Inotropy and L-type Ca2+ current, activated by beta1- and beta2-adrenoceptors, are differently controlled by phosphodiesterases 3 and 4 in rat heart

Modelling hemodynamics regulation in rats and dogs to facilitate drugs safety risk assessment

Pharmaceutical companies routinely screen compounds for hemodynamics related safety risk. In vitro secondary pharmacology is initially used to prioritize compounds while in vivo studies are later used to quantify and translate risk to humans. This strategy has shown limitations but could be improved via the incorporation of molecular findings in the animal-based toxicological risk assessment. The aim of this study is to develop a mathematical model for rat and dog species that can integrate secondary pharmacology modulation and therefore facilitate the overall pre-clinical safety translation assessment. Following an extensive literature review, we built two separate models recapitulating known regulation processes in dogs and rats. We describe the resulting models and show that they can reproduce a variety of interventions in both species. We also show that the models can incorporate the mechanisms of action of a pre-defined list of 50 pharmacological mechanisms whose modulation predict results consistent with known pharmacology. In conclusion, a mechanistic model of hemodynamics regulations in rat and dog species has been developed to support mechanism-based safety translation in drug discovery and development.

Diverse pathways in GPCR-mediated activation of Ca2+ mobilization in HEK293 cells

G protein-coupled receptors transduce extracellular stimuli into intracellular signaling. Ca2+ is a well-known second messenger that can be induced by G protein-coupled receptor activation through the primary canonical pathways involving Gαq- and Gβγ-mediated activation of phospholipase C-β (PLCβ). While some Gs-coupled receptors are shown to trigger Ca2+ mobilization, underlying mechanisms remain elusive. Here, we evaluated whether Gs-coupled receptors including the β2-adrenergic receptor (β2AR) and the prostaglandin EP2 and EP4 receptors (EP2R and EP4R) that are endogenously expressed in human embryonic kidney 293 (HEK293) cells utilize common pathways for mediating Ca2+ mobilization. For the β2AR, we found an essential role for Gq in agonist-promoted Ca2+ mobilization while genetic or pharmacological inhibition of Gs or Gi had minimal effect. β-agonist-promoted Ca2+ mobilization was effectively blocked by the Gq-selective inhibitor YM-254890 and was not observed in ΔGαq/11 or ΔPLCβ cells. Bioluminescence resonance energy transfer analysis also suggests agonist-dependent association of the β2AR with Gq. For the EP2R, which couples to Gs, agonist treatment induced Ca2+ mobilization in a pertussis toxin-sensitive but YM-254890-insensitive manner. In contrast, EP4R, which couples to Gs and Gi, exhibited Ca2+ mobilization that was sensitive to both pertussis toxin and YM-254890. Interestingly, both EP2R and EP4R were largely unable to induce Ca2+ mobilization in ΔGαs or ΔPLCβ cells, supporting a strong dependency on Gs signaling in HEK293 cells. Taken together, we identify differences in the signaling pathways that are used to mediate Ca2+ mobilization in HEK293 cells where the β2AR primarily uses Gq, EP2R uses Gs and Gi, and EP4R uses Gs, Gi, and Gq.

A molecular mechanism to diversify Ca2+ signaling downstream of Gs protein-coupled receptors

A long-held tenet in inositol-lipid signaling is that cleavage of membrane phosphoinositides by phospholipase Cβ (PLCβ) isozymes to increase cytosolic Ca2+ in living cells is exclusive to Gq- and Gi-sensitive G protein-coupled receptors (GPCRs). Here we extend this central tenet and show that Gs-GPCRs also partake in inositol-lipid signaling and thereby increase cytosolic Ca2+. By combining CRISPR/Cas9 genome editing to delete Gαs, the adenylyl cyclase isoforms 3 and 6, or the PLCβ1-4 isozymes, with pharmacological and genetic inhibition of Gq and G11, we pin down Gs-derived Gβγ as driver of a PLCβ2/3-mediated cytosolic Ca2+ release module. This module does not require but crosstalks with Gαs-dependent cAMP, demands Gαq to release PLCβ3 autoinhibition, but becomes Gq-independent with mutational disruption of the PLCβ3 autoinhibited state. Our findings uncover the key steps of a previously unappreciated mechanism utilized by mammalian cells to finetune their calcium signaling regulation through Gs-GPCRs.

Phosphodiesterase in heart and vessels: from physiology to diseases

Phosphodiesterases (PDEs) are a superfamily of enzymes that hydrolyze cyclic nucleotides, including cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Both cyclic nucleotides are critical secondary messengers in the neurohormonal regulation in the cardiovascular system. PDEs precisely control spatiotemporal subcellular distribution of cyclic nucleotides in a cell- and tissue-specific manner, playing critical roles in physiological responses to hormone stimulation in the heart and vessels. Dysregulation of PDEs has been linked to the development of several cardiovascular diseases, such as hypertension, aneurysm, atherosclerosis, arrhythmia, and heart failure. Targeting these enzymes has been proven effective in treating cardiovascular diseases and is an attractive and promising strategy for the development of new drugs. In this review, we discuss the current understanding of the complex regulation of PDE isoforms in cardiovascular function, highlighting the divergent and even opposing roles of PDE isoforms in different pathogenesis.

Investigation on the positive chronotropic action of 6-nitrodopamine in the rat isolated atria

6-Nitrodopamine (6-ND) is released from rat isolated atria being 100 times more potent than noradrenaline and adrenaline, and 10,000 times more potent than dopamine as a positive chronotropic agent. The present study aimed to investigate the interactions of 6-ND with the classical catecholamines, phosphodiesterase (PDE)-3 and PDE4, and the protein kinase A in rat isolated atria. Atrial incubation with 1 pM of dopamine, noradrenaline, or adrenaline had no effect on atrial frequency. Similar results were observed when the atria were incubated with 0.01 pM of 6-ND. However, co-incubation of 6-ND (0.01 pM) with dopamine, noradrenaline, or adrenaline (1 pM each) resulted in significant increases in atrial rate, which persisted over 30 min after washout of the agonists. The increased atrial frequency induced by co-incubation of 6-ND with the catecholamines was significantly reduced by the voltage-gated sodium channel blocker tetrodotoxin (1 µM, 30 min), indicating that the positive chronotropic effect of 6-ND is due in part to activation of nerve terminals. Pre-treatment of the animals with reserpine had no effect on the positive chronotropic effect induced by dopamine, noradrenaline, or adrenaline; however, reserpine markedly reduced the 6-ND (1 pM)-induced positive chronotropic effect. Incubation of the rat isolated atria with the protein kinase A inhibitor H-89 (1 µM, 30 min) abolished the increased atrial frequency induced by dopamine, noradrenaline, and adrenaline, but only attenuated the increases induced by 6-ND. 6-ND induces catecholamine release from adrenergic terminals and increases atrial frequency independently of PKA activation.

Regulation of APD and Force by the Na+/Ca2+ Exchanger in Human-Induced Pluripotent Stem Cell-Derived Engineered Heart Tissue

The physiological importance of NCX in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is not well characterized but may depend on the relative strength of the current, compared to adult cardiomyocytes, and on the exact spatial arrangement of proteins involved in Ca2+ extrusion. Here, we determined NCX currents and its contribution to action potential and force in hiPSC-CMs cultured in engineered heart tissue (EHT). The results were compared with data from rat and human left ventricular tissue. The NCX currents in hiPSC-CMs were larger than in ventricular cardiomyocytes isolated from human left ventricles (1.3 ± 0.2 pA/pF and 3.2 ± 0.2 pA/pF for human ventricle and EHT, respectively, p < 0.05). SEA0400 (10 µM) markedly shortened the APD90 in EHT (by 26.6 ± 5%, p < 0.05) and, to a lesser extent, in rat ventricular tissue (by 10.7 ± 1.6%, p < 0.05). Shortening in human left ventricular preparations was small and not different from time-matched controls (TMCs; p > 0.05). Force was increased by the NCX block in rat ventricle (by 31 ± 5.4%, p < 0.05) and EHT (by 20.8 ± 3.9%, p < 0.05), but not in human left ventricular preparations. In conclusion, hiPSC-CMs possess NCX currents not smaller than human left ventricular tissue. Robust NCX block-induced APD shortening and inotropy makes EHT an attractive pharmacological model.

Dual Activation of Phosphodiesterase 3 and 4 Regulates Basal Cardiac Pacemaker Function and Beyond

The sinoatrial (SA) node is the physiological pacemaker of the heart, and resting heart rate in humans is a well-known risk factor for cardiovascular disease and mortality. Consequently, the mechanisms of initiating and regulating the normal spontaneous SA node beating rate are of vital importance. Spontaneous firing of the SA node is generated within sinoatrial nodal cells (SANC), which is regulated by the coupled-clock pacemaker system. Normal spontaneous beating of SANC is driven by a high level of cAMP-mediated PKA-dependent protein phosphorylation, which rely on the balance between high basal cAMP production by adenylyl cyclases and high basal cAMP degradation by cyclic nucleotide phosphodiesterases (PDEs). This diverse class of enzymes includes 11 families and PDE3 and PDE4 families dominate in both the SA node and cardiac myocardium, degrading cAMP and, consequently, regulating basal cardiac pacemaker function and excitation-contraction coupling. In this review, we will demonstrate similarities between expression, distribution, and colocalization of various PDE subtypes in SANC and cardiac myocytes of different species, including humans, focusing on PDE3 and PDE4. Here, we will describe specific targets of the coupled-clock pacemaker system modulated by dual PDE3 + PDE4 activation and provide evidence that concurrent activation of PDE3 + PDE4, operating in a synergistic manner, regulates the basal cardiac pacemaker function and provides control over normal spontaneous beating of SANCs through (PDE3 + PDE4)-dependent modulation of local subsarcolemmal Ca²⁺ releases (LCRs).

