agonist, mimicked the autoantibody effect on rat atria β1-AR apoptosis. Our results suggest that autoantibodies from the sera of periodontitis patients bind and interact with rat atria β1-AR, provoking apoptosis. This implicates a series of modulatory cardiac signaling events that could alter normal heart function and may occur with chronic stimulation of the atria β1-AR, which could lead to heart failure. These results suggest an important link between periodontitis and cardiovascular disease.

Nitric oxide modulates intensity of non-quantal acetylcholine release in myocardium of the right atrium of rat

The main parasympathetic neurotransmitter acetylcholine (ACh) is released in the myocardium from the intramural postganglionic parasympathetic nerve endings. The mechanism of non-quantal ACh release has been recently demonstrated in these neurons. Non-quantal ACh release does not depend on exocytosis of ACh-containing vesicles in response to nerve impulse activity but is assumed to be mediated by the high-affinity choline uptake system. The intensity of non-quantal ACh release in the myocardium correlates with the degree of manifestation of the effects of acetylcholinesterase inhibitors inducing the accumulation of non-quantal ACh in the myocardium. The present study deals with the influence of putative modulators of non-quantal ACh release: nitric oxide (NO) and ATP, on the intensity of cholinergic effects induced by organophosphorous acetylcholinesterase inhibitor paraoxon. Intracellular registration of bioelectrical activity in isolated right atrium preparations from rats was used. Under normal conditions, paraoxon (10−7–10−5 M) induced a marked decrease in the action potential (AP) duration at a level of 50 and 90% repolarization in the working right atrial myocardium and slowed down the sinus rhythm. ATP, which is known to suppress nonquantal ACh release in the neuromuscular junction, did not induce significant reduction or augmentation of the effects of paraoxon (5 × 10−6 M). The NO donors, sodium nitroprusside (10−5 M) and SNAP (10−4 M), significantly reduced the paraoxon-induced AP shortening. Moreover, sodium nitroprusside decreased the negative chronotropic effect of paraoxon by 43.7%. On the contrary, NO synthase inhibitor L-NAME (10−4 M), which is known to suppress endogenous NO production, augmented the AP shortening caused by paraoxon. It may be deduced that NO is a universal regulator of non-quantal ACh release intensity both in the myocardium and in the neuromuscular junction.

Upregulation of β3-Adrenergic Receptor Expression in the Atrium of Rats With Chronic Heart Failure

To investigate the expression of β(3)-adrenergic receptor (β(3)-AR) in the atrium of rats with chronic heart failure (CHF). The heart failure rat model was established by aortic constriction. Thirty-six male Wistar rats were divided into Sham group (n = 10) and heart failure model group (n = 26), which were further divided into CHF control (CHF group) and BRL group. The rats in the BRL group were treated with a selective β(3)-AR agonist, BRL-37344 (4.0 nmol/kg, twice weekly) for 4 weeks. In the BRL group, the left ventricular end-systolic pressure (83.21 ± 13.0 vs 101.50 ± 12.12 mm Hg) and the absolute values of the maximal rate of rise and fall of left ventricular pressure ([±dP/dtmax] 2.81 ± 0.04 vs 0.35 ± 0.04 and -2.72 ± 0.06 vs -3.33 ± 0.06) were lower than in the CHF group (P < .01). The left atrial mass index (LAMI) in the BRL group (0.4132 ± 0.0306) was higher than that in the CHF (0.3212 ± 0.0136) or Sham group (0.2683 ± 0.0145; P < .01). The levels of the left atrial β(3)-AR messenger RNA (mRNA) expression in the BRL group (0.932 ± 0.055) was higher than that in the CHF (0.706 ± 0.043) or Sham group (0.310 ± 0.020; P < .01). In all animals, there was a positive correlation between the level of β(3)-AR mRNA expression and the left or right atrial mass index (correlation coefficient ranged from 0.744 to 0.937). There is a significant increase in the β(3)-AR mRNA expression in the atrium of rats with heart failure. The level of β(3)-AR mRNA expression was associated with the AMI and was enhanced by a selective β(3)-AR agonist, BRL-37344.

