Changes underlying arrhythmia in the transgenic heart overexpressing Refsum disease gene-associated protein

Previously, we identified a novel neuron-specific protein (PAHX-AP1) that binds to Refsum disease gene product (PAHX), and we developed transgenic (TG) mice that overexpress heart-targeted PAHX-AP1. These mice have atrial tachycardia and increased susceptibility to aconitine-induced arrhythmia. This study was undertaken to elucidate the possible changes in ion channels underlying the susceptibility to arrhythmia in these mice. RT-PCR analyses revealed that the cardiac expression of adrenergic beta(1)-receptor (ADRB1) was markedly lower, whereas voltage-gated potassium channel expression (Kv2.1) was higher in PAHX-AP1 TG mice compared with non-TG mice. However, the expression of voltage-sensitive sodium and calcium channels, and muscarinic receptor was not significantly different. Propranolol pretreatment, a non-specific beta-adrenoceptor antagonist, blocked aconitine-induced arrhythmia in non-TG mice, but not in PAHX-AP1 TG mice. Our results indicate that, in the PAHX-AP1 TG heart, the modulation of voltage-gated potassium channel and ADRB1 expression seem to be important in the electrophysiological changes associated with altered ion channel functions, but ADRB1 is not involved in the greater susceptibility to aconitine-induced arrhythmia.     

Thromboxane A2 and prostaglandin F2alpha mediate inflammatory tachycardia

Systemic inflammation induces various adaptive responses including tachycardia. Although inflammation-associated tachycardia has been thought to result from increased sympathetic discharge caused by inflammatory signals of the immune system, definitive proof has been lacking. Prostanoids, including prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2) and thromboxane (TX) A(2), exert their actions through specific receptors: DP, EP (EP(1), EP(2), EP(3), EP(4)), FP, IP and TP, respectively. Here we have examined the roles of prostanoids in inflammatory tachycardia using mice that lack each of these receptors individually. The TXA(2) analog I-BOP and PGF(2alpha) each increased the beating rate of the isolated atrium of wild-type mice in vitro through interaction with TP and FP receptors, respectively. The cytokine-induced increase in beating rate was markedly inhibited in atria from mice lacking either TP or FP receptors. The tachycardia induced in wild-type mice by injection of lipopolysaccharide (LPS) was greatly attenuated in TP-deficient or FP-deficient mice and was completely absent in mice lacking both TP and FP. The beta-blocker propranolol did not block the LPS-induced increase in heart rate in wild-type animals. Our results show that inflammatory tachycardia is caused by a direct action on the heart of TXA(2) and PGF(2alpha) formed under systemic inflammatory conditions.     

Effects of hyperthyroidism on delayed rectifier K+ currents in left and right murine atria

Hyperthyroidism has been associated with atrial fibrillation (AF); however, hyperthyroidism-induced ion channel changes that may predispose to AF have not been fully elucidated. To understand the electrophysiological changes that occur in left and right atria with hyperthyroidism, the patch-clamp technique was used to compare action potential duration (APD) and whole cell currents in myocytes from left and right atria from both control and hyperthyroid mice. Additionally, RNase protection assays and immunoblotting were performed to evaluate the mRNA and protein expression levels of K(+) channel alpha-subunits in left and right atria. The results showed that 1) in control mice, the APD was shorter and the ultra-rapid delayed rectifier K(+) conductance (I(Kur)) and the sustained delayed rectifier K(+) conductance (I(ss)) were larger in the left than in the right atrium; also, mRNA and protein expression levels of Kv1.5 and Kv2.1 were higher in the left atrium; 2) in hyperthyroid mice, the APD was shortened and I(Kur) and I(ss) were increased in both left and right atrial myocytes, and the protein expression levels of Kv1.5 and Kv2.1 were increased significantly in both atria; and 3) the influence of hyperthyroidism on APD and delayed rectifier K(+) currents was more prominent in right than in left atrium, which minimized the interatrial APD difference. In conclusion, hyperthyroidism resulted in more significant APD shortening and greater delayed rectifier K(+) current increases in the right vs. the left atrium, which can contribute to the propensity for atrial arrhythmia in hyperthyroid heart.     