Dual Activation of Phosphodiesterase 3 and 4 Regulates Basal Cardiac Pacemaker Function and Beyond

The sinoatrial (SA) node is the physiological pacemaker of the heart, and resting heart rate in humans is a well-known risk factor for cardiovascular disease and mortality. Consequently, the mechanisms of initiating and regulating the normal spontaneous SA node beating rate are of vital importance. Spontaneous firing of the SA node is generated within sinoatrial nodal cells (SANC), which is regulated by the coupled-clock pacemaker system. Normal spontaneous beating of SANC is driven by a high level of cAMP-mediated PKA-dependent protein phosphorylation, which rely on the balance between high basal cAMP production by adenylyl cyclases and high basal cAMP degradation by cyclic nucleotide phosphodiesterases (PDEs). This diverse class of enzymes includes 11 families and PDE3 and PDE4 families dominate in both the SA node and cardiac myocardium, degrading cAMP and, consequently, regulating basal cardiac pacemaker function and excitation-contraction coupling. In this review, we will demonstrate similarities between expression, distribution, and colocalization of various PDE subtypes in SANC and cardiac myocytes of different species, including humans, focusing on PDE3 and PDE4. Here, we will describe specific targets of the coupled-clock pacemaker system modulated by dual PDE3 + PDE4 activation and provide evidence that concurrent activation of PDE3 + PDE4, operating in a synergistic manner, regulates the basal cardiac pacemaker function and provides control over normal spontaneous beating of SANCs through (PDE3 + PDE4)-dependent modulation of local subsarcolemmal Ca²⁺ releases (LCRs).

Regulation of basal and norepinephrine-induced cAMP and ICa in hiPSC-cardiomyocytes: Effects of culture conditions and comparison to adult human atrial cardiomyocytes

BACKGROUND

There is ongoing interest in generating cardiomyocytes derived from human induced pluripotent stem cells (hiPSC) to study human cardiac physiology and pathophysiology. Recently we found that norepinephrine-stimulated calcium currents (I_Ca) in hiPSC-cardiomyocytes were smaller in conventional monolayers (ML) than in engineered heart tissue (EHT). In order to elucidate culture specific regulation of β₁-adrenoceptor (β₁-AR) responses we investigated whether action of phosphodiesterases (PDEs) may limit norepinephrine effects on I_Ca and on cytosolic cAMP in hiPSC-cardiomyocytes. Results were compared to adult human atrial cardiomyocytes.

METHODS

Adult human atrial cardiomyocytes were isolated from tissue samples obtained during open heart surgery. All patients were in sinus rhythm. HiPSC-cardiomyocytes were dissociated from ML and EHT. Förster-resonance energy transfer (FRET) was used to monitor cytosolic cAMP (Epac1-camps sensor, transfected by adenovirus). I_Ca was recorded by whole-cell patch clamp technique. Cilostamide (300 nM) and rolipram (10 μM) were used to inhibit PDE3 and PDE4, respectively. β₁-AR were stimulated with the physiological agonist norepinephrine (100 μM).

RESULTS

In adult human atrial cardiomyocytes, norepinephrine increased cytosolic cAMP FRET ratio by +13.7 ± 1.5% (n = 10/9, mean ± SEM, number of cells/number patients) and I_Ca by +10.4 ± 1.5 pA/pF (n = 15/10). This effect was not further increased in the concomitant presence of rolipram, cilostamide and norepinephrine, indicating saturation by norepinephrine alone. In ML hiPSC-cardiomyocytes, norepinephrine exerted smaller increases in cytosolic cAMP and I_Ca (FRET +9.6 ± 0.5% n = 52/21, number of cells/number of ML or EHT, and I_Ca + 1.4 ± 0.2 pA/pF n = 34/7, p < 0.05 each) and both were augmented in the presence of the PDE4 inhibitor rolipram (FRET +16.7 ± 0.8% n = 94/26 and I_Ca + 5.6 ± 1.4 pA/pF n = 11/5, p < 0.05 each). Cilostamide increased the response to norepinephrine on FRET (+12.7 ± 0.5% n = 91/19, p < 0.05), but not on I_Ca. In EHT hiPSC-cardiomyocytes, norepinephrine responses on both, FRET and I_Ca, were larger than in ML (FRET +12.1 ± 0.3% n = 87/32 and I_Ca + 3.3 ± 0.2 pA/pF n = 13/5, p < 0.05 each). Rolipram augmented the norepinephrine effect on I_Ca (+6.2 ± 1.6 pA/pF; p < 0.05 vs. norepinephrine alone, n = 10/4), but not on FRET.

CONCLUSION

Our results show culture-dependent differences in hiPSC-cardiomyocytes. In conventional ML but not in EHT, maximum norepinephrine effects on cytosolic cAMP depend on PDE3 and PDE4, suggesting immaturity when compared to the situation in adult human atrial cardiomyocytes. The smaller I_Ca responses to norepinephrine in ML and EHT vs. adult human atrial cardiomyocytes depend at least partially on a non-physiological large impact of PDE4 in hiPSC-cardiomyocytes.

No impact of sex and age on beta-adrenoceptor-mediated inotropy in human right atrial trabeculae

AIM

There is an increasing awareness of the impact of age and sex on cardiovascular diseases (CVDs). Differences in physiology are suspected. Beta-adrenoceptors (beta-ARs) are an important drug target in CVD and potential differences might have significant impact on the treatment of many patients. To investigate whether age and sex affects beta-AR function, we analysed a large data set on beta-AR-induced inotropy in human atrial trabeculae.

METHODS

We performed multivariable analysis of individual atrial contractility data from trabeculae obtained during heart surgery of patients in sinus rhythm (535 trabeculae from 165 patients). Noradrenaline or adrenaline were used in the presence of the beta₂ -selective antagonist (ICI 118 551, 50 nmol/L) or the beta₁ -selective antagonist (CGP 20712A, 300 nmol/L) to stimulate beta₁ -AR or beta₂ -AR respectively. Agonist concentration required to achieve half-maximum inotropic effects (EC₅₀ ) was taken as a measure of beta-AR sensitivity.

RESULTS

Impact of clinical variables was modelled using multivariable mixed model regression. As previously reported, chronic treatment with beta-blockers sensitized beta-AR. However, there was no significant interaction between basal force, maximum force and beta-AR sensitivity when age and sex were modelled continuously. In addition, there was no statistically significant effect of body mass index or diabetes on atrial contractility.

CONCLUSION

Our large, multivariable analysis shows that neither age nor sex affects beta-AR-mediated inotropy or catecholamine sensitivity in human atrial trabeculae. These findings may have important clinical implications because beta-ARs, as a common drug target in CVD and heart failure, do not behave differently in women and men across age decades.

Compartmentation of β2 -adrenoceptor stimulated cAMP responses by phosphodiesterase types 2 and 3 in cardiac ventricular myocytes

BACKGROUND AND PURPOSE

In cardiac myocytes, cyclic AMP (cAMP) produced by both β₁ - and β₂ -adrenoceptors increases L-type Ca²⁺ channel activity and myocyte contraction. However, only cAMP produced by β₁ -adrenoceptors enhances myocyte relaxation through phospholamban-dependent regulation of the sarco/endoplasmic reticulum Ca²⁺ ATPase 2 (SERCA2). Here we have tested the hypothesis that stimulation of β₂ -adrenoceptors produces a cAMP signal that is unable to reach SERCA2 and determine what role, if any, phosphodiesterase (PDE) activity plays in this compartmentation.

EXPERIMENTAL APPROACH

The cAMP responses produced by β₁ -and β₂ -adrenoceptor stimulation were studied in adult rat ventricular myocytes using two different fluorescence resonance energy transfer (FRET)-based biosensors, the Epac2-camps, which is expressed uniformly throughout the cytoplasm of the entire cell and the Epac2-αKAP, which is targeted to the SERCA2 signalling complex.

KEY RESULTS

Selective activation of β₁ - or β₂ -adrenoceptors produced cAMP responses detected by Epac2-camps. However, only stimulation of β₁ -adrenoceptors produced a cAMP response detected by Epac2-αKAP. Yet, stimulation of β₂ -adrenoceptors was able to produce a cAMP signal detected by Epac2-αKAP in the presence of selective inhibitors of PDE2 or PDE3, but not PDE4.

CONCLUSION AND IMPLICATIONS

These results support the conclusion that cAMP produced by β₂ -adrenoceptor stimulation was not able to reach subcellular locations where the SERCA2 pump is located. Furthermore, this compartmentalized response is due at least in part to PDE2 and PDE3 activity. This discovery could lead to novel PDE-based therapeutic treatments aimed at correcting cardiac relaxation defects associated with certain forms of heart failure.