β1‐Adrenoceptor antibody‐induced increase in soluble CD40 ligand release in chronic periodontitis patients: role of prostaglandin E2

In this paper, we demonstrate that circulating antibodies from chronic periodontitis patients reacting with atrial β(1)-adrenoceptors (β(1)-ARs) act as an inducer of soluble CD40 ligand (sCD40L) release and prostaglandin E(2) (PGE(2)) generation. By enzyme-linked immunosorbent assay using β(1) synthetic peptide (with an amino acid sequence identical to the second loop of human myocardial β(1)-ARs) as a coating antigen, we demonstrated reactivity against the second extracellular loop on human myocardial β(1)-ARs. This autoantibody present in the serum of chronic periodontitis patients was significantly correlated with the release of sCD40L and PGE(2). The release of sCD40L was blunted by atenolol, SP600125 and β(1) synthetic peptide, and PGE(2) generation was inhibited by DuP 697 and slightly by FR122049. The effects of the antibody incubated with isolated rat atria upregulated sCD40L release with an increase of PGE(2) production and c-Jun N-terminal kinase phosphorylation. These results indicate that in chronic periodontitis patients, there is a positive association between sCD40L release and PGE(2) generation via the action of β(1)-AR antibodies.

Both neuronal and non-neuronal acetylcholine take part in non-quantal acetylcholine release in the rat atrium

AimsIn mammalian myocardium acetylcholine (ACh), neurotransmitter which strikingly affects the cardiomyocytes, can be released from the neurons both via quantal (vesicular) and nonquantal (non-vesicular) mechanism of secretion. Non-quantal release is continuous, independent on vagus activity and provides accumulation of ACh in myocardium in the presence of acetylcholinesterase (AChE) inhibitors. The aim of the present study was to determine the source of non-quantal ACh in isolated atrial myocardium of adult and newborn rats. Main methodsStandard microelectrode technique was used to determine the cholinergic changes of electrical activity under the action of AChE inhibitor paraoxon, which correlates with the intensity of nonquantal ACh release. Key findingsIn adult rats selective inhibitor of neuronal choline uptake system hemicholinium III (10–5M) decreased all effects of paraoxon (5×10–6M) more than twofold. Inhibitor of polyspecific 3 organic cation transporters corticosterone (10–4M) also significantly decreased effects of paraoxon in adult rats, indicating that non-neuronal ACh, which is synthesized by cardiomyocytes, takes part in accumulation of ACh in the myocardium. When hemicholinium III and corticosterone were applied together, paraoxon effects in adult atrial myocardium were suppressed almost completely. In newborn rats cardiomyocytes do not excrete ACh. In accordance with this fact hemicholinium III completely abolished effects of paraoxon in newborn myocardium, while corticosterone was ineffective. Thus, non-quantal ACh is released both from cholinergic nerves and cardiomyocytes in adult rat myocardium, while it has exclusively neuronal nature in newborns. SignificanceThe study demonstrates dual neuronal and non-neuronal nature of non-quantal ACh in the heart.

Oleanolic acid‐induced regulation of atrial natriuretic peptide secretion in perfused beating atrial model

The heart is an endocrine organ involved in the regulation of blood pressure and body fluid homeostasis through the secretion of atrial natriuretic peptide (ANP). Oleanolic acid (OA) is a well known triterpenoid compound that exist widely in food and medicinal herbs. The purpose of present study was to examine the effect of OA in the regulation of ANP secretion. Experments were performed in beating rat atria. OA (30 mg/kg/day) was administrated orally in Sprague‐Dawley rats : Atrial dynamics, pulse pressure, and ANP secretion were measured. OA administration increased ANP levels in plasma and atria. To identify the effects of OA on the ANP secretion by activation of ¥â‐adrenergic and muscarinic receptor, isoproterenol and acetylcholine were administration in the atria from OA‐ and sham‐treatie rats. Oral intake of OA accenturated the acetylcholine‐induced increase of ANP secretion. However, OA attenuated the isoproterenol‐induced decrease of ANP secretion. These findings demonstrate that OA attenuates the decrease of ANP secretion induced by activation of ¥â‐adrenergic receptor and accentuates the increase of ANP secretion induced by muscarinic receptor. These results suggest that oral intake of OA induces an activation of the ANP system in rats.

Alteration of Purinergic Neurotransmission in Isolated Atria of Streptozotocin-induced Diabetic Rats

