Potentiation of slow component of delayed rectifier K(+) current by cGMP via two distinct mechanisms: inhibition of phosphodiesterase 3 and activation of protein kinase G

Deciphering cellular signals in adult mouse sinoatrial node cells

Sinoatrial node (SAN) cells are the pacemakers of the heart. This study describes a method for culturing and infection of adult mouse SAN cells with FRET-based biosensors that can be exploited to examine signaling events. SAN cells cultured in media with blebbistatin or (S)-nitro-blebbistatin retain their morphology, protein distribution, action potential (AP) waveform, and cAMP dynamics for at least 40 h. SAN cells expressing targeted cAMP sensors show distinct β-adrenergic-mediated cAMP pools. Cyclic GMP, protein kinase A, Ca2+/CaM kinase II, and protein kinase D in SAN cells also show unique dynamics to different stimuli. Heart failure SAN cells show a decrease in cAMP and cGMP levels. In summary, a reliable method for maintaining adult mouse SAN cells in culture is presented, which facilitates studies of signaling networks and regulatory mechanisms during physiological and pathological conditions.

Carbon monoxide activation of delayed rectifier potassium currents of human cardiac fibroblasts through diverse pathways

To identify the effect and mechanism of carbon monoxide (CO) on delayed rectifier K⁺ currents (I_K) of human cardiac fibroblasts (HCFs), we used the wholecell mode patch-clamp technique. Application of CO delivered by carbon monoxidereleasing molecule-3 (CORM3) increased the amplitude of outward K⁺ currents, and diphenyl phosphine oxide-1 (a specific I_K blocker) inhibited the currents. CORM3- induced augmentation was blocked by pretreatment with nitric oxide synthase blockers (L-NG-monomethyl arginine citrate and L-NG-nitro arginine methyl ester). Pretreatment with KT5823 (a protein kinas G blocker), 1H-[1,-2,-4] oxadiazolo-[4,-3-a] quinoxalin-1-on (ODQ, a soluble guanylate cyclase blocker), KT5720 (a protein kinase A blocker), and SQ22536 (an adenylate cyclase blocker) blocked the CORM3 stimulating effect on I_K. In addition, pretreatment with SB239063 (a p38 mitogen-activated protein kinase [MAPK] blocker) and PD98059 (a p44/42 MAPK blocker) also blocked the CORM3's effect on the currents. When testing the involvement of S-nitrosylation, pretreatment of N-ethylmaleimide (a thiol-alkylating reagent) blocked CO-induced I_K activation and DL-dithiothreitol (a reducing agent) reversed this effect. Pretreatment with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)-21H,23H porphyrin manganese (III) pentachloride and manganese (III) tetrakis (4-benzoic acid) porphyrin chloride (superoxide dismutase mimetics), diphenyleneiodonium chloride (an NADPH oxidase blocker), or allopurinol (a xanthine oxidase blocker) also inhibited CO-induced I_K activation. These results suggest that CO enhances I_K in HCFs through the nitric oxide, phosphorylation by protein kinase G, protein kinase A, and MAPK, S-nitrosylation and reduction/oxidation (redox) signaling pathways.

Cyclic nucleotide signaling and pacemaker activity

The sinoatrial node (SAN) is the natural pacemaker of the heart, producing the electrical impulse that initiates every heart beat. Its activity is tightly controlled by the autonomic nervous system, and by circulating and locally released factors. Neurohumoral regulation of heart rate plays a crucial role in the integration of vital functions and influences behavior and ability to respond to changing environmental conditions. At the cellular level, modulation of SAN activity occurs through intracellular signaling pathways involving cyclic nucleotides: cyclic AMP (cAMP) and cyclic GMP (cGMP). In this Review, dedicated to Professor Dario DiFrancesco and his accomplishements in the field of cardiac pacemaking, we summarize all findings on the role of cyclic nucleotides signaling in regulating the key actors of cardiac automatism, and we provide an up-to-date review on cAMP- and cGMP-phosphodiesterases (PDEs), compellingly involved in this modulation.

Functional modulation of sarcolemmal KATP channels by atrial natriuretic peptide-elicited intracellular signaling in adult rabbit ventricular cardiomyocytes