Cardiac Cyclic Nucleotide Phosphodiesterases: Roles and Therapeutic Potential in Heart Failure

The cyclic nucleotides cyclic adenosine-3′,5′-monophosphate (cAMP) and cyclic guanosine-3′,5′-monophosphate (cGMP) maintain physiological cardiac contractility and integrity. Cyclic nucleotide–hydrolysing phosphodiesterases (PDEs) are the prime regulators of cAMP and cGMP signalling in the heart. During heart failure (HF), the expression and activity of multiple PDEs are altered, which disrupt cyclic nucleotide levels and promote cardiac dysfunction. Given that the morbidity and mortality associated with HF are extremely high, novel therapies are urgently needed. Herein, the role of PDEs in HF pathophysiology and their therapeutic potential is reviewed. Attention is given to PDEs 1–5, and other PDEs are briefly considered. After assessing the role of each PDE in cardiac physiology, the evidence from pre-clinical models and patients that altered PDE signalling contributes to the HF phenotype is examined. The potential of pharmacologically harnessing PDEs for therapeutic gain is considered.

Signal profiling of the β1AR reveals coupling to novel signalling pathways and distinct phenotypic responses mediated by β1AR and β2AR

A comprehensive understanding of signalling downstream of GPCRs requires a broad approach to capture novel signalling modalities in addition to established pathways. Here, using an array of sixteen validated BRET-based biosensors, we analyzed the ability of seven different β-adrenergic ligands to engage five distinct signalling pathways downstream of the β₁-adrenergic receptor (β₁AR). In addition to generating signalling signatures and capturing functional selectivity for the different ligands toward these pathways, we also revealed coupling to signalling pathways that have not previously been ascribed to the βAR. These include coupling to G_z and G₁₂ pathways. The signalling cascade linking the β₁AR to calcium mobilization was also characterized using a combination of BRET-based biosensors and CRISPR-engineered HEK 293 cells lacking the Gαs subunit or with pharmacological or genetically engineered pathway inhibitors. We show that both G_s and G₁₂ are required for the full calcium response. Our work highlights the power of combining signal profiling with genome editing approaches to capture the full complement of GPCR signalling activities in a given cell type and to probe their underlying mechanisms.

Cardiac Overexpression of PDE4B Blunts β-Adrenergic Response and Maladaptive Remodeling in Heart Failure

BACKGROUND

The cyclic AMP (adenosine monophosphate; cAMP)-hydrolyzing protein PDE4B (phosphodiesterase 4B) is a key negative regulator of cardiac β-adrenergic receptor stimulation. PDE4B deficiency leads to abnormal Ca²⁺ handling and PDE4B is decreased in pressure overload hypertrophy, suggesting that increasing PDE4B in the heart is beneficial in heart failure.

METHODS

We measured PDE4B expression in human cardiac tissues and developed 2 transgenic mouse lines with cardiomyocyte-specific overexpression of PDE4B and an adeno-associated virus serotype 9 encoding PDE4B. Myocardial structure and function were evaluated by echocardiography, ECG, and in Langendorff-perfused hearts. Also, cAMP and PKA (cAMP dependent protein kinase) activity were monitored by Förster resonance energy transfer, L-type Ca²⁺ current by whole-cell patch-clamp, and cardiomyocyte shortening and Ca²⁺ transients with an Ionoptix system. Heart failure was induced by 2 weeks infusion of isoproterenol or transverse aortic constriction. Cardiac remodeling was evaluated by serial echocardiography, morphometric analysis, and histology.

RESULTS

PDE4B protein was decreased in human failing hearts. The first PDE4B-transgenic mouse line (TG15) had a ≈15-fold increase in cardiac cAMP-PDE activity and a ≈30% decrease in cAMP content and fractional shortening associated with a mild cardiac hypertrophy that resorbed with age. Basal ex vivo myocardial function was unchanged, but β-adrenergic receptor stimulation of cardiac inotropy, cAMP, PKA, L-type Ca²⁺ current, Ca²⁺ transients, and cell contraction were blunted. Endurance capacity and life expectancy were normal. Moreover, these mice were protected from systolic dysfunction, hypertrophy, lung congestion, and fibrosis induced by chronic isoproterenol treatment. In the second PDE4B-transgenic mouse line (TG50), markedly higher PDE4B overexpression, resulting in a ≈50-fold increase in cardiac cAMP-PDE activity caused a ≈50% decrease in fractional shortening, hypertrophy, dilatation, and premature death. In contrast, mice injected with adeno-associated virus serotype 9 encoding PDE4B (10¹² viral particles/mouse) had a ≈50% increase in cardiac cAMP-PDE activity, which did not modify basal cardiac function but efficiently prevented systolic dysfunction, apoptosis, and fibrosis, while attenuating hypertrophy induced by chronic isoproterenol infusion. Similarly, adeno-associated virus serotype 9 encoding PDE4B slowed contractile deterioration, attenuated hypertrophy and lung congestion, and prevented apoptosis and fibrotic remodeling in transverse aortic constriction.

CONCLUSIONS

Our results indicate that a moderate increase in PDE4B is cardioprotective and suggest that cardiac gene therapy with PDE4B might constitute a new promising approach to treat heart failure.

Regulation of ICa,L and force by PDEs in human-induced pluripotent stem cell-derived cardiomyocytes

Background and Purpose

Phosphodiesterases (PDEs) are important regulators of β‐adrenoceptor signalling in the heart. While PDE4 is the most important isoform that regulates I_Ca,L and force in rodent cardiomyocytes, the dominant isoform in adult human cardiomyocytes is PDE3.

Experimental Approach

Given the potential of human‐induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CMs) for biomedical research, this study characterized the contribution of PDE3 and PDE4 isoforms to the regulation of I_Ca,L and force in hiPSC‐CMs in an engineered heart tissue (EHT) model.

Key Results

There was a lower abundance of mRNA for PDE3A and 4A in hiPSC‐CM EHT than in non‐failing human heart samples. Selective inhibition of PDE3 and 4 with cilostamide and rolipram, respectively, showed that, in hiPSC‐CM, PDE4 was the predominant isoform for the regulation of I_Ca,L (cilostamide: +1.44‐fold; rolipram: +1.77‐fold)_. Furthermore, in contrast to cilostamide, rolipram decreased the EC₅₀ of isoprenaline about 15‐fold.

Conclusion and implications

The predominance of PDE4 over PDE3 is a peculiarity of hiPSC‐CMs and is probably an indicator of immaturity. This finding has implications for the use of hiPSC‐CM as pharmacological models to investigate and assess the effects of PDE inhibitors.

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.

Divergent off-target effects of RSK N-terminal and C-terminal kinase inhibitors in cardiac myocytes

P90 ribosomal S6 kinases (RSK) are ubiquitously expressed and regulate responses to neurohumoral stimulation. To study the role of RSK signalling on cardiac myocyte function and protein phosphorylation, pharmacological RSK inhibitors were tested. Here, the ATP competitive N-terminal kinase domain-targeting compounds D1870 and SL0101 and the allosteric C-terminal kinase domain-targeting FMK were evaluated regarding their ability to modulate cardiac myocyte protein phosphorylation. Exposure to D1870 and SL0101 significantly enhanced phospholamban (PLN) Ser16 and cardiac troponin I (cTnI) Ser22/23 phosphorylation in response to D1870 and SL0101 upon exposure to phenylephrine (PE) that activates RSK. In contrast, FMK pretreatment significantly reduced phosphorylation of both proteins in response to PE. D1870-mediated enhancement of PLN Ser16 phosphorylation was also observed after exposure to isoprenaline or noradrenaline (NA) stimuli that do not activate RSK. Inhibition of β-adrenoceptors by atenolol or cAMP-dependent protein kinase (PKA) by H89 prevented the D1870-mediated increase in PLN phosphorylation, suggesting that PKA is the kinase responsible for the observed phosphorylation. Assessment of changes in cAMP formation by FRET measurements revealed increased cAMP formation in vicinity to PLN after exposure to D1870 and SL0101. D1870 inhibited phosphodiesterase activity similarly as established PDE inhibitors rolipram or 3-isobutyl-1-methylxanthine. Assessment of catecholamine-mediated force development in rat ventricular muscle strips revealed significantly reduced EC₅₀ for NA after D1870 pretreatment (DMSO/NA: 2.33 μmol/L vs. D1870/NA: 1.30 μmol/L). The data reveal enhanced cardiac protein phosphorylation by D1870 and SL0101 that was not detectable in response to FMK. This disparate effect might be attributed to off-target inhibition of PDEs with impact on muscle function as demonstrated for D1870.

β3 -adrenergic receptor activation plays an important role in the depressed myocardial contractility via both elevated levels of cellular free Zn2+ and reactive nitrogen species

Role of β₃ -AR dysregulation, as either cardio-conserving or cardio-disrupting mediator, remains unknown yet. Therefore, we examined the molecular mechanism of β₃ -AR activation in depressed myocardial contractility using a specific agonist CL316243 or using β₃ -AR overexpressed cardiomyocytes. Since it has been previously shown a possible correlation between increased cellular free Zn²⁺ ([Zn²⁺ ]_i ) and depressed cardiac contractility, we first demonstrated a relation between β₃ -AR activation and increased [Zn²⁺ ]_i , parallel to the significant depolarization in mitochondrial membrane potential in rat ventricular cardiomyocytes. Furthermore, the increased [Zn²⁺ ]_i induced a significant increase in messenger RNA (mRNA) level of β₃ -AR in cardiomyocytes. Either β₃ -AR activation or its overexpression could increase cellular reactive oxygen species (ROS) and reactive nitrogen species (RNS) levels, in line with significant changes in nitric oxide (NO)-pathway, including increases in the ratios of pNOS3/NOS3 and pGSK-3β/GSK-3β, and PKG expression level in cardiomyocytes. Although β₃ -AR activation induced depression in both Na⁺ - and Ca²⁺ -currents, the prolonged action potential (AP) seems to be associated with a marked depression in K⁺ -currents. The β₃ -AR activation caused a negative inotropic effect on the mechanical activity of the heart, through affecting the cellular Ca²⁺ -handling, including its effect on Ca²⁺ -leakage from sarcoplasmic reticulum (SR). Our cellular level data with β₃ -AR agonism were supported with the data on high [Zn²⁺ ]_i and β₃ -AR protein-level in metabolic syndrome (MetS)-rat heart. Overall, our present data can emphasize the important deleterious effect of β₃ -AR activation in cardiac remodeling under pathological condition, at least, through a cross-link between β₃ -AR activation, NO-signaling, and [Zn²⁺ ]_i pathways. Moreover, it is interesting to note that the recovery in ER-stress markers with β₃ -AR agonism in hyperglycemic cardiomyocytes is favored. Therefore, how long and to which level the β₃ -AR agonism would be friend or become foe remains to be mystery, yet.