Age-dependent biochemical and contractile properties in atrium of transgenic mice overexpressing junctin

Junctin is a transmembrane protein of the cardiac junctional sarcoplasmic reticulum (SR) that binds to the ryanodine receptor, calsequestrin, and triadin 1. This quaternary protein complex is thought to facilitate SR Ca2+ release. To improve our understanding of the contribution of junctin to the regulation of SR function, we examined the age-dependent effects of junctin overexpression in the atrium of 3-, 6-, and 18-wk-old transgenic mice. The ratio of atrial weight and body weight was unchanged between junctin-overexpressing (JCN) and wild-type (WT) mice at all ages investigated (n=6-8). The protein expression of triadin 1 was decreased starting in 3-wk-old JCN atria (by 69%), whereas the expression of the ryanodine receptor was diminished in 6- (by 48%) and 18-wk-old (by 57%) JCN atria compared with age-matched WT atria. Force of contraction was decreased by 35% in 18-wk-old JCN compared with age-matched WT left atrial muscle strips, which was accompanied by a prolonged time of relaxation (48.1 +/- 0.9 vs. 44.2 +/- 0.8 ms, respectively, n=6-8, P <0.05). The spontaneous beating rate of isolated right atria was higher in 18-wk-old JCN mice compared with age-matched WT mice (389 +/- 10 vs. 357 +/- 6 beats/min, respectively, n=6-8, P <0.05). Heart rate was lower by 9% in telemetric ECG recordings in 18-wk-old JCN mice during stress tests. Three-week-old JCN atria exhibited a higher potentiation of force of contraction at rest pauses of 30 s (by 13%) and of 300 s (by 35%), suggesting increased SR Ca2+ content. This was consistent with the higher force of contraction in 3-wk-old JCN atria (by 29%) compared with age-matched WT atria (by 10%) under the administration of caffeine. We conclude that in 3-wk-old atria, junctin overexpression was associated with a reduced expression of triadin 1 resulting in a higher SR Ca2+ load without changes in contractility or heart rate. In 6-wk-old JCN atria, the compensatory downregulation of the ryanodine receptor may offset the effects of junctin overexpression. Finally, the progressive decrease in ryanodine receptor density may contribute to the decreased atrial contractility and lower heart rate during stress in 18-wk-old JCN mice.     

Beta adrenergic receptor-mediated atrial specific up-regulation of RGS5

Previous investigations had suggested that signaling from the overexpressed beta(2) adrenergic in the heart of transgenic TG4 mice was dampened in the atria. Using an RT-PCR based strategy, we have identified Regulator of G-protein Signaling 5 (RGS5) as being up-regulated in the atria of TG4 mice. Northern blot analysis demonstrated that RGS5 levels were 3 fold higher in the atria of TG4 mice. Western blot analysis of a panel of rat tissues demonstrated that basal expression of RGS5 protein was confined to the heart and skeletal muscle. Furthermore, RGS5 protein was detected in skeletal muscle C2C12 and cardiomyocyte HL-1 cultured cell lines. As observed for RGS5 mRNA levels in TG4 mice, RGS5 protein levels were increased in the atria of rats that were administered the beta adrenergic agonist isoproterenol during a 14 day period. Taken together, these results indicate that RGS5 is a housekeeping RGS in the heart and in skeletal muscle while its beta adrenergic-mediated induction in the atrium suggests that it also has a highly specialized function.     

Comparative aspects of adrenergic receptors in the hearts of lower vertebrates

The cardiac adrenoceptors of lower vertebrates were characterized in atrial preparations. Adrenaline (A) potentiated the force and frequency of contraction in the spontaneously beating atria of the frog, trout and flounder and in electrically paced atrial strips from the shark. The inotropic responses of A were most pronounced at the lower temperatures for the frog and trout, while A enhanced frequency to a greater extent at higher temperatures in the frog atria. Atrial alpha-receptors activated by A at 8 degrees C could not be detected in any of the species under study. The apparent affinities for the inotropic and chronotropic responses of agonist in the frog (15 degrees C) and trout (8 degrees C) atria were: Iso greater than Sal greater than or equal to A greater than NA. A cocaine-sensitive uptake for A and NA was apparent in these atria, consistent with sympathetic innervation. The affinities for the catecholamines in the flounder and shark atria were not increased by cocaine, in accordance with absence of sympathetic innervation of the atria in these species. These atria were also insensitive to corticosterone. The affinities for A and NA were on the other hand higher in the sympathetically non-innervated atria of the flounder than in the innervated atria of the frog and trout. The apparent orders of relative affinities for agonists were Iso greater than A = NA greater than Sal for the flounder, and of the relative potencies Iso = A greater than NA greater than Sal for the shark atrium. The results are consistent with the hypothesis that catecholamines enhance cardiac performance in lower vertebrates chiefly via "adrenaline" receptors which resemble the beta 2-type of mammalian adrenoceptors in many respects. Unlike that in mammals, cardiac adrenaline receptors in the frog and trout are activated by the sympathetic neurotransmitter ("innervated" receptors). On the other hand, the adrenaline receptors of the flounder and shark are responding to the circulating catecholamines ("humoral" receptors). However, the flounder atrium, with equal affinities for A and NA, appears as an exception to the rule by having a mixed population of humoral beta 1- and beta 2-adrenoceptors, indicating a role for circulating NA in cardiac regulation in this species.     