ATP-sensitive potassium (K_ATP) channels couple cell metabolic status to membrane excitability and are crucial for stress adaptation and cytoprotection in the heart. Atrial natriuretic peptide (ANP), a cardiac peptide important for cardiovascular homeostasis, also exhibits cytoprotective features including protection against myocardial ischemia-reperfusion injuries. However, how ANP modulates cardiac K_ATP channels is largely unknown. In the present study we sought to address this issue by investigating the role of ANP signaling in functional modulation of sarcolemmal K_ATP (sarcK_ATP) channels in ventricular myocytes freshly isolated from adult rabbit hearts. Single-channel recordings were performed in combination with pharmacological approaches in the cell-attached patch configuration. Bath application of ANP markedly potentiated sarcK_ATP channel activities induced by metabolic inhibition with sodium azide, whereas the K_ATP-stimulating effect of ANP was abrogated by selective inhibition of the natriuretic peptide receptor type A (NPR-A), cGMP-dependent protein kinase (PKG), reactive oxygen species (ROS), extracellular signal-regulated protein kinase (ERK)1/2, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), or the ryanodine receptor (RyR). Blockade of RyRs also nullified hydrogen peroxide (H₂O₂)-induced stimulation of sarcK_ATP channels in intact cells. Furthermore, single-channel kinetic analyses revealed that ANP enhanced the function of ventricular sarcK_ATP channels through destabilizing the long closures and facilitating the opening transitions, without affecting the single-channel conductance. In conclusion, here we report that ANP positively modulates the activity of ventricular sarcK_ATP channels via an intracellular signaling mechanism consisting of NPR-A, PKG, ROS, ERK1/2, CaMKII, and RyR2. This novel mechanism may regulate cardiac excitability and contribute to cytoprotection, in part, by opening myocardial K_ATP channels.

Neurohumoral Control of Sinoatrial Node Activity and Heart Rate: Insight From Experimental Models and Findings From Humans

The sinoatrial node is perhaps one of the most important tissues in the entire body: it is the natural pacemaker of the heart, making it responsible for initiating each-and-every normal heartbeat. As such, its activity is heavily controlled, allowing heart rate to rapidly adapt to changes in physiological demand. Control of sinoatrial node activity, however, is complex, occurring through the autonomic nervous system and various circulating and locally released factors. In this review we discuss the coupled-clock pacemaker system and how its manipulation by neurohumoral signaling alters heart rate, considering the multitude of canonical and non-canonical agents that are known to modulate sinoatrial node activity. For each, we discuss the principal receptors involved and known intracellular signaling and protein targets, highlighting gaps in our knowledge and understanding from experimental models and human studies that represent areas for future research.

Effects of Nitric Oxide on Voltage-Gated K⁺ Currents in Human Cardiac Fibroblasts through the Protein Kinase G and Protein Kinase A Pathways but Not through S-Nitrosylation

This study investigated the expression of voltage-gated K⁺ (K_V) channels in human cardiac fibroblasts (HCFs), and the effect of nitric oxide (NO) on the K_V currents, and the underlying phosphorylation mechanisms. In reverse transcription polymerase chain reaction, two types of K_V channels were detected in HCFs: delayed rectifier K⁺ channel and transient outward K⁺ channel. In whole-cell patch-clamp technique, delayed rectifier K⁺ current (I_K) exhibited fast activation and slow inactivation, while transient outward K⁺ current (I_to) showed fast activation and inactivation kinetics. Both currents were blocked by 4-aminopyridine. An NO donor, S-nitroso-N-acetylpenicillamine (SNAP), increased the amplitude of I_K in a concentration-dependent manner with an EC₅₀ value of 26.4 µM, but did not affect I_to. The stimulating effect of SNAP on I_K was blocked by pretreatment with 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or by KT5823. 8-bromo-cyclic GMP stimulated the I_K. The stimulating effect of SNAP on I_K was also blocked by pretreatment with KT5720 or by SQ22536. Forskolin and 8-bromo-cyclic AMP each stimulated I_K. On the other hand, the stimulating effect of SNAP on I_K was not blocked by pretreatment of N-ethylmaleimide or by DL-dithiothreitol. Our data suggest that NO enhances I_K, but not I_to, among K_V currents of HCFs, and the stimulating effect of NO on I_K is through the PKG and PKA pathways, not through S-nitrosylation.

IKs Gain- and Loss-of-Function in Early-Onset Lone Atrial Fibrillation

INTRODUCTION

Atrial fibrillation (AF) is the most frequent cardiac arrhythmia. The potassium current IKs is essential for cardiac repolarization. Gain-of-function mutation in KCNQ1, the gene encoding the pore-forming α-subunit of the IKs channel (KV 7.1), was the first ion channel dysfunction to be associated with familial AF. We hypothesized that early-onset lone AF is associated with a high prevalence of mutations in KCNQ1.

METHODS AND RESULTS

We bidirectionally sequenced the entire coding sequence of KCNQ1 in 209 unrelated patients with early-onset lone AF (<40 years) and investigated the identified mutations functionally in a heterologous expression system. We found 4 nonsynonymous KCNQ1 mutations (A46T, R195W, A302V, and R670K) in 4 unrelated patients (38, 31, 39, and 36 years, respectively). None of the mutations were present in the control group (n = 416 alleles). No other mutations were found in genes previously associated with AF. The mutations A46T, R195W, and A302V have previously been associated with long-QT syndrome. In line with previous reports, we found A302V to display a pronounced loss-of-function of the IKs current, while the other mutants exhibited a gain-of-function phenotype.