Phosphodiesterase PDE2 activity, increased by isoprenaline, does not reduce β-adrenoceptor-mediated chronotropic and inotropic effects in rat heart

Myocardial PDE2 activity increases in terminal human heart failure and after isoprenaline infusion in rat heart. PDE2 inhibitors do not potentiate the murine sinoatrial tachycardia produced by noradrenaline. We investigated whether isoprenaline infusion induces PDE2 to decrease the chronotropic and inotropic effects of catecholamines in rat heart. Sprague-Dawley rats were infused with isoprenaline (2.4 mg kg^-1 day^-1) for 3 days. We used spontaneously beating right atria, paced right ventricular strips and left ventricular papillary muscles. The effects of the PDE2 inhibitors EHNA (10 μM) and Bay 60-7550 (0.1-1 μM) were investigated on the cardiostimulation produced by noradrenaline (ICI118551 50 nM present to block β₂-adrenoceptors) and adrenaline (CGP20712A 300 nM present to block β₁-adrenoceptors). Hydrolysis of cAMP by PDE2 was measured by radioenzyme assay. Bay 60-7550 but not EHNA increased sinoatrial beating. A stable tachycardia elicited by noradrenaline (10 nM) or adrenaline (1 μM) was not increased by the PDE2 inhibitors. Isoprenaline infusion increased the hydrolytic PDE2 activity threefold in left ventricle, reduced the chronotropic and inotropic effects and potency of noradrenaline and abolished the effects of adrenaline. The potency of the catecholamines was not increased by the PDE2 inhibitors. Neither EHNA nor Bay 60-7550 potentiated the effects of the catecholamines. Rat PDE2 decreased basal sinoatrial beating but did not reduce the sinoatrial tachycardia or increases of ventricular force mediated through β₁- and β₂-adrenoceptors. The β-adrenoceptor desensitization induced by the isoprenaline infusion was not reversed by the PDE2 inhibitors despite the increased hydrolysis of cAMP by PDE2.

Functional β2-adrenoceptors in rat left atria: effect of foot-shock stress

Altered sensitivity to the chronotropic effect of catecholamines and a reduction in the β1/β2-adrenoceptor ratio have previously been reported in right atria of stressed rats, human failing heart, and aging. In this report, we investigated whether left atrial inotropism was affected by foot-shock stress. Male rats were submitted to 3 foot-shock sessions and the left atrial inotropic response, adenylyl cyclase activity, and β-adrenoceptor expression were investigated. Left atria of stressed rats were supersensitive to isoprenaline when compared with control rats and this effect was abolished by ICI118,551, a selective β2-receptor antagonist. Schild plot slopes for the antagonism between CGP20712A (a selective β1-receptor antagonist) and isoprenaline differed from unity in atria of stressed but not control rats. Atrial sensitivity to norepinephrine, as well as basal and forskolin- or isoprenaline-stimulated adenylyl cyclase activities were not altered by stress. The effect of isoprenaline on adenylyl cyclase stimulation was partially blocked by ICI118,551 in atrial membranes of stressed rats. These findings indicate that foot-shock stress equally affects inotropism and chronotropism and that β2-adrenoceptor upregulation contributes to the enhanced inotropic response to isoprenaline.

C1q/tumor necrosis factor-related protein-3 enhances the contractility of cardiomyocyte by increasing calcium sensitivity

C1q/tumor necrosis factor-related protein-3 (CTRP3) is an adipokine that protects against myocardial infarction-induced cardiac dysfunction through its pro-angiogenic, anti-apoptotic, and anti-fibrotic effects. However, whether CTRP3 can directly affect the systolic and diastolic function of cardiomyocytes remains unknown. Adult rat cardiomyocytes were isolated and loaded with Fura-2AM. The contraction and Ca²⁺ transient data was collected and analyzed by IonOptix system. 1 and 2μg/ml CTRP3 significantly increased the contraction of cardiomyocytes. However, CTRP3 did not alter the diastolic Ca²⁺ content, systolic Ca²⁺ content, Ca²⁺ transient amplitude, and L-type Ca²⁺ channel current. To reveal whether CTRP3 affects the Ca²⁺ sensitivity of cardiomyocytes, the typical phase-plane diagrams of sarcomere length vs. Fura-2 ratio was performed. We observed a left-ward shifting of the late relaxation trajectory after CTRP3 perfusion, as quantified by decreased Ca²⁺ content at 50% sarcomere relaxation, and increased mean gradient (μm/Fura-2 ratio) during 500-600ms (-0.163 vs. -0.279), 500-700ms (-0.159 vs. -0.248), and 500-800ms (-0.148 vs. -0.243). Consistently, the phosphorylation level of cardiac troponin I at Ser23/24 was reduced by CTRP3, which could be eliminated by preincubation of okadaic acid, a type 2A protein phosphatase inhibitor. In summary, CTRP3 increases the contraction of cardiomyocytes by increasing the myofilament Ca²⁺ sensitivity. CTRP3 might be a potential endogenous Ca²⁺ sensitizer that modulates the contractility of cardiomyocytes.

Purinergic Receptor Transactivation by the β2-Adrenergic Receptor Increases Intracellular Ca2+ in Nonexcitable Cells

The β₂ adrenergic receptor (β₂AR) increases intracellular Ca²⁺ in a variety of cell types. By combining pharmacological and genetic manipulations, we reveal a novel mechanism through which the β₂AR promotes Ca²⁺ mobilization (pEC₅₀ = 7.32 ± 0.10) in nonexcitable human embryonic kidney (HEK)293S cells. Downregulation of Gs with sustained cholera toxin pretreatment and the use of Gs-null HEK293 (∆Gs-HEK293) cells generated using the clustered regularly interspaced short palindromic repeat-associated protein-9 nuclease (CRISPR/Cas9) system, combined with pharmacological modulation of cAMP formation, revealed a Gs-dependent but cAMP-independent increase in intracellular Ca²⁺ following β₂AR stimulation. The increase in cytoplasmic Ca²⁺ was inhibited by P2Y purinergic receptor antagonists as well as a dominant-negative mutant form of Gq, a Gq-selective inhibitor, and an inositol 1,4,5-trisphosphate (IP₃) receptor antagonist, suggesting a role for this Gq-coupled receptor family downstream of the β₂AR activation. Consistent with this mechanism, β₂AR stimulation promoted the extracellular release of ATP, and pretreatment with apyrase inhibited the β₂AR-promoted Ca²⁺ mobilization. Together, these data support a model whereby the β₂AR stimulates a Gs-dependent release of ATP, which transactivates Gq-coupled P2Y receptors through an inside-out mechanism, leading to a Gq- and IP₃-dependent Ca²⁺ mobilization from intracellular stores. Given that β₂AR and P2Y receptors are coexpressed in various tissues, this novel signaling paradigm could be physiologically important and have therapeutic implications. In addition, this study reports the generation and validation of HEK293 cells deleted of Gs using the CRISPR/Cas9 genome editing technology that will undoubtedly be powerful tools to study Gs-dependent signaling.

Phosphodiesterase inhibitor KMUP-3 displays cardioprotection via protein kinase G and increases cardiac output via G-protein-coupled receptor agonist activity and Ca(2+) sensitization