Expression of atrial natriuretic factor (ANF) during Xenopus cardiac development

We have isolated the Xenopus orthologue of the atrial natriuretic factor (ANF) gene. Characterization of embryonic expression indicates that the ANF gene is initially expressed throughout the developing myocardium at the late heart tube stage (about stage 32). This is in contrast to all previously characterized Xenopus cardiac differentiation markers that are first expressed in the cardiogenic plate at approximately stage 27. ANF expression becomes restricted exclusively to the atrium at about stage 47, long after the commencement of beating and the original formation of the atrial and ventricular compartments, but shortly after septation of the single atrium into two distinct atria. Received: 5 May 2000 / Accepted: 3 August 2000     

Eplerenone inhibits atrial fibrosis in mutant TGF-β1 transgenic mice

The purpose of the present study was to study the impacts of eplerenone(EPL), an antagonist of mineralocorticoid receptors(MR), on atrial fibrosis in a mouse model with selective fibrosis in the atrium, and to explore the possible mechanisms. Using mutant TGF-β1 transgenic(Tx) mice, we first demonstrated that EPL inhibited atrial fibrosis specifically and decreased macrophage accumulation in the atria of these mice. Results from immunohistochemistry and western blotting showed that EPL attenuated protein expression of fibrosis-related molecules such as connective tissue growth factor(CTGF) and fibronectin in the atria of Tx mice. In culture, EPL inhibited gene expression of fibrosis-related molecules such as fibronectin, α-SMA, and CTGF in TGF-β1-stimulated atrial fibroblasts. Finally, using a co-culture system, we showed that TGF-β1-stimulated atrial fibroblasts induced migration of macrophages and this was blocked by EPL. EPL also blocked TGF-β1-induced gene expression of intedeukin-6(IL-6) in atrial fibroblasts. Therefore, we conclude that EPL attenuated atrial fibrosis and macrophage infiltration in Tx mice. TGF-β1 and IL-6 were involved in the impacts of EPL on activation of atrial fibroblasts and interactions between fibroblasts and macrophages.     

4:2:1 conduction of an AF initiating trigger

A 44 year old male with idiopathic dilated cardiomyopathy was undergoing persistent atrial fibrillation (AF) ablation. Following antral ablation, AF terminated into a regular narrow complex rhythm. Earliest activation was mapped to a focus in the superior vena cava (SVC) which was conducted in a 2:1 ratio to the atria which in turn was conducted with 2:1 ratio to the ventricles, resulting in an unusual 4:2:1 conduction of the SVC tachycardia. 1:1 conduction of the SVC tachycardia to the atrium preceded initiation of AF. During AF, SVC tachycardia continued unperturbed. Sinus rhythm was restored following catheter ablation of the focus.     