CONCLUSIONS

Mutations in the IKs channel leading to gain-of-function have previously been described in familial AF, yet this is the first time a loss-of-function mutation in KCNQ1 is associated with early-onset lone AF. These findings suggest that both gain-of-function and loss-of-function of cardiac potassium currents enhance the susceptibility to AF.

G-Protein-Dependent and -Independent Pathways in Denatonium Signal Transduction

To clarify the involvement of G protein in denatonium signal transduction, we carried out a whole-cell patch-clamp analysis with isolated taste cells in mice. Two different responses were observed by applying GDP-beta-S, a G-protein inhibitor. One response to denatonium was reduced by GDP-beta-S (G-protein-dependent), whereas the other was not affected (G-protein-independent). These different patterns were also observed by concurrently inhibiting the phospholipase C beta2 and phosphodiesterase pathways via G protein. These data suggest dual, G-protein-dependent and -independent mechanisms for denatonium. Moreover, the denatonium responses were not attenuated by singly inhibiting the phospholipase C beta2 or phosphodiesterase pathway, implying that both pathways were involved in G-protein-dependent transduction. In the G-protein-independent cells, the response was abolished by the depletion of calcium ions within the intracellular store. These results suggest that Ca2+ release from the intracellular store is an important factor. Our data demonstrate multiple transduction pathways for denatonium in mammalian taste cells.

Distinct patterns of constitutive phosphodiesterase activity in mouse sinoatrial node and atrial myocardium

Phosphodiesterases (PDEs) are critical regulators of cyclic nucleotides in the heart. In ventricular myocytes, the L-type Ca(2+) current (I(Ca,L)) is a major target of regulation by PDEs, particularly members of the PDE2, PDE3 and PDE4 families. Conversely, much less is known about the roles of PDE2, PDE3 and PDE4 in the regulation of action potential (AP) properties and I(Ca,L) in the sinoatrial node (SAN) and the atrial myocardium, especially in mice. Thus, the purpose of our study was to measure the effects of global PDE inhibition with Isobutyl-1-methylxanthine (IBMX) and selective inhibitors of PDE2, PDE3 and PDE4 on AP properties in isolated mouse SAN and right atrial myocytes. We also measured the effects of these inhibitors on I(Ca,L) in SAN and atrial myocytes in comparison to ventricular myocytes. Our data demonstrate that IBMX markedly increases spontaneous AP frequency in SAN myocytes and AP duration in atrial myocytes. Spontaneous AP firing in SAN myocytes was also increased by the PDE2 inhibitor erythro-9-[2-hydroxy-3-nonyl] adenine (EHNA), the PDE3 inhibitor milrinone (Mil) and the PDE4 inhibitor rolipram (Rol). In contrast, atrial AP duration was increased by EHNA and Rol, but not by Mil. IBMX also potently, and similarly, increased I(Ca,L) in SAN, atrial and ventricular myocytes; however, important differences emerged in terms of which inhibitors could modulate I(Ca,L) in each myocyte type. Consistent with our AP measurements, EHNA, Mil and Rol each increased I(Ca,L) in SAN myocytes. Also, EHNA and Rol, but not Mil, increased atrial I(Ca,L). In complete contrast, no selective PDE inhibitors increased I(Ca,L) in ventricular myocytes when given alone. Thus, our data show that the effects of selective PDE2, PDE3 and PDE4 inhibitors are distinct in the different regions of the myocardium indicating important differences in how each PDE family constitutively regulates ion channel function in the SAN, atrial and ventricular myocardium.

cGMP-cAMP interplay in cardiac myocytes: a local affair with far-reaching consequences for heart function

cAMP and cGMP signalling pathways are common targets in the pharmacological treatment of heart failure, and often drugs that modulate the level of these second messengers are simultaneously administered to patients. cGMP can potentially affect cAMP levels by modulating the activity of PDEs (phosphodiesterases), the enzymes that degrade cyclic nucleotides. This biochemical cross-talk provides the means for drugs that increase cGMP to concomitantly affect cAMP signals. Recent studies using FRET (fluorescence resonance energy transfer) reporters and real-time imaging show that, in cardiac myocytes, the interplay between cGMP and cAMP has different outcomes depending on the specific location where the cross-modulation occurs. cGMP can either increase or decrease the cAMP response to catecholamines, based on the cyclase that generates it and on the PDEs associated with each subcellular compartment. cGMP-mediated modulation of cAMP signals has functional relevance as it affects protein phosphorylation downstream of protein kinase A and myocyte contractility. The physical separation of positive and negative modulation of cAMP levels by cGMP offers the previously unrecognized possibility to selectively modulate local cAMP signals to improve the efficacy of therapy.

Mutations in the potassium channel subunit KCNE1 are associated with early-onset familial atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia. The potassium current IKs is essential for cardiac repolarization. Gain-of-function mutations in KV7.1, the pore-forming α-subunit of the IKs channel, have been associated with AF. We hypothesized that early-onset lone AF is associated with mutations in the IKs channel regulatory subunit KCNE1.