KMUP-3 (7-{2-[4-(4-nitrobenzene) piperazinyl]ethyl}-1, 3-dimethylxanthine) displays cardioprotection and increases cardiac output, and is suggested to increase cardiac performance and improve myocardial infarction. To determine whether KMUP-3 improves outcomes in hypoperfused myocardium by inducing Ca(2+) sensitization to oppose protein kinase (PK)G-mediated Ca(2+) blockade, we measured left ventricular systolic blood pressure, maximal rates of pressure development, mean arterial pressure and heart rate in rats, and measured contractility and expression of PKs/RhoA/Rho kinase (ROCK)II in beating guinea pig left atria. Hemodynamic changes induced by KMUP-3 (0.5-3.0 mg/kg, intravenously) were inhibited by Y27632 [(R)-(+)-trans-4-1-aminoethyl)-N-(4-Pyridyl) cyclohexane carboxamide] and ketanserin (1 mg/kg, intravenously). In electrically stimulated left guinea pig atria, positive inotropy induced by KMUP-3 (0.1-100μM) was inhibited by the endothelial NO synthase (eNOS) inhibitors N-nitro-l-arginine methyl ester (L-NAME) and 7-nitroindazole, cyclic AMP antagonist SQ22536 [9-(terahydro-2-furanyl)-9H-purin-6-amine], soluble guanylyl cyclase (sGC) antagonist ODQ (1H-[1,2,4] oxadiazolo[4,3-a] quinoxalin-1-one), RhoA inhibitor C3 exoenzyme, β-blocker propranolol, 5-hydroxytryptamine 2A antagonist ketanserin, ROCK inhibitor Y27632 and KMUP-1 (7-{2-[4-(2-chlorobenzene) piperazinyl]ethyl}-1, 3-dimethylxanthine) at 10μM. Western blotting assays indicated that KMUP-3 (0.1-10μM) increased PKA, RhoA/ROCKII, and PKC translocation and CIP-17 (an endogenous 17-kDa inhibitory protein) activation. In spontaneous right atria, KMUP-3 induced negative chronotropy that was blunted by 7-nitroindazole and atropine. In neonatal myocytes, L-NAME inhibited KMUP-3-induced eNOS phosphorylation and RhoA/ROCK activation. In H9c2 cells, Y-27632 (50μM) and PKG antagonist KT5823 [2,3,9,10,11,12-hexahydro-10R- methoxy-2,9-dimethyl-1-oxo-9S,12R-epoxy-1H-diindolo(1,2,3-fg:3',2',1'-kl) pyrrolo(3,4-i)(1,6)benzodiazocine-10-carboxylic acid, methyl ester] (3μM) reversed KMUP-3 (1-100μM)-induced Ca(2+)-entry blockade. GPCR agonist activity of KMUP-3 appeared opposed to KMUP-1, and increased cardiac output via Ca(2+) sensitization, and displayed cardioprotection via cyclic GMP/PKG-mediated myocardial preconditioning in animal studies.

Inhibitors of phosphodiesterases PDE2, PDE3, and PDE4 do not increase the sinoatrial tachycardia of noradrenaline and prostaglandin PGE₁ in mice

Phosphodiesterases PDE2, PDE3, and PDE4 are expressed in murine sinoatrial cells. PDE3 and/or PDE4 reduce heart rate but apparently do not influence the tachycardia mediated through sinoatrial β1- and β2-adrenoceptors despite the high content of sinoatrial cAMP. The function of PDE2 is, however, uncertain. Prostaglandin PGE1 elicits sinoatrial tachycardia through EP receptors, but the control by phosphodiesterases is unknown. We investigated on spontaneously beating right atria of mice the effects of the PDE2 inhibitors Bay 60-7550 and EHNA on basal beating and the tachycardia produced by noradrenaline (3 nM) and PGE1 (1 μM). Bay 60-7550 (1 μM), but not EHNA (10 μM), increased basal sinoatrial beating. EHNA also failed to produce tachycardia in the presence of the adenosine deaminase inhibitor 2'-deoxycoformycin (10 μM), remaining inconclusive whether PDE2 reduces basal sinoatrial beating. Rolipram (10 μM) and cilostamide (300 nM) caused moderate tachycardia. The tachycardia evoked by Bay 60-7550 was similar in the absence and presence of rolipram. Noradrenaline elicited stable tachycardia that was not increased by Bay 60-7550. A stable tachycardia caused by PGE1 was not increased by the inhibitors of PDE2, PDE3, and PDE4. Unlike PDE3 and PDE4 which reduce murine basal sinoatrial beating, a possible effect of PDE2 needs further research. The stable tachycardia produced by noradrenaline and PGE1, together with the lack potentiation by the inhibitors of PDE2, PDE3, and PDE4, suggests that cAMP generated at the receptor compartments is hardly hydrolyzed by these phophodiesterases. Evidence from human volunteers is consistent with this proposal.

Interaction between phosphodiesterases in the regulation of the cardiac β-adrenergic pathway

In cardiac myocytes, the second messenger cAMP is synthesized within the β-adrenergic signaling pathway upon sympathetic activation. It activates Protein Kinase A (PKA) mediated phosphorylation of multiple target proteins that are functionally critical to cardiac contractility. The dynamics of cAMP are also controlled indirectly by cGMP-mediated regulation of phosphodiesterase isoenzymes (PDEs). The nature of the interactions between cGMP and the PDEs, as well as between PDE isoforms, and how these ultimately transduce the cGMP signal to regulate cAMP remains unclear. To better understand this, we have developed mechanistically detailed models of PDEs 1-4, the primary cAMP-hydrolyzing PDEs in cardiac myocytes, and integrated them into a model of the β-adrenergic signaling pathway. The PDE models are based on experimental studies performed on purified PDEs which have demonstrated that cAMP and cGMP bind competitively to the cyclic nucleotide (cN)-binding domains of PDEs 1, 2, and 3, while PDE4 regulation occurs via PKA-mediated phosphorylation. Individual PDE models reproduce experimentally measured cAMP hydrolysis rates with dose-dependent cGMP regulation. The fully integrated model replicates experimentally observed whole-cell cAMP activation-response relationships and temporal dynamics upon varying degrees of β-adrenergic stimulation in cardiac myocytes. Simulations reveal that as a result of network interactions, reduction in the level of one PDE is partially compensated for by increased activation of others. PDE2 and PDE4 exert the strongest compensatory roles among all PDEs. In addition, PDE2 competes with other PDEs to bind and hydrolyze cAMP and is a strong regulator of PDE interactions. Finally, an increasing level of cGMP gradually out-competes cAMP for the catalytic sites of PDEs 1, 2, and 3, suppresses their cAMP hydrolysis rates, and results in amplified cAMP signaling. These results provide insights into how PDEs transduce cGMP signals to regulate cAMP and how PDE interactions affect cardiac β-adrenergic response.

β 1 Adrenoceptor antagonistic effects of the supposedly selective β 2 adrenoceptor antagonist ICI 118,551 on the positive inotropic effect of adrenaline in murine hearts

Studies on the relative contribution of β₁- and β₂-adrenoceptors (AR) generally employ selective β₁- and β₂-AR antagonists such as CGP 20712A and ICI 118,551, respectively, and assume that antagonism by one of these compounds indicates mediation by the respective AR subtype. Here, we evaluated the β₂-AR-selectivity of ICI 118,551 in ventricular muscle strips of transgenic mice lacking β₁-AR (β₁-KO), β₂-AR (β₂-KO), or both (β₁/β₂-KO). Strips were electrically driven and force development was measured. In wild type (WT), ICI 118,551 (100 nmol/L) shifted the concentration–response curve (CRC) for adrenaline by about 0.5 log units to the right, corresponding to the known affinity of ICI 118,551 to β₁-AR but not to β₂-AR. Conversely, the phosphodiesterase inhibitor rolipram (10 μmol/L) shifted the CRC to the left, but did not enlarge the ICI 118,551 shift, indicating exclusive β₁-AR mediation even when PDE4 is inactive. In line with this, rolipram and ICI 118,551 had similar effects in β₂-KO than in WT. In contrast, β₁-KO did not show any inotropic reaction to adrenaline (+/− rolipram). In WT, the β₁-AR selective antagonist CGP 20712A (100 nmol/L) shifted the CRC for isoprenaline by 2.1 log units, corresponding to the affinity of CGP 20712A to β₁-AR. Rolipram increased the sensitivity to adrenaline independently of the presence of CGP 20712A. We conclude that effects sensitive to the β₂-AR antagonist ICI 118,551 are not necessarily β₂-AR-mediated and CGP 20712A-resistant effects cannot be simply interpreted as β₂-AR-mediated. Catecholamine effects in murine ventricles strictly depend on β₁-AR, even if PDE 4 is blocked.

Epinephrine reversed high-concentration bupivacaine- induced inhibition of calcium channels and transient outward potassium current channels, but not on sodium channel in ventricular myocytes of rats

Background

Epinephrine is a first-line drug for cardiopulmonary resuscitation, but its efficacy in the treatment of bupivacaine-induced cardiac toxicity is still in question. We hypothesized that epinephrine can reverse cardiac inhibition of bupivacaine by modulating ion flows through the ventricular myocyte membrane channels of rats. The aim of this study was to observe and report the effects of epinephrine on high-concentration bupivacaine-induced inhibition of sodium (I_Na), L-type calcium (I_Ca-L), and transient outward potassium (I_to) currents in the ventricular myocytes of rats.

Methods

The ventricular myocytes were isolated from Sprague-Dawley rats (250-300 g) by acute enzymatic dissociation. The whole-cell patch clamp technique was used to record the ion channel currents in single ventricular myocytes both before and after administration of medications.

Result

Administration of bupivacaine 100 μmol/L significantly reduced I_Na, (P < 0.05). However, administration of bupivacaine 100 μmol/L in conjunction with epinephrine 0.15 μg/ml had no effect in restoring I_Na to its previous state. Similarly, a sharp decline of I_Ca-L and I_to was observed after administration of bupivacaine 100 μmol/L (P < 0.05). In contrast to I_Na, I_Ca-L and I_to were significantly improved after the administration of the aforementioned combination of bupivacaine and epinephrine (P < 0.05).

Conclusion

Epinephrine can reverse high-concentration bupivacaine induced inhibition of I_Ca-L and I_to, but not I_Na. Thus, epinephrine’s effectiveness in reversal of bupivacaine-induced cardiac toxicity secondary to sodium channel inhibition may be limited.