起搏器术后新发房性心律失常的影响因素分析

目的:探讨起搏器术后新发房性心律失常的发生情况及其相关影响因素。方法:选择2006年1月至2007年12月于沈阳军区总医院首次植入永久起搏器的107例患者,男性50例,平均年龄65.0±11.9岁,术前通过追问病史及相关检查均排除房性心律失常(房颤、房扑、房速),术后平均随访3.9年,观察新发房性心律失常情况。按术后是否出现房性心律失常,将患者分为新发房性心律失常组和无房性心律失常组,比较两组患者术前和术后心脏超声结果的变化、心室起搏比例、起搏部位及起搏模式,并通过logistic回归分析起搏器术后发生房性心律失常的影响因素。结果:新发房性心律失常组26例(24.3%),其中房颤17例(15.9%),房扑2例(1.9%),房速7例(6.5%);无房性心律失常组81例。与无房性心律失常组比较,新发房性心律失常组左房内径明显增加(P=0.040)、二尖瓣返流程度较重(P=0.032)及左室射血分数明显下降(P=0.001),心室起搏百分比(VP%)显著升高(P=0.017)。心尖部起搏患者房性心律失常的发生率明显高于间隔部起搏(33.3%vs 16.9%,P<0.05),双腔起搏组患者房性心律失常发生率明显低于单腔起搏器组(18.7%vs 37.5%,P<0.05)。Logistic回归分析显示术后新发房性心律失常的发生与高比例的心室起搏(P=0.006)、VVI(R)起搏模式(P=0.014)及右心室起搏电极导线植于心尖部(P=0.024)显著相关。结论:起搏模式、心室起搏百分比、起搏部位是起搏器术后发生房性心律失常的影响因素。     

The positive inotropic effect of the aqueous extract of Convallaria keiskei in beating rabbit atria

The positive inotropic effect of the aqueous extract of Convallaria keiskei (ACK) and the possible mechanisms responsible for this effect were investigated in beating rabbit atria. ACK significantly increased atrial stroke volume, pulse pressure, and cAMP efflux in beating rabbit atria. The effects were not altered by pre-treatment with staurosporine and diltiazem, a non-selective protein kinase inhibitor and an L-type Ca2+ channel blocker, respectively. In addition, ACK markedly increased the K+ concentration in the beating atria-derived perfusate. Convallatoxin, a well-known digitalis-like cardiac glycosidic constituent of ACK, also increased atrial stroke volume and pulse pressure but did not alter the cAMP efflux level. The increases in atrial stroke volume and pulse pressure induced by convallatoxin were not also altered by pre-treatment with diltiazem. These results suggest that the ACK-induced positive inotropic effect in beating rabbit atria may, at least in part, be due to the digitalis-like activity of convallatoxin.     

Comparison of time- and voltage-dependent K+ currents in myocytes from left and right atria of adult mice

Consistent differences in K+ currents in left and right atria of adult mouse hearts have been identified by the application of current- and voltage-clamp protocols to isolated single myocytes. Left atrial myocytes had a significantly (P < 0.05) larger peak outward K+ current density than myocytes from the right atrium. Detailed analysis revealed that this difference was due to the rapidly activating sustained K+ current, which is inhibited by 100 muM 4-aminopyridine (4-AP); this current was almost three times larger in the left atrium than in the right atrium. Accordingly, 100 muM 4-AP caused a significantly (P < 0.05) larger increase in action potential duration in left than in right atrial myocytes. Inward rectifier K+ current density was also significantly (P < 0.05) larger in left atrial myocytes. There was no difference in the voltage-dependent L-type Ca2+ current between left and right atria. As expected from this voltage-clamp data, the duration of action potentials recorded from single myocytes was significantly (P < 0.05) shorter in myocytes from left atria, and left atrial tissue was found to have a significantly (P < 0.05) shorter effective refractory period than right atrial tissue. These results reveal similarities between mice and other mammalian species where the left atrium repolarizes more quickly than the right, and provide new insight into cellular electrophysiological mechanisms responsible for this difference. These findings, and previous results, suggest that the atria of adult mice may be a suitable model for detailed studies of atrial electrophysiology and pharmacology under control conditions and in the context of induced atrial rhythm disturbances.     

IP3 type 1 receptors in the heart: their predominance in atrial walls with ganglion cells

Previously we have shown that inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are abundantly expressed in the atria of rat hearts. Since arrangement of atria is very heterogeneous, in this work we focused on the precise localization of IP3 receptors in the left atrium, where the gene expression of the type 1 IP3R was the highest. The mRNA levels of the IP3 type 1 receptors in the left atrium, left ventricle and myocytes were determined using real-time polymerase chain reaction and Taqman probe. For precise localization, immunohistochemistry with the antibody against type 1 IP3Rs was performed. The mRNA of type 1 IP3 receptor was more than three times higher in the left atrium than in the left ventricle, as determined by real-time PCR. Expression of the type 1 IP3 receptor mRNA was higher in the atria, especially in parts containing cardiac ganglion cells. The atrial auricles, which are particularly free of ganglion cells, and the ventricles (wall of the right and left ventricle and ventricular septum) contained four to five times less IP3 receptors than atrial samples with ganglia. IP3R type 1 immunoreactivity detected by a confocal microscope attributed the most condensed signal on ganglionic cells, although light immunoreactivity was also seen in cardiomyocytes. These results show that type 1IP3 receptors predominate in intrinsic neuronal ganglia of cardiac atria.     