METHODS

In 209 unrelated early-onset lone AF patients (< 40 years) the entire coding sequence of KCNE1 was bidirectionally sequenced. We analyzed the identified KCNE1 mutants electrophysiologically in heterologous expression systems.

RESULTS

Two non-synonymous mutations G25V and G60D were found in KCNE1 that were not present in the control group (n = 432 alleles) and that have not previously been reported in any publicly available databases or in the exom variant server holding exom data from more than 10.000 alleles. Proband 1 (female, age 45, G25V) had onset of paroxysmal AF at the age of 39 years. Proband 2 (G60D) was diagnosed with lone AF at the age of 33 years. The patient has inherited the mutation from his mother, who also has AF. Both probands had no mutations in genes previously associated with AF. In heterologous expression systems, both mutants showed significant gain-of-function for IKs both with respect to steady-state current levels, kinetic parameters, and heart rate-dependent modulation.

CONCLUSIONS

Mutations in KV7.1 leading to gain-of-function of IKs current have previously been described in lone AF, yet this is the first time a mutation in the beta-subunit KCNE1 is associated with the disease. This finding further supports the hypothesis that increased potassium current enhances AF susceptibility.

The role of atrial natriuretic peptide in modulating cardiac electrophysiology

Since the discovery of atrial natriuretic peptide (ANP) in 1981, significant progress has been made in understanding the mechanism of its release and its role in salt and water balance in the body. It has also become clear that ANP plays a key role in cardiac electrophysiology, modulating the autonomic nervous system and regulating the function of cardiac ion channels. The clinical importance of this role was established when mutations in NPPA, the gene encoding ANP, were identified as a cause of familial atrial fibrillation. This review examines our current understanding of the electrophysiological effects of ANP, and their physiological relationship to clinical studies linking ANP and atrial fibrillation.

Phosphodiesterase inhibition in heart failure

Compounds that inhibit the catalytic activity of cyclic nucleotide phosphodiesterases are used as therapeutic agents to increase intracellular cAMP and/or cGMP content in cells or tissues of interest. In patients with heart failure, inhibitors of enzymes in the PDE3 family of cyclic nucleotide phosphodiesterases are used to raise intracellular cAMP content in cardiac muscle, with inotropic actions. These drugs are effective in acute applications, but their long-term use has been complicated by an increase in cardiovascular mortality in clinical trials. Inhibitors of enzymes in the PDE5 family have been used to raise cGMP content in cardiac muscle in animal models of pressure overload, chronic β-adrenergic receptor stimulation, ischemic injury, and doxorubicin toxicity, and have been shown to have antihypertrophic and cardioprotective actions. Recent experimental results raise some question as to the likely applicability of these findings to humans, in whose hearts PDE5 is present at much lower levels than those seen in animal models, and raise the possibility of PDE1, a dual-specificity phosphodiesterase present at high levels in human myocardium, as an alternative target for inotropic and cardioprotective actions.

Modulation of cardiac ERG1 K(+) channels by cGMP signaling

Different K(+) currents have been implicated in the myocardial action potential repolarization including the I(Kr). ERG1 alpha subunits, identified as the molecular correlate of I(Kr), have been shown to form heteromultimeric channels in the heart and their activity is modulated by a complex interplay of signal transduction events. Using electrophysiological techniques, we examined the effects of the cGMP-analogue 8-Br-cGMP on rat and guinea-pig papillary action potential duration (APD), on the biophysical properties of heterologously expressed homo- and heteromeric ERG1 channels, and on cardiac I(Kr). 8-Br-cGMP prolonged APD by about 25% after pharmacological inhibition of L-type Ca(2+) currents and I(Ks). The prolongation was completely abolished by prior application of the hERG channel blocker E-4031 or the protein kinase G (PKG) inhibitor Rp-8-Br-cGMPS. Expression analysis revealed the presence of both ERG1a and -1b subunits in rat papillary muscle. Both 8-Br-cGMP and ANP inhibited heterologously expressed ERG1b and even stronger ERG1a/1b channels, whereas ERG1a channels remained unaffected. The inhibitory 8-Br-cGMP effects were PKG-dependent and involved a profound ERG current reduction, which was also observed with cardiac AP clamp recordings. Measurements of I(Kr) from isolated mouse cardiomyocytes using Cs(+) as charge carrier exhibited faster deactivation kinetics in atrial than in ventricular myocytes consistent with a higher relative expression of ERG1b transcripts in atria than in ventricles. 8-Br-cGMP significantly reduced I(Kr) in atrial, but not in ventricular myocytes. These findings provide first evidence that through heteromeric assembly ERG1 channels become a critical target of cGMP-PKG signaling linking cGMP accumulation to cardiac I(Kr) modulation.