PDE3, but not PDE4, reduces β₁ - and β₂-adrenoceptor-mediated inotropic and lusitropic effects in failing ventricle from metoprolol-treated patients

BACKGROUND AND PURPOSE

PDE3 and/or PDE4 control ventricular effects of catecholamines in several species but their relative effects in failing human ventricle are unknown. We investigated whether the PDE3-selective inhibitor cilostamide (0.3-1 μM) or PDE4 inhibitor rolipram (1-10 μM) modified the positive inotropic and lusitropic effects of catecholamines in human failing myocardium.

EXPERIMENTAL APPROACH

Right and left ventricular trabeculae from freshly explanted hearts of 5 non-β-blocker-treated and 15 metoprolol-treated patients with terminal heart failure were paced to contract at 1 Hz. The effects of (-)-noradrenaline, mediated through β₁ adrenoceptors (β₂ adrenoceptors blocked with ICI118551), and (-)-adrenaline, mediated through β₂ adrenoceptors (β₁ adrenoceptors blocked with CGP20712A), were assessed in the absence and presence of PDE inhibitors. Catecholamine potencies were estimated from -logEC₅₀s.

KEY RESULTS

Cilostamide did not significantly potentiate the inotropic effects of the catecholamines in non-β-blocker-treated patients. Cilostamide caused greater potentiation (P = 0.037) of the positive inotropic effects of (-)-adrenaline (0.78 ± 0.12 log units) than (-)-noradrenaline (0.47 ± 0.12 log units) in metoprolol-treated patients. Lusitropic effects of the catecholamines were also potentiated by cilostamide. Rolipram did not affect the inotropic and lusitropic potencies of (-)-noradrenaline or (-)-adrenaline on right and left ventricular trabeculae from metoprolol-treated patients.

CONCLUSIONS AND IMPLICATIONS

Metoprolol induces a control by PDE3 of ventricular effects mediated through both β₁ and β₂ adrenoceptors, thereby further reducing sympathetic cardiostimulation in patients with terminal heart failure. Concurrent therapy with a PDE3 blocker and metoprolol could conceivably facilitate cardiostimulation evoked by adrenaline through β₂ adrenoceptors. PDE4 does not appear to reduce inotropic and lusitropic effects of catecholamines in failing human ventricle.

Non-classical regulation of β1- and β 2-adrenoceptor-mediated inotropic responses in rat heart ventricle by the G protein Gi

Studies suggest that increased activity of Gi contributes to the reduced β-adrenoceptor-mediated inotropic response (βAR-IR) in failing cardiomyocytes and that β2AR-IR but not β1AR-IR is blunted by dual coupling to Gs and Gi. We aimed to clarify the role of Gi upon the β1AR-IR and β2AR-IR in Sham and failing myocardium by directly measuring contractile force and cAMP accumulation. Contractility was measured ex vivo in left ventricular strips and cAMP accumulation in cardiomyocytes from rats with post-infarction heart failure (HF) or sham operates (Sham). The β2AR-IR in Sham and HF was small and was amplified by simultaneously inhibiting phosphodiesterases 3 and 4 (PDE3&4). In HF, the inotropic response and cAMP accumulation evoked by β1AR- or β2AR-stimulation were reduced. Inactivation of Gi with pertussis toxin (PTX) did not restore the β1AR-IR or β2AR-IR in HF to Sham levels but did enhance the maximal β2AR-IR. PTX increased both β1AR- and β2AR-evoked cAMP accumulation more in Sham than that in HF, and HF levels approached those in untreated Sham. The potency of agonists at β1 and at β2ARs (only under PDE3&4 inhibition) was increased in HF and by PTX in both HF and Sham. Without PDE3&4 inhibition, PTX increased only the maximal β2AR-IR, not potency. We conclude that Gi regulates both β1AR- and β2AR-IR independent of receptor coupling with Gi. Gi together with PDE3&4 tonically restrict the β2AR-IR. Gi inhibition did not restore the βAR-IR in HF despite increasing cAMP levels, suggesting that the mechanism of impairment resides downstream to cAMP signalling.

Effects of proarrhythmic drugs on relaxation time and beating pattern in rat engineered heart tissue

The assessment of proarrhythmic risks of drugs remains challenging. To evaluate the suitability of rat engineered heart tissue (EHT) for detecting proarrhythmic effects. We monitored drug effects on spontaneous contractile activity and, in selected cases, on action potentials (sharp microelectrode) and Ca²⁺ transients (Fura-2) and contraction under electrical pacing. The I_to-blocker inhibitor 4-aminopyridine increased action potential duration and T2 and caused aftercontractions, which were abolished by inhibitors of ryanodine receptors (RyR2; JTV-519) or sodium calcium exchanger (NCX; SEA0400). 77 Drugs were then tested at 1-10-100× free therapeutic plasma concentrations (FTPC): Inhibitors of I_Kr, I_Ks, I_to, antiarrhythmics (8), drugs withdrawn from market for torsades des pointes arrhythmias (TdP, 5), drugs with measurable (7) or isolated TdP incidence (13), drugs considered safe (14), 28 new chemical entities (NCE). Inhibitors of I_Kr or I_Ks had no effect alone, but substantially prolonged relaxation time (T2) when combined at high concentration. 15/33 drugs associated with TdP and 6/14 drugs considered non-torsadogenic (cibenzoline, diltiazem, ebastine, ketoconazole, moxifloxacin, and phenytoin) induced concentration-dependent T2 prolongations (10-100× FTPC). Bepridil, desipramine, imipramine, thioridazine, and erythromycin induced irregular beating. Three NCE prolonged T2, one reduced force. Drugs inhibiting repolarization prolong relaxation in rat EHTs and cause aftercontractions involving RyR2 and NCX. Insensitivity to I_Kr inhibitors makes rat EHTs unsuitable as general proarrhythmia screen, but favors detection of effects on I_to, I_Ks + I_to or I_Ks + I_Kr. Screening a large panel of drugs suggests that effects on these currents, in addition to I_Kr, are more common than anticipated.

Prevention of cardiac events caused by surgical stress in aged rats: simultaneously activating β2-adrenoceptor and inhibiting β1-adrenoceptor

Increased plasma catecholamine levels are associated with a high risk of perioperative cardiac events in aged individuals undergoing non-cardiac surgical interventions. Given the different effects of β1-adrenoreceptor (β1AR) and β2-adrenoreceptor (β2AR) stimulation by catecholamine in cardiomyocytes, this study evaluated whether simultaneous inhibition of β1AR and activation of β2AR is better than separate application in reducing the risk of perioperative cardiac events in aged rats undergoing non-cardiac surgery. Male aged Sprague-Dawley (SD) rats were divided into five groups. Normal group received no treatment. Surgery group received an abdominal surgery with hypoxia. β1- group, β2+ group, β2+ group and β1+β2+ group received surgery and hypoxia with metoprolol (100 mg/kg·d), fenoterol (250 μg/kg·d) or both, respectively. The drugs were given three days before surgery with treatment continued through post-surgical day 7. The results showed that simultaneous activation of β2AR with a β2AR agonist and inhibition of β1AR with a selective β1AR blocker normalized myocardial oxygen consumption, decreased myocardial damage, augmented cardiomyocyte survival, improved cardiac function, reduced the incidence of arrhythmia, thus decreasing the occurrence of cardiac events in perioperative aged rats undergoing non-cardiac surgery. The results demonstrated that combined use of β2AR agonist and β1AR blocker achieved better general effects than use of either one alone. Our results provide a new insight into preventing perioperative cardiac events for elderly patients undergoing surgical stress.

Differential regulation of cardiac excitation-contraction coupling by cAMP phosphodiesterase subtypes

AIMS

Multiple phosphodiesterases (PDEs) hydrolyze cAMP in cardiomyocytes, but the functional significance of this diversity is not well understood. Our goal here was to characterize the involvement of three different PDEs (PDE2-4) in cardiac excitation-contraction coupling (ECC).

METHODS AND RESULTS

Sarcomere shortening and Ca(2+) transients were recorded simultaneously in adult rat ventricular myocytes and ECC protein phosphorylation by PKA was determined by western blot analysis. Under basal conditions, selective inhibition of PDE2 or PDE3 induced a small but significant increase in Ca(2+) transients, sarcomere shortening, and troponin I phosphorylation, whereas PDE4 inhibition had no effect. PDE3 inhibition, but not PDE2 or PDE4, increased phospholamban phosphorylation. Inhibition of either PDE2, 3, or 4 increased phosphorylation of the myosin-binding protein C, but neither had an effect on L-type Ca(2+) channel or ryanodine receptor phosphorylation. Dual inhibition of PDE2 and PDE3 or PDE2 and PDE4 further increased ECC compared with individual PDE inhibition, but the most potent combination was obtained when inhibiting simultaneously PDE3 and PDE4. This combination also induced a synergistic induction of ECC protein phosphorylation. Submaximal β-adrenergic receptor stimulation increased ECC, and this effect was potentiated by individual PDE inhibition with the rank order of potency PDE4 = PDE3 > PDE2. Identical results were obtained on ECC protein phosphorylation.

CONCLUSION

Our results demonstrate that PDE2, PDE3, and PDE4 differentially regulate ECC in adult cardiomyocytes. PDE2 and PDE3 play a more prominent role than PDE4 in regulating basal cardiac contraction and Ca(2+) transients. However, PDE4 becomes determinant when cAMP levels are elevated, for instance, upon β-adrenergic stimulation or PDE3 inhibition.