CYP2J2 Overexpression Protects against Arrhythmia Susceptibility in Cardiac Hypertrophy

Maladaptive cardiac hypertrophy predisposes one to arrhythmia and sudden death. Cytochrome P450 (CYP)-derived epoxyeicosatrienoic acids (EETs) promote anti-inflammatory and antiapoptotic mechanisms, and are involved in the regulation of cardiac Ca²⁺-, K⁺- and Na⁺-channels. To test the hypothesis that enhanced cardiac EET biosynthesis counteracts hypertrophy-induced electrical remodeling, male transgenic mice with cardiomyocyte-specific overexpression of the human epoxygenase CYP2J2 (CYP2J2-TG) and wildtype littermates (WT) were subjected to chronic pressure overload (transverse aortic constriction, TAC) or β-adrenergic stimulation (isoproterenol infusion, ISO). TAC caused progressive mortality that was higher in WT (42% over 8 weeks after TAC), compared to CYP2J2-TG mice (6%). In vivo electrophysiological studies, 4 weeks after TAC, revealed high ventricular tachyarrhythmia inducibility in WT (47% of the stimulation protocols), but not in CYP2J2-TG mice (0%). CYP2J2 overexpression also enhanced ventricular refractoriness and protected against TAC-induced QRS prolongation and delocalization of left ventricular connexin-43. ISO for 14 days induced high vulnerability for atrial fibrillation in WT mice (54%) that was reduced in CYP-TG mice (17%). CYP2J2 overexpression also protected against ISO-induced reduction of atrial refractoriness and development of atrial fibrosis. In contrast to these profound effects on electrical remodeling, CYP2J2 overexpression only moderately reduced TAC-induced cardiac hypertrophy and did not affect the hypertrophic response to β-adrenergic stimulation. These results demonstrate that enhanced cardiac EET biosynthesis protects against electrical remodeling, ventricular tachyarrhythmia, and atrial fibrillation susceptibility during maladaptive cardiac hypertrophy.     

An Immunohistochemical Analysis of Tissue Thrombin Expression in the Human Atria

Objective

Thrombin, the final coagulation product of the coagulation cascade, has been demonstrated to have many physiological effects, including pro-fibrotic actions via protease-activated receptor (PAR)-1. Recent investigations have demonstrated that activation of the cardiac local coagulation system was associated with atrial fibrillation. However, the distribution of thrombin in the heart, especially difference between the atria and the ventricle, remains to be clarified. We herein investigated the expression of thrombin and other related proteins, as well as tissue fibrosis, in the human left atria and left ventricle.

Methods

We examined the expression of thrombin and other related molecules in the autopsied hearts of patients with and without atrial fibrillation. An immunohistochemical analysis was performed in the left atria and the left ventricle.

Results

The thrombin was immunohistologically detected in both the left atria and the left ventricles. Other than in the myocardium, the expression of thrombin was observed in the endocardium and the subendocardium of the left atrium. Thrombin was more highly expressed in the left atrium compared to the left ventricle, which was concomitant with more tissue fibrosis and inflammation, as detected by CD68 expression, in the left atrium. We also confirmed the expression of prothrombin in the left atrium. The expression of PAR-1 was observed in the endocardium, subendocardium and myocardium in the left atrium. In patients with atrial fibrillation, strong thrombin expression was observed in the left atrium.

Conclusions

The strong expression levels of thrombin, prothrombin and PAR-1 were demonstrated in the atrial tissues of human autopsied hearts.     