Augmented potassium current is a shared phenotype for two genetic defects associated with familial atrial fibrillation

Mutations in multiple genes have been implicated in familial atrial fibrillation (AF), but the underlying mechanisms, and thus implications for therapy, remain ill-defined. Among 231 participants in the Vanderbilt AF Registry, we found a mutation in KCNQ1 (encoding the alpha-subunit of slow delayed rectifier potassium current [I(Ks)]) and separately a mutation in natriuretic peptide precursor A (NPPA) gene (encoding atrial natriuretic peptide, ANP), both segregating with early onset lone AF in different kindreds. The functional effects of these mutations yielded strikingly similar I(Ks) "gain-of-function." In Chinese Hamster Ovary (CHO) cells, coexpression of mutant KCNQ1 with its ancillary subunit KCNE1 generated approximately 3-fold larger currents that activated much faster than wild-type (WT)-I(Ks). Application of the WT NPPA peptide fragment produced similar changes in WT-I(Ks), and these were exaggerated with the mutant NPPA S64R peptide fragment. Anantin, a competitive ANP receptor antagonist, completely inhibited the changes in I(Ks) gating observed with NPPA S64R. Computational simulations identified accelerated transitions into open states as the mechanism for variant I(Ks) gating. Incorporating these I(Ks) changes into computed human atrial action potentials (AP) resulted in 37% shortening (120 vs. 192 ms at 300 ms cycle length), reflecting loss of the phase II dome which is dependent on L-type calcium channel current. We found striking functional similarities due to mutations in KCNQ1 and NPPA genes which led to I(Ks) "gain-of-function", atrial AP shortening, and consequently altered calcium current as a common mechanism between diverse familial AF syndromes.

Milrinone inhibits hypoxia or hydrogen dioxide-induced persistent sodium current in ventricular myocytes

Much evidence indicates that increased persistent sodium current (I(Na.P)) is associated with cellular calcium overload and I(Na.P) is considered to be a potential target for therapeutic intervention in ischaemia and heart failure. By inhibiting type III phosphodiesterase, milrinone increases intracellular cyclic adenosine monophosphate (cAMP), with a positive inotropic effect. However, the effect of milrinone on increased I(Na.P) under pathological conditions remains unknown. Accordingly, we investigated the effect of milrinone on increased I(Na.P) induced by hypoxia or hydrogen dioxide in guinea pig ventricular myocytes. While milrinone (0.01 mM or 0.1mM) or cAMP (0.1 mM) decreased I(Na.P) respectively in control condition, application of 1 microM H-89, a selective cAMP-dependant protein kinase inhibitor, prevented the effect of 0.1mM milrinone in control condition. Milrinone (0.1 mM) reduced the increased I(Na.P) induced by hypoxia. Furthermore, 0.01 mM or 0.1mM milrinone reduced the enhanced I(Na.P) induced by 0.3 mM hydrogen peroxide. In addition, 0.01 mM or 0.1 mM milrinone shortened action potential duration at 90% repolarization (APD(90)). Bath application of 0.3 mM hydrogen dioxide markedly prolonged APD(90), while 2 microM tetrodotoxin (TTX) reversed the prolonged APD(90). In the other two groups, 0.01 mM or 0.1 mM milrinone shortened the prolonged APD(90) induced by 0.3 mM hydrogen peroxide, ultimately 2 microM TTX causing a further decurtation of APD(90). These findings demonstrate that milrinone inhibited I(Na.P) under normal condition, hypoxia or hydrogen dioxide-induced I(Na.P), and the APD(90) prolonged by hydrogen dioxide-induced I(Na.P) in ventricular myocytes, which is associated with the mechanism of milrinone increasing intracellular cAMP.

The cardiac IKs potassium channel macromolecular complex includes the phosphodiesterase PDE4D3

The cardiac I(Ks) potassium channel is a macromolecular complex consisting of alpha-(KCNQ1) and beta-subunits (KCNE1) and the A kinase-anchoring protein (AKAP) Yotiao (AKAP-9), which recruits protein kinase A) and protein phosphatase 1 to the channel. Here, we have tested the hypothesis that specific cAMP phosphodiesterase (PDE) isoforms of the PDE4D family that are expressed in the heart are also part of the I(Ks) signaling complex and contribute to its regulation by cAMP. PDE4D isoforms co-immunoprecipitated with I(Ks) channels in hearts of mice expressing the I(Ks) channel. In myocytes isolated from these mice, I(Ks) was increased by pharmacological PDE inhibition. PDE4D3, but not PDE4D5, co-immunoprecipitated with the I(Ks) channel only in Chinese hamster ovary cells co-expressing AKAP-9, and PDE4D3, but not PDE4D5, co-immunoprecipitated with AKAP-9. Functional experiments in Chinese hamster ovary cells expressing AKAP-9 and either PDE4D3 or PDE4D5 isoforms revealed modulation of the I(Ks) response to cAMP by PDE4D3 but not PDE4D5. We conclude that PDE4D3, like protein kinase A and protein phosphatase 1, is recruited to the I(Ks) channel via AKAP-9 and contributes to its critical regulation by cAMP.