Cardiac dysfunctions are described in diabetes. However, the role of purinergic neurotransmission in diabetes-related cardiovascular diseases is unknown. The purpose of this study was to evaluate the purinergic neurotransmission in isolated atria from streptozotocin-induced diabetic rats. The animals were grouped as control and diabetic with 30 days (D30) and 60 days (D60) after streptozotocin-induced diabetes. The isolated left and right atria were used in functional experiments. The effects of adenosine triphosphate, uridine diphosphate, and adenosine were evaluated on atrial inotropism and chronotropism. The antagonists 8-cyclopentyl-1,3-dipropylxanthine and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate were also used, as blockers of P1 and P2 receptors, respectively. A negative inotropic effect followed by a positive inotropic effect was induced by adenosine triphosphate in isolated atria. This negative inotropic effect was decreased by 25% in left atria of D30. Additionally, the apparent affinity for adenosine was diminished in left atria of D30, suggesting changes in P1 receptor function. No changes were found in the right atria of D30 stimulated by adenosine. The left atria and right atria stimulated by uridine diphosphate showed an increased inotropic effect of 92% and 17%, respectively. No changes were observed in left and right atria of D30 stimulated by uridine diphosphate. Our data showed the involvement of purinergic neurotransmission in diabetes-related cardiovascular changes.

Impact of Obesity on the Atrial Substrate is Compounded by Age

Introduction: Obesity is a recognised risk factor of human atrial fibrillation (AF) and its prevalence has been projected to increase perhaps concomitantly with the AF epidemic. The impact of the aging population on the effects of obesity on the atria is not known. Methods: Intracellular membrane potential recordings were obtained from the isolated right atria of Zucker lean (n = 12) and obese rats (n = 12) aged 3, 7 and 12 months respectively (n = 4 per age group). An aluminosilicate glass electrode (100 MΩ, filled with 3 M KCl) was used to record intracellular action potential parameters during atrial pacing. Atrial orientation and pacing site and glass electrode position were kept consistent throughout each experiment. Results: Obese animals were significantly heavier than age-matched lean animals across all age groups (p < 0.01). Blood pressure was significantly higher in obese animals aged >7 months (p < 0.0001) compared to lean animals, and further increased with age (p < 0.05). In lean animals, APD demonstrated a gradual decrease and plateau with age alone. In contrast, in obese animals there was a progressive increase in APD (p < 0.05). At each pacing cycle length (200, 300 and 400 ms), the APD was significantly longer in obese rats aged 7 and 10 months compared to their age-matched lean rats (p < 0.001). Conclusion: The impact of obesity on the substrate for AF is greater with increasing age. These findings have significant implications to the consequences of the obesity epidemic in an aging population.

Regulation of atrial catecholamine enzymes and adrenoceptor gene expression after chronic stress

Chronic stress is a risk factor for the development of numerous psychopathological conditions in humans including depression. Changes in gene expression of tyrosine-hydroxylase (TH), dopamine-β-hydroxylase (DBH) phenylethanolamine N-methyltransferase (PNMT), β1-, β2- and β3-adrenoceptors in right and left rat atria upon chronic unpredictable mild stress (CMS) were investigated. CMS decreased TH and DBH gene expression levels both in right and left atria and increased PNMT mRNA in left atria. No changes in mRNA levels of β1- and β2-adrenoceptors were recorded, whereas β3-adrenoreceptor mRNA level was significantly elevated in right atria of CMS rats. At the same time, CMS produced a significant increase of β1- and β2-adrenoreceptor mRNA levels in left atria, but did not affect β3-adrenoceptor mRNA level.The results presented here suggest that stress-induced depression expressed differential effects on catecholamine biosynthetic enzymes and β-adrenoceptors at molecular level in right and left atria of adult rat males. Elevated gene expression of PNMT in left atria of rats exposed to CMS can lead to altered physiological response and may play a role in the pathophysiology of cardiovascular function.

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 2O 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.

Investigation of the Cardiac Effects of Pancuronium, Rocuronium, Vecuronium, and Mivacurium on the Isolated Rat Atrium