Various phosphodiesterase activities in different regions of the heart alter the cardiac effects of nitric oxide

The modulation of cardiac functions by nitric oxide (NO) was established. This study examined the influences of phosphodiesterase (PDE) inhibitors on the action of NO in the different regions of the rat heart. NO donor diethylamine nonoate (DEA/NO) (0.1-100 μM) decreased functions of the right atrium. DEA/NO-induced depression of the developed tension of the right atrium was inhibited by [erythro-9-(2-hydroxy-3-nonyl)adenine] (PDE2 inhibitor), augmented by milrinone (PDE3 inhibitor), and upturned by rolipram (PDE4 inhibitor). A DEA/NO-induced decrease in the resting tension was inhibited by vinpocetine (PDE1 inhibitor) and [erythro-9-(2-hydroxy-3-nonyl)adenine] but reversed by rolipram. The decreased sinus rate by DEA/NO was prevented by vinpocetine and rolipram. DEA/NO increased cyclic guanosine monophosphate and cyclic adenosine monophosphate (cAMP) concentrations in the right atrium, and rolipram enhanced increased cAMP level. DEA/NO had no effect on the contraction of the papillary muscle. However, unchanged contraction under DEA/NO stimulation was decreased by vinpocetine, milrinone, and rolipram. DEA/NO increased cyclic guanosine monophosphate concentration but has no effect on cAMP in the papillary muscle. However, in the presence of vinpocetine and milrinone, DEA/NO reduced cAMP level. The PDE5 inhibitor sildenafil has no effect on DEA/NO actions. This study indicates that a variety of PDE activities in different regions of the rat heart shapes the action of NO on the myocardium.

Cardiac PDEs and crosstalk between cAMP and cGMP signalling pathways in the regulation of contractility

Elucidation of cAMP and cGMP signalling in the heart remains a hot topic, and new regulatory mechanisms continue to appear. Studying the influence of phosphodiesterases on 5-HT4 receptor signalling in porcine atrium, a paper from this issue of the journal expands findings of a crosstalk between cardiac cGMP and cAMP signalling recently discovered in failing rat ventricle to a different species and cardiac region. The overall data suggest that cGMP, produced following stimulation of the NPR-B receptor for C-type natriuretic peptide (CNP), inhibits cAMP degradation by phosphodiesterase 3 and thereby enhances cAMP-mediated signalling from β-adrenoceptors and 5-HT4 receptors to inotropic effects. In porcine atrium, this effect can be seen both as an increase in inotropic effect and as a reduced fade of the inotropic effect with time. Thus, accumulating evidence brings together several active fields of research, including cardiac phosphodiesterases, compartmentation of cyclic nucleotide signalling and the field of natriuretic peptides. If present in human hearts, this effect of CNP may have clinical implications.

Cardiac cyclic nucleotide phosphodiesterases: function, regulation, and therapeutic prospects

The second messengers cAMP and cGMP exist in multiple discrete compartments and regulate a variety of biological processes in the heart. The cyclic nucleotide phosphodiesterases, by catalyzing the hydrolysis of cAMP and cGMP, play crucial roles in controlling the amplitude, duration, and compartmentalization of cyclic nucleotide signaling. Over 60 phosphodiesterase isoforms, grouped into 11 families, have been discovered to date. In the heart, both cAMP- and cGMP-hydrolyzing phosphodiesterases play important roles in physiology and pathology. At least 7 of the 11 phosphodiesterase family members appear to be expressed in the myocardium, and evidence supports phosphodiesterase involvement in regulation of many processes important for normal cardiac function including pacemaking and contractility, as well as many pathological processes including remodeling and myocyte apoptosis. Pharmacological inhibitors for a number of phosphodiesterase families have also been used clinically or preclinically to treat several types of cardiovascular disease. In addition, phosphodiesterase inhibitors are also being considered for treatment of many forms of disease outside the cardiovascular system, raising the possibility of cardiovascular side effects of such agents. This review will discuss the roles of phosphodiesterases in the heart, in terms of expression patterns, regulation, and involvement in physiological and pathological functions. Additionally, the cardiac effects of various phosphodiesterase inhibitors, both potentially beneficial and detrimental, will be discussed.

Phosphodiesterases reduce spontaneous sinoatrial beating but not the 'fight or flight' tachycardia elicited by agonists through Gs-protein-coupled receptors

Cyclic AMP (cAMP) steers the generation of basal heart beat in the sinoatrial node. It also induces sinoatrial tachycardia and increased cardiac force, elicited through activation of Gs-protein-coupled receptors (GsPCRs). Phosphodiesterases (PDEs) hydrolyse cAMP. In the heart mainly PDE3 and PDE4 would be expected to limit those functions, and the PDE isoenzymes do indeed reduce basal sinoatrial beating rate and blunt the positive inotropic effects of agonists, mediated by GsPCRs. By contrast, recent evidence shows that GsPCR-mediated sinoatrial tachycardia is not controlled by PDE1-5. A PDE-resistant cAMP pool in sinoatrial cells, generated through activation of GsPCRs, including β(1)- and β(2)-adrenoceptors, appears to guarantee unrestrained tachycardia during fight or flight stress.

Differential regulation of β2 -adrenoceptor-mediated inotropic and lusitropic response by PDE3 and PDE4 in failing and non-failing rat cardiac ventricle

BACKGROUND AND PURPOSE

β-Adrenoceptors play a major role in regulating myocardial function through cAMP-dependent pathways. Different phosphodiesterases (PDEs) regulate intracellular cAMP-pools and thereby contribute to the compartmentalization of cAMP-dependent effects. We explored the involvement of PDEs in limiting the β(2) adrenoceptor-mediated positive inotropic (PIR) and lusitropic (LR) responses in sham-operated (Sham) and failing rat hearts.

EXPERIMENTAL APPROACH

Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats. Rats developing heart failure were studied 6 weeks after surgery. Contractility was measured in left ventricular strips from failing and Sham hearts. cAMP was quantified by RIA.

KEY RESULTS

In ventricular strips, stimulation of β(2) -adrenoceptors with (-)-adrenaline (300 nM CGP20712A present) exerted a small PIR and LR. In Sham hearts, β(2) -adrenoceptor-mediated as well as β(1) -adrenoceptor-mediated PIR and LR were increased by selective inhibition of either PDE3 (1 µM cilostamide) or PDE4 (10 µM rolipram). In failing rat hearts, PDE3 inhibition enhanced PIR and LR to both β(1) - and β(2) -adrenoceptor stimulation while PDE4 inhibition had no effect on these responses despite a significant increase in cAMP levels. Combined PDE3/4 inhibition further enhanced the PIR and LR of β(2) - and β(1) -adrenoceptor activation both in Sham and failing hearts, compared with PDE3 inhibition alone. PDE4 enzyme activity was reduced in failing hearts.

CONCLUSIONS AND IMPLICATIONS

Both PDE3 and PDE4 attenuated β(2) - and β(1) -adrenoceptor-mediated contractile responses in Sham hearts. In failing hearts, these responses are attenuated solely by PDE3 and thus even selective PDE3 inhibitors may provide a profound enhancement of β-adrenoceptor-mediated responses in heart failure.

Conserved expression and functions of PDE4 in rodent and human heart

PDE4 isoenzymes are critical in the control of cAMP signaling in rodent cardiac myocytes. Ablation of PDE4 affects multiple key players in excitation–contraction coupling and predisposes mice to the development of heart failure. As little is known about PDE4 in human heart, we explored to what extent cardiac expression and functions of PDE4 are conserved between rodents and humans. We find considerable similarities including comparable amounts of PDE4 activity expressed, expression of the same PDE4 subtypes and splicing variants, anchoring of PDE4 to the same subcellular compartments and macromolecular signaling complexes, and downregulation of PDE4 activity and protein in heart failure. The major difference between the species is a fivefold higher amount of non-PDE4 activity in human hearts compared to rodents. As a consequence, the effect of PDE4 inactivation is different in rodents and humans. PDE4 inhibition leads to increased phosphorylation of virtually all PKA substrates in mouse cardiomyocytes, but increased phosphorylation of only a restricted number of proteins in human cardiomyocytes. Our findings suggest that PDE4s have a similar role in the local regulation of cAMP signaling in rodent and human heart. However, inhibition of PDE4 has ‘global’ effects on cAMP signaling only in rodent hearts, as PDE4 comprises a large fraction of the total cardiac PDE activity in rodents but not in humans. These differences may explain the distinct pharmacological effects of PDE4 inhibition in rodent and human hearts.