Phosphodiesterase inhibition in heart failure

Drugs that inhibit cyclic nucleotide phosphodiesterase activity act to increase intracellular cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) content. In total, 11 families of these enzymes-which differ with respect to affinity for cAMP and cGMP, cellular expression, intracellular localization, and mechanisms of regulation-have been identified. Inhibitors of enzymes in the PDE3 family of cyclic nucleotide phosphodiesterases raise intracellular cAMP content in cardiac and vascular smooth muscle, with inotropic and, to a lesser extent, vasodilatory actions. These drugs have been used for many years in the treatment of patients with heart failure, but their long-term use has generally been shown to increase mortality through mechanisms that remain unclear. More recently, inhibitors of PDE5 cyclic nucleotide phosphodiesterases have been used as cGMP-raising agents in vascular smooth muscle. With respect to cardiovascular disease, there is evidence that these drugs are more efficacious in the pulmonary than in the systemic vasculature, for which reason they are used principally in patients with pulmonary hypertension. Effects attributable to inhibition of myocardial PDE5 activity are less well characterized. New information indicating that enzymes from the PDE1 family of cyclic nucleotide phosphodiesterases constitute the majority of cAMP- and cGMP-hydrolytic activity in human myocardium raises questions as to their role in regulating these signaling pathways in heart failure.

Cyclic nucleotide phosphodiesterase PDE1C1 in human cardiac myocytes

Isoforms in the PDE1 family of cyclic nucleotide phosphodiesterases were recently found to comprise a significant portion of the cGMP-inhibited cAMP hydrolytic activity in human hearts. We examined the expression of PDE1 isoforms in human myocardium, characterized their catalytic activity, and quantified their contribution to cAMP hydrolytic and cGMP hydrolytic activity in subcellular fractions of this tissue. Western blotting with isoform-selective anti-PDE1 monoclonal antibodies showed PDE1C1 to be the principal isoform expressed in human myocardium. Immunohistochemical analysis showed that PDE1C1 is distributed along the Z-lines and M-lines of cardiac myocytes in a striated pattern that differs from that of the other major dual-specificity cyclic nucleotide phosphodiesterase in human myocardium, PDE3A. Most of the PDE1C1 activity was recovered in soluble fractions of human myocardium. It binds both cAMP and cGMP with K(m) values of approximately 1 microm and hydrolyzes both substrates with similar catalytic rates. PDE1C1 activity in subcellular fractions was quantified using a new PDE1-selective inhibitor, IC295. At substrate concentrations of 0.1 microm, PDE1C1 constitutes the great majority of cAMP hydrolytic and cGMP hydrolytic activity in soluble fractions and the majority of cGMP hydrolytic activity in microsomal fractions, whereas PDE3 constitutes the majority of cAMP hydrolytic activity in microsomal fractions. These results indicate that PDE1C1 is expressed at high levels in human cardiac myocytes with an intracellular distribution distinct from that of PDE3A and that it may have a role in the integration of cGMP-, cAMP- and Ca(2+)-mediated signaling in these cells.

cAMP and cGMP signaling cross-talk: role of phosphodiesterases and implications for cardiac pathophysiology

Cyclic nucleotide phosphodiesterases regulate cAMP-mediated signaling by controlling intracellular cAMP content. The cAMP-hydrolyzing activity of several families of cyclic nucleotide phosphodiesterases found in human heart is regulated by cGMP. In the case of PDE2, this regulation primarily involves the allosteric stimulation of cAMP hydrolysis by cGMP. For PDE3, cGMP acts as a competitive inhibitor of cAMP hydrolysis. Several cGMP-mediated responses in cardiac cells, including a potentiation of Ca(2+) currents and a diminution of the responsiveness to beta-adrenergic receptor agonists, have been shown to result from the effects of cGMP on cAMP hydrolysis. These effects appear to be dependent on the specific spatial distribution of the cGMP-generating and cAMP-hydrolyzing proteins, as well as on the intracellular concentrations of the two cyclic nucleotides. Gaining a more precise understanding of how these cross-talk mechanisms are individually regulated and coordinated is an important direction for future research.

Role of kinases and G-proteins in the hyposmotic stimulation of cardiac IKs

Exposure of cardiac myocytes to hyposmotic solution stimulates slowly-activating delayed-rectifying K(+) current (I(Ks)) via unknown mechanisms. In the present study, I(Ks) was measured in guinea-pig ventricular myocytes that were pretreated with modulators of cell signaling processes, and then exposed to hyposmotic solution. Pretreatment with compounds that (i) inhibit serine/threonine kinase activity (10-100 microM H89; 200 microM H8; 50 microM H7; 1 microM bisindolylmaleimide I; 10 microM LY294002; 50 microM PD98059), (ii) stimulate serine/threonine kinase activity (1-5 microM forskolin; 0.1 microM phorbol-12-myristate-13-acetate; 10 microM acetylcholine; 0.1 microM angiotensin II; 20 microM ATP), (iii) suppress G-protein activation (10 mM GDPbetaS), or (iv) disrupt the cytoskeleton (10 microM cytochalasin D), had little effect on the stimulation of I(Ks) by hyposmotic solution. In marked contrast, pretreatment with tyrosine kinase inhibitor tyrphostin A25 (20 microM) strongly attenuated both the hyposmotic stimulation of I(Ks) in myocytes and the hyposmotic stimulation of current in BHK cells co-expressing Ks channel subunits KCNQ1 and KCNE1. Since attenuation of hyposmotic stimulation was not observed in myocytes and cells pretreated with inactive tyrphostin A1, we conclude that TK has an important role in the response of cardiac Ks channels to hyposmotic solution.