BackgroundPancuronium, vecuronium, rocuronium, and mivacurium are nondepolarizing neuromuscular blocking agents that affect the cardiovascular system with different potencies. Their cardiovascular effects are clinically significant in the anesthetic management of patients, particularly those undergoing cardiac surgery. ObjectiveWe aimed to compare the cardiac effects of these compounds, such as heart rate and developed force, in one species under identical experimental conditions in isolated rat atria. MethodsThe left or right atria of rats were removed and suspended in organ baths. Pancuronium, vecuronium, rocuronium, or mivacurium were added cumulatively (10–9–10–5 M) in the presence and absence of the nonselective β-blocker propranolol (10–8 M) and the noradrenaline reuptake inhibitor desipramine (10–7 M), and heart rate changes were recorded in spontaneously beating right atria. Left atrial preparations were stimulated by electrical field stimulation using a bipolar platinum electrode, and the effects of cumulative concentrations of these nondepolarizing neuromuscular blocking agents on the developed force in the presence and absence of propranolol (10–8 M) and desipramine (10–7 M) were recorded. ResultsPancuronium increased heart rate in a dose-dependent manner compared with the control group (P < 0.027). Vecuronium, rocuronium, and mivacurium also increased heart rate in a dose-dependent manner, but the changes were not statistically significant. Although propranolol decreased the pancuronium heart rate effect (P < 0.05), it did not change the heart rate effects with vecuronium, rocuronium, or mivacurium. Desipramine did not change the heart rate effects of vecuronium, rocuronium, mivacurium, or pancuronium. All 4 drugs increased developed force in a dose-dependent manner; the increases were significant at 10–5 M concentration for pancuronium and at 10–6 and 10–5 M concentrations for vecuronium, rocuronium, and mivacurium (P < 0.038). These increases in developed force were abolished with the addition of propranolol. Desipramine did not change the developed force effects of any of the 4 drugs. ConclusionsThe heart rate effect of pancuronium and developed force effects of pancuronium, vecuronium, rocuronium, and mivacurium may occur via direct stimulation of β receptors. Although our investigation was an in vitro study, the effects found may be important especially under pathologic conditions, such as hypertension, in which patients usually use β-blocking agents, which cause β receptor upregulation.

Novel cilostamide analogs, PDE3 inhibitors, produce positive inotropic but differential lusitropic and chronotropic effects on isolated rat atria

Novel cilostamide analogs, PDE3 inhibitors, produce positive inotropic but differential lusitropic and chronotropic effects on isolated rat atria

Angiotensin II type 1 receptor blocker attenuates diabetes-induced atrial structural remodeling

Diabetes mellitus promotes atrial structural remodeling, thereby producing atrial arrhythmogenicity, where advanced glycation endproducts (AGEs) and their receptor (RAGE) are implicated to play a role in the pathogenesis. We investigated the effects of candesartan, an angiotensin type II receptor blocker, on the diabetes-induced atrial structural change. Diabetes was induced in 8-week-old female Sprague-Dawley rats by intraperitoneal injection of streptozotocin at 70 mg/kg. Osmotic pumps were simultaneously set to infuse candesartan at a subdepressor dose of 0.05 mg/kg/day. Twelve weeks after the induction of diabetes, the blood glucose and glycated hemoglobin A1c were significantly higher in streptozotocin-injected rats than those in control rats, and were not affected by candesartan treatment. The atria of diabetic rats showed remarkable diffuse interstitial fibrosis with more enhanced protein expressions of RAGE and connective tissue growth factor (CTGF) compared with control ones. The treatment with candesartan significantly reduced CTGF expression and effectively suppressed the development of fibrotic deposition in diabetic animals. Candesartan reduced CTGF expression and attenuated the fibrosis in diabetic rat atria. These results implied the protective effects of candesartan on diabetes-related atrial arrhythmias.

Effect ofMucuna pruriensSeed Extract Pretreatment on the Responses of Spontaneously Beating Rat Atria and Aortic Ring toNaja sputatrix(Javan Spitting Cobra) Venom

Mucuna pruriens Linn. (velvet bean) has been used by native Nigerians as a prophylactic for snakebite. Rats pretreated with M. pruriens seed extract (MPE) have been shown to protect against the lethal and cardiovascular depressant effects of Naja sputatrix (Javan spitting cobra) venoms, and the protective effect involved immunological neutralization of the venom toxins. To investigate further the mechanism of the protective effect of MPE pretreatment against cobra venom toxicity, the actions of Naja sputatrix venom on spontaneously beating rat atria and aortic rings isolated from both MPE pretreated and untreated rats were studied. Our results showed that the MPE pretreatment conferred protection against cobra venom-induced depression of atrial contractility and atrial rate in the isolated atrial preparations, but it had no effect on the venom-induced contractile response of aortic ring preparation. These observations suggested that the protective effect of MPE pretreatment against cobra venom toxicity involves a direct protective action of MPE on the heart function, in addition to the known immunological neutralization mechanism, and that the protective effect does not involve action on blood vessel contraction. The results also suggest that M. pruriens seed may contain novel cardioprotective agent with potential therapeutic value.