Rat corin gene: molecular cloning and reduced expression in experimental heart failure

Stored cardiac pro-atrial natriuretic peptide (pro-ANP) is converted to ANP and released upon stretch from the atria into the circulation. Corin is a serin protease with pro-ANP-converting properties and may be the rate-limiting enzyme in ANP release. This study was aimed to clone and sequence corin in the rat and to analyze corin mRNA expression in heart failure when ANP release upon stretch is blunted. Full-length cDNA of rat corin was obtained from atrial RNA by RT-PCR and sequenced. Tissue distribution as well as regulation of corin mRNA expression in the atria were determined by RT-PCR and RNase protection assay. Heart failure was induced by an infrarenal aortocaval shunt. Stretch was applied to the left atrium in a working heart modus, and ANP was measured in the perfusates. The sequence of rat corin cDNA was found to be 93.6% homologous to mouse corin cDNA. Corin mRNA was expressed almost exclusively in the heart with highest concentrations in both atria. The aortocaval shunt led to cardiac hypertrophy and heart failure. Stretch-induced ANP release was blunted in shunt animals (control 1,195 +/- 197 fmol.min(-1).g(-1); shunt: 639 +/- 99 fmol.min(-1).g(-1), P < 0.05). Corin mRNA expression was decreased in both atria in shunt animals [right atrium: control 0.638 +/- 0.004 arbitrary units (AU), shunt 0.566 +/- 0.014 AU, P < 0.001; left atrium: control 0.564 +/- 0.009 AU, shunt 0.464 +/- 0.009 AU, P < 0.001]. Downregulation of atrial corin mRNA expression may be a novel mechanism for the blunted ANP release in heart failure.     

Loss of atrial contractility is primary cause of atrial dilatation during first days of atrial fibrillation

Atrial fibrillation (AF) induces a progressive dilatation of the atria which in turn might promote the arrhythmia. The mechanism of atrial dilatation during AF is not known. To test the hypothesis that loss of atrial contractile function is a primary cause of atrial dilatation during the first days of AF, eight goats were chronically instrumented with epicardial electrodes, a pressure transducer in the right atrium, and piezoelectric crystals to measure right atrial diameter. AF was induced with the use of repetitive burst pacing. Atrial contractility was assessed during sinus rhythm, atrial pacing (160-, 300-, and 400-ms cycle length), and electrically induced AF. The compliance of the fibrillating right atrium was measured during unloading the atria with diuretics and loading with 1 liter of saline. All measurements were repeated after 6, 12, and 24 h of AF and then once a day during the first 5 days of AF. Recovery of the observed changes after spontaneous cardioversion was also studied. After 5 days of AF, atrial contractility during sinus rhythm or slow atrial pacing was greatly reduced. During rapid pacing (160 ms) or AF, the amplitude of the atrial pressure waves had declined to 20% of control. The compliance of the fibrillating atria increased twofold, whereas the right atrial pressure was unchanged. As a result, the mean right atrial diameter increased by approximately 12%. All changes were reversible within 3 days of sinus rhythm. We conclude that atrial dilatation during the first days of AF is due to an increase in atrial compliance caused by loss of atrial contractility during AF. Atrial compliance and size are restored when atrial contractility recovers after cardioversion of AF.     

Histamine inhibits atrial myocytic ANP release via H2 receptor-cAMP-protein kinase signaling

Changes in cyclic nucleotide production and atrial dynamics have been known to modulate atrial natriuretic peptide (ANP) release. Although cardiac atrium expresses histamine receptors and contains histamine, the role of histamine in the regulation of ANP release has to be defined. The purpose of the present study was to define the effect of histamine on the regulation of ANP release in perfused beating rabbit atria. Histamine decreased ANP release concomitantly with increases in cAMP efflux and atrial dynamics in a concentration-dependent manner. Histamine-induced decrease in ANP release was a function of an increase in cAMP production. Blockade of histamine H2 receptor with cimetidine but not of H1 receptor with triprolidine abolished the responses of histamine. Cell-permeable cAMP analog, 8-Br-cAMP, mimicked the effects of histamine, and the responses were dose-dependent and blocked by a protein kinase A (PKA)-selective inhibitor, KT5720. Nifedipine failed to modulate histamine-induced decrease in ANP release. Protein kinase nonselective inhibitor staurosporine blocked histamine-induced changes in a concentration-dependent manner. KT5720 and RP-adenosine 3',5'-cyclic monophosphorothioate, another PKA-selective inhibitor, attenuated histamine-induced changes. These results suggest that histamine decreases atrial ANP release by H2 receptor-cAMP signaling via PKA-dependent and -independent pathways.