Isoforms of cyclic nucleotide phosphodiesterase PDE3 and their contribution to cAMP hydrolytic activity in subcellular fractions of human myocardium

Three isoforms of PDE3 (cGMP-inhibited) cyclic nucleotide phosphodiesterase regulate cAMP content in different intracellular compartments of cardiac myocytes in response to different signals. We characterized the catalytic activity and inhibitor sensitivity of these isoforms by using recombinant proteins. We determined their contribution to cAMP hydrolysis in cytosolic and microsomal fractions of human myocardium at 0.1 and 1.0 microm cAMP in the absence and presence of Ca(2+)/calmodulin. We examined the effects of cGMP on cAMP hydrolysis in these fractions. PDE3A-136, PDE3A-118, and PDE3A-94 have similar K(m) and k(cat) values for cAMP and are equal in their sensitivities to inhibition by cGMP and cilostazol. In microsomes, PDE3A-136, PDE3A-118, and PDE3A-94 comprise the majority of cAMP hydrolytic activity under all conditions. In cytosolic fractions, PDE3A-118 and PDE3A-94 comprise >50% of the cAMP hydrolytic activity at 0.1 microm cAMP, in the absence of Ca(2+)/calmodulin. At 1.0 microm cAMP, in the presence of Ca(2+)/calmodulin, activation of Ca(2+)/calmodulin-activated (PDE1) and other non-PDE3 phosphodiesterases reduces their contribution to <20% of cAMP hydrolytic activity. cGMP inhibits cAMP hydrolysis in microsomal fractions by inhibiting PDE3 and in cytosolic fractions by inhibiting both PDE3 and PDE1. These findings indicate that the contribution of PDE3 isoforms to the regulation of cAMP hydrolysis in intracellular compartments of human myocardium and the effects of PDE3 inhibition on cAMP hydrolysis in these compartments are highly dependent on intracellular [Ca(2+)] and [cAMP], which are lower in failing hearts than in normal hearts. cGMP may amplify cAMP-mediated signaling in intracellular compartments of human myocardium by PDE3-dependent and PDE3-independent mechanisms.

Nitric oxide-dependent modulation of the delayed rectifier K+ current and the L-type Ca2+ current by ginsenoside Re, an ingredient of Panax ginseng, in guinea-pig cardiomyocytes

1 Ginsenoside Re, a major ingredient of Panax ginseng, protects the heart against ischemia-reperfusion injury by shortening action potential duration (APD) and thereby prohibiting influx of excessive Ca2+. Ginsenoside Re enhances the slowly activating component of the delayed rectifier K+ current (IKs) and suppresses the L-type Ca2+ current (I(Ca,L)), which may account for APD shortening. 2 We used perforated configuration of patch-clamp technique to define the mechanism of enhancement of IKs and suppression of I(Ca,L) by ginsenoside Re in guinea-pig ventricular myocytes. 3 S-Methylisothiourea (SMT, 1 microm), an inhibitor of nitric oxide (NO) synthase (NOS), and N-acetyl-L-cystein (LNAC, 1 mm), an NO scavenger, inhibited IKs enhancement. Application of an NO donor, sodium nitroprusside (SNP, 1 mm), enhanced IKs with a magnitude similar to that by a maximum dose (20 microm) of ginseonside Re, and subsequent application of ginsenoside Re failed to enhance IKs. Conversely, after IKs had been enhanced by ginsenoside Re (20 microm), subsequently applied SNP failed to further enhance IKs. 4 An inhibitor of guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microm), barely suppressed IKs enhancement, while a thiol-alkylating reagent, N-ethylmaleimide (NEM, 0.5 mm), clearly suppressed it. A reducing reagent, di-thiothreitol (DTT, 5 mm), reversed both ginsenoside Re- and SNP-induced IKs enhancement. 5 I(Ca,L) suppression by ginsenoside Re (3 microm) was abolished by SMT (1 microm) or LNAC (1 mm). NEM (0.5 mm) did not suppress I(Ca,L) inhibition and DTT (5 mm) did not reverse I(Ca,L) inhibition, whereas in the presence of ODQ (10 microm), ginsenoside Re (3 microm) failed to suppress I(Ca,L). 6 These results indicate that ginsenoside Re-induced IKs enhancement and I(Ca,L) suppression involve NO actions. Direct S-nitrosylation of channel protein appears to be the main mechanism for IKs enhancement, while a cGMP-dependent pathway is responsible for I(Ca,L) inhibition.