The Angiotensin-(1-7)/Mas Receptor Axis Is Expressed in Sinoatrial Node Cells of Rats

The authors' previous studies have indicated that angiotensin(Ang)-(1-7) protects the heart against reperfusion arrhythmias. The aim of this study was to determine whether a functional angiotensin-converting enzyme2 (ACE2)/Ang-(1-7)/Mas receptor axis is present in the sinoatrial node (SAN) of Wistar rats. SAN cells were identified by Masson's trichrome staining, HCN4 expression, and lack of connexin43 expression. Immunohistochemistry technique was used to detect the expression of ACE2, Ang-(1-7), and Mas in the SAN. To evaluate the role of this axis in the SAN function, atrial tachyarrhythmias (ATs) were induced in isolated rat atria perfused with Krebs-Ringer solution (KRS) alone (control) or KRS containing Ang-(1-7). The specific Mas antagonist, A-779, was used to evaluate the role of Mas in the Ang-(1-7) effects. The findings showed that all components of the ACE2/Ang-(1-7)/Mas branch are present in the SAN of rats. Importantly, it was found that this axis is functional because perfusion of atria with Ang-(1-7) significantly reduced the duration of ATs. Additionally, this anti-arrhythmogenic effect was attenuated by A-779. No significant changes were observed in heart rate, contractile tension, or ±dT/dt. These observations demonstrate that the ACE2/Ang-(1-7)/Mas axis is expressed in SAN cells of rats. They provide the morphological support to the anti-arrhythmogenic effect of Ang-(1-7).

Hypotensive effect of the water extract of the leaves of Pseuderanthemum palatiferum

The cardiovascular effects of the water extract from fresh leaves of P. palatiferum (PP) were evaluated in rats in both in vivo and in vitro models. The PP extract consisted of alkaloids, coumarins and sterols. The extract (5-25mg/kg, I.V.) decreased both the mean arterial blood pressure and heart rate of normotensive and L: -N-(ω)-nitro-arginine methyl ester (L: -NAME)-induced hypertensive anesthetized rats. The extract caused relaxation of the norepinephrine-contracted endothelium-intact aorta ring (EC(50)=81.0μg/mL). Functional removal of the endothelium attenuated the vasodilatation effect of the extract (EC(50)=136.4μg/mL). Pretreatment with L: -NAME or atropine did not alter the vasodilation effect of the extract. Pre-incubation with atropine did not alter bradycardic effect of the extract on isolated rat atrium. The effects of isoproterenol on isolated atrium were attenuated by the extract (2.5-5mg/mL). In conclusion, the water extract from fresh leaves of P. palatiferum possessed hypotensive and bradycardic effects. The vasorelaxant effect of the extract was dependent partly on the vascular endothelium but was not dependent on the synthesis of nitric oxide and did not act through activation of the muscarinic receptor.

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 0.1 nM) 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; 0.1 μM), but not by BIM 23056 (SSTR type 5 antagonist; 0.1 μM) and urantide (urotensin II receptor antagonist; 0.1 μM). When pretreated with an agonist for SSTR type 2 (Seglitide, 0.1 nM) and SSTR type 5 (L 817818, 0.1 nM), 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, 5 μM) or protein kinase A (KT 5720, 0.1 μM). 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.

Possible involvement of protein kinase C in the modulation of inotropic and chronotropic effects induced by ouabain in rat right atrial muscles.

Modulations of the inotropic and chronotropic effects of ouabain and protein kinase C (PKC) stimulation with phorbol esters in rat right atria were examined. Cumulative administration of ouabain (3-30 microM) caused a positive inotropic effect in a concentration-dependent manner, but did not produce a chronotropic effect. A single administration of ouabain (30 microM) also had similar effects: + 74.4 +/- 8.4% (n = 23, P < 0.01) in the contractile force and -0.7 +/- 1.3% (n = 23, P > 0.05) in the sinus rate. Addition of phorbol esters reinforced the ouabain-evoked positive inotropic effect: 26.5 +/- 8.9% (n = 6, P < 0.05) with 100 microM 4-beta-phorbol-12,13-dibutyrate (PDB), and 6.4 +/- 3.3% (n = 6, P > 0.05) with 100 microM 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Simultaneously, the mixture of ouabain and phorbol ester raised the resting tension. Phorbol esters alone caused a positive inotropic effect (by about 21-27%). Non-PKC activating phorbol ester, 4-alpha-phorbol-12,13-didecanoate (PDD, 100 microM), did not have any effect. Pretreatment with the PKC inhibitor (staurosporine 100 microM) significantly decreased the ouabain-induced positive inotropic effect and caused a negative chronotropic effect, but H-7 (1-(5-isoquinolinylsulphonyl)-2-methylpiperazine dihydrochloride) (5 microM) had no effect. These results suggest that PKC stimulation may be involved in the ouabain-evoked responses in the right atria of rat as seen by increased cellular Ca2+ concentration (and Ca(2+)-sensitivity); thus the positive inotropic effect may not be due only to modulation of Na+/K+ pump activity.