ANP inhibits TNF-alpha-induced endothelial MCP-1 expression--involvement of p38 MAPK and MKP-1

Atrial natriuretic peptide (ANP) has been shown to reduce tumor necrosis factor-alpha (TNF-alpha)-induced activation of endothelial cells via inhibition of p38 mitogen-activated protein kinase (MAPK) and nuclear factor (NF)-kappaB pathways. The aim of this study was to determine whether ANP is able to inhibit TNF-alpha-induced expression of monocyte chemoattractant protein-1 (MCP-1) in endothelial cells and to elucidate the mechanisms involved. Pretreatment of human umbilical vein endothelial cells (HUVEC) with ANP significantly reduced TNF-alpha-induced expression of MCP-1 protein and mRNA. The effects of ANP were shown to be mediated via the guanylyl-cyclase (GC)-coupled A receptor. Activation of the other GC-coupled receptor (natriuretic peptide receptor-B) by the C-type natriuretic peptide as well as activation of soluble GC with S-nitroso-L-glutathione (GSNO) exerted similar effects as ANP, supporting a role for cyclic guanosine monophosphate (cGMP) in the signal transduction. Antisense experiments showed a requirement of MAPK phosphatase-1 (MKP-1) induction and therefore, inhibition of p38 MAPK in the ANP-mediated inhibition of TNF-alpha-induced expression of MCP-1. To investigate a potential interplay between TNF-alpha-induced activation of p38 MAPK and NF-kappaB, the p38 MAPK inhibitor SB203580 and a dominant-negative p38 MAPK mutant were used. The results indicated that the blockade of p38 MAPK activity leads to an increased activation of NF-kappaB and therefore, suggest a counter-regulatory action of p38 MAPK and NF-kappaB. As antisense experiments revealed a pivotal role for MKP-1 induction and therefore, p38 MAPK inhibition in ANP-mediated attenuation of MCP-1 expression, this action seems to be rather independent of NF-kappaB inhibition.

Role of calcium-activated potassium currents in CNP-induced relaxation of gastric antral circular smooth muscle in guinea pigs

AIM: To investigate ion channel mechanism in CNP-induced relaxation of gastric circular smooth muscle in guinea pigs.

METHODS: Spontaneous contraction of gastric smooth muscle was recorded by a four-channel physiograph. The whole cell patch-clamp technique was used to record calcium-activated potassium currents and membrane potential in the gastric myocytes isolated by collagenase.

RESULTS: C-type natriuretic peptide (CNP) markedly inhibited the spontaneous contraction in a dose-dependent manner in gastric circular smooth muscle in guinea pigs. Ly83583, an inhibitor of guanylate cyclase, weakened CNP-induced inhibition on spontaneous contraction but Zaparinast, an inhibitor of cGMP sensitive phosphoesterase, potentiated CNP-induced inhibition in gastric circular smooth muscles. The inhibitory effects of CNP on spontaneous contraction were blocked by tetrathylammonium (TEA), a nonselective potassium channel blocker. C N P hyperpolarized membrane potential from -60.0 mV ± 2.0 mV to -68.3 mV ± 3.0 mV in a single gastric myocyte. CNP increased calcium-activated potassium currents (I_K(ca)) in a dose-dependent manner in gastric circular myocytes. CNP also increased the spontaneously transient outward currents (STOCs). Ly83583 partly blocked CNP-induced increase of calcium-activated potassium currents, but Zaparinast potented the effect.

CONCLUSION: CNP inhibits spontaneous contraction, and potassium channel may be involved in the process in gastric circular smooth muscle of guinea pigs. CNP-induced increase of I_K(ca) is mediated by a cGMP dependent pathway.

Electrophysiological effects of ginseng and ginsenoside Re in guinea pig ventricular myocytes

Panax ginseng is a folk medicine with various cardiovascular actions; however, its underlying mechanisms of action are not well known. In the present study, we examined the effects of ginseng and its main component, ginsenoside Re, on action potentials and membrane currents recorded from isolated guinea pig ventricular myocytes with the whole-cell patch clamp technique. Ginseng (1 mg/ml) shortened the action potential duration in a rate-dependent manner. Ginseng depressed the L-type Ca2+ current (I(Ca-L)) in a mode of both tonic block and use-dependent block, and enhanced the slowly activating component of the delayed rectifier K+ current (I(Ks)). Ginsenoside Re 3 microM exhibited similar electrophysiological effects to those of 1 mg/ml ginseng, but of slightly smaller magnitude. Inhibition of I(Ca,L) and enhancement of I(Ks) by ginsenoside Re appear to be one of the main electrophysiological actions of ginseng in the heart, although contributions from other ingredients should be considered.