Function of cardiac beta1- and beta2-adrenoceptors of newborn piglets: role of phosphodiesterases PDE3 and PDE4

The structures of porcine and human beta(2)-adrenoceptors differ but the repercussions for porcine cardiac function are unknown. We investigated the function of porcine beta(2)-adrenoceptors in 3 cardiac regions, sinoatrial node, left atrium and right ventricle of newborn piglets. Both (-)-noradrenaline and (-)-adrenaline caused sinoatrial tachycardia: 60+/-10% and 62+/-7% of the maximum response (E(max)) to (-)-noradrenaline (-logEC(50)=9.0) and (-)-adrenaline (-logEC(50)=7.5) respectively, were resistant to antagonism by the beta(1)-selective CGP20712A (2-hydroxy-5-[2-[[2-hydroxy-3-[4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]phenoxy]propyl]amino]ethoxy]-benzamide) (300 nM) but antagonized by beta(2)-selective ICI118551 (erythro(+/-)-[1-(2,3-dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol) (50 nM), consistent with mediation through beta(2)-adrenoceptors. The phosphodiesterase3-selective inhibitor cilostamide and phosphodiesterase4-selective inhibitor rolipram did not affect catecholamine chronotropic potencies. Only small CGP20712A-resistant positive inotropic effects of (-)-adrenaline were detected in the left atria (13+/-2% of E(max)) and ventricular trabeculae (14+/-5% of E(max)). The atrial inotropic responses to (-)-noradrenaline and (-)-adrenaline faded; fades were prevented by rolipram but not cilostamide or concurrent cilostamide+rolipram respectively. (-)-Noradrenaline (ICI118551 present) increased left atrial cAMP levels through beta(1)-adrenoceptors that were markedly enhanced by rolipram but unaffected by cilostamide. Concurrent cilostamide+rolipram uncovered inotropic and cAMP responses to (-)-adrenaline (CGP20712A present). We conclude that sinoatrial beta(2)-adrenoceptors are more important than beta(1)-adrenoceptors in the mediation of tachycardia caused by both (-)-noradrenaline and (-)-adrenaline in the newborn piglet. beta(2)-adrenoceptors have only a minor role in the mediation of left atrial and ventricular inotropic effects of (-)-adrenaline. Catecholamine-evoked tachycardia is not controlled by PDE3 or PDE4. PDE4, but not PDE3, controls the atrial inotropic and cAMP beta(1)-adrenoceptor-mediated responses to (-)-noradrenaline. Both PDE3 and PDE4 blunt left atrial inotropic and cAMP responses to (-)-adrenaline through beta(2)-adrenoceptors.

Increased spontaneous activity and reduced inotropic response to catecholamines in ventricular myocytes from footshock-stressed rats

Exposure to stressors has been shown to change atrial responsiveness to catecholamines, but it is not clear yet how it affects the ventricular myocardium, which plays a major role in the catecholamine-stimulated increase in cardiac output. Adult male rats were submitted to restraint (RST) or footshock (FS) sessions for 3 days. Reactivity to agonists of the beta-adrenergic pathway was analyzed in left ventricular myocytes isolated from stressed and control rats (CTR). Whereas no significant changes were detected after RST, enhancement of catecholamine-induced spontaneous activity, accompanied by decrease in inotropic maximal response, was observed in myocytes from FS rats. Changes were reversed by beta(1)-, but not by alpha(1)-or beta(2)-adrenoceptor (AR) blockade. Similar alterations were seen in response to forskolin. However, responsiveness to 3-isobutyl-1-methylxanthine and CaCl(2) was comparable in control and FS groups. A significant negative correlation was observed between the maximally stimulated spontaneous activity rate and contraction amplitude. Results indicate that: (a) enhanced automatism during adrenergic stimulation of myocytes from FS rats is mediated by beta(1)-ARs and seems to involve post-receptor mechanisms, probably decreased cAMP degradation; (b) the exaggerated spontaneous activity, which may contribute to generation of catecholaminergic arrhythmias, might limit the development of the inotropic response.

Phosphodiesterases do not limit beta1-adrenoceptor-mediated sinoatrial tachycardia: evidence with PDE3 and PDE4 in rabbits and PDE1-5 in rats

The mammalian heart expresses at least five phosphodiesterases (PDE1-5). Catecholamines produce surges of inotropically relevant cAMP through beta(1)-adrenoceptor stimulation. cAMP is mainly hydrolysed by PDE3 and/or PDE4 thereby blunting contractility. Basal sinoatrial beating rate in mouse, rat, piglet and rabbit sinoatrial cells is reduced by PDE3 and/or PDE4 through hydrolysis of cAMP. However, in rodents, the tachycardia elicited by catecholamines through production of cAMP by beta-adrenoceptor activation is not controlled by PDE3 and PDE4, despite a blunting effect of PDE3 or/and PDE4 on basal sinoatrial beating, but it is unknown whether PDE3 limits catecholamine-evoked tachycardia in the rabbit. Since rabbit sinoatrial cells are an important model for pacemaker research, we investigated whether the positive chronotropic effects of (-)-noradrenaline on spontaneously beating right atria of the rabbit are potentiated by inhibition of PDE3 with cilostamide (300 nM). We also studied the sinoatrial effects of the PDE4 inhibitor rolipram (10 microM) and its influence on the responses to (-)-noradrenaline. For comparison, we investigated the influence of cilostamide and rolipram on the positive inotropic responses to (-)-noradrenaline on rabbit left atria and right ventricular papillary muscles. Cilostamide and concurrent cilostamide + rolipram, but not rolipram alone, increased sinoatrial rate by 15% and 31% of the effect of (-)-isoprenaline (200 microM) but the PDE inhibitors did not significantly change the chronotropic potency of (-)-noradrenaline. In contrast in papillary muscle, the positive inotropic effects of (-)-noradrenaline were potentiated 2.4-, 2.6- and 44-fold by cilostamide, rolipram and concurrent cilostamide + rolipram, respectively. In left atrium, the positive inotropic effects of (-)-noradrenaline were marginally potentiated by cilostamide, as well as potentiated 2.7- and 32-fold by rolipram and by concurrent cilostamide and rolipram respectively. To compare the influence of PDE1-5 on basal sinoatrial rate and (-)-noradrenaline-evoked tachycardia, we investigated on rat right atria the effects of selective inhibitors. The PDE4 inhibitor rolipram and non-selective inhibitor isobutyl-methylxanthine caused tachycardia with -logEC(50)s of 7.2 and 5.0 and E(max) of 18% and 102% of (-)-isoprenaline, respectively. Rolipram did not change the chronotropic potency of (-)-noradrenaline. At high concentrations (10-30 microM), the PDE1, PDE3 and PDE5 inhibitors 8-methoxymethyl-3-isobutyl-1-methylxanthine, cilostamide and sildenafil, respectively, caused marginal tachycardia but did not significantly change the chronotropic potency of (-)-noradrenaline. The PDE2-selective inhibitor erythro-9-[2-hydroxy-3-nonyl]adenine caused marginal bradycardia at 30 microM and tended to reduce the chronotropic potency of (-)-noradrenaline. Rabbit PDE3 reduces basal sinoatrial rate. Although PDE4 only marginally reduces rate, under conditions of PDE3 inhibition, it further reduces sinoatrial rate. Both PDE3 and PDE4 control atrial and ventricular positive inotropic effects of (-)-noradrenaline. In contrast, neither PDE3 nor PDE4 limit the sinoatrial tachycardia induced by (-)-noradrenaline. In the rat, only PDE4, but not PDE1, PDE2, PDE3 and PDE5, reduces basal sinoatrial rate. None of the five rat PDEs limits the (-)-noradrenaline-evoked tachycardia. Taken together, these results confirm and expand evidence for our proposal that the cAMP-compartment modulating basal sinoatrial rate, controlled by PDE3 and/or PDE4, is different from the PDE-resistant cAMP compartment involved in beta(1)-adrenoceptor-mediated sinoatrial tachycardia.

Ontogenic changes of the control by phosphodiesterase-3 and -4 of 5-HT responses in porcine heart and relevance to human atrial 5-HT(4) receptors

BACKGROUND AND PURPOSE

Atrial inotropic responses to 5-HT mediated through 5-HT(4) receptors fade, presumably through phosphodiesterase (PDE) activity. We investigated the influence of a selective inhibitor of PDE3 (cilostamide) or of PDE4 (rolipram) on the fade of 5-HT responses in atrial muscle.

EXPERIMENTAL APPROACH

5-HT responses were compared, ex vivo, on sinoatrial beating rate of newborn piglets, porcine atrial and ventricular force, and human atrial force. cAMP levels were assessed in piglet atrium.

KEY RESULTS

5-HT-evoked sinoatrial tachycardia did not fade and was not potentiated by cilostamide (300 nmol.L(-1)) or rolipram (1 micromol.L(-1)). Inotropic responses to 5-HT faded in atria from piglets, adolescent pigs and humans. Cilostamide reduced atrial fade of 5-HT responses in adolescent pigs and humans but not in newborn piglets. Cilostamide disclosed 5-HT ventricular responses in newborn, but not adolescent pigs. Rolipram reduced fade of atrial 5-HT responses in newborn and adolescent pigs but not in humans. Concurrent cilostamide + rolipram abolished fade of 5-HT responses in porcine left atria and facilitated ventricular 5-HT responses, but did not reduce residual fade in human atrium in the presence of cilostamide. 5-HT-evoked increases in cAMP faded; fade was abolished by concurrent cilostamide + rolipram.

CONCLUSIONS AND IMPLICATIONS

PDE3-induced control of porcine 5-HT responses differed in atrium and ventricle and changed with age. PDE3 and PDE4 jointly prevented fade of inotropic and cAMP responses to 5-HT in porcine atrium. Unlike porcine atria, only PDE3 induced fade of 5-HT responses in human atria.

Phosphodiesterases PDE3 and PDE4 jointly control the inotropic effects but not chronotropic effects of (-)-CGP12177 despite PDE4-evoked sinoatrial bradycardia in rat atrium

Acting through a low-affinity site of the beta(1)-adrenoceptor (beta(1L)AR), CGP12177 causes sinoatrial tachycardia and positive inotropic effects in left atrium but not in the ventricle of the rat. However, inhibition of either PDE3 or PDE4 also uncovers positive inotropic effects of CGP12177 in ventricle, but whether these phosphodiesterases also control the atrial agonist effects of CGP12177 was unknown. We, therefore, investigated the effects of the PDE3-selective inhibitor cilostamide (300 nM) and PDE4 inhibitor rolipram (1 microM) on the (-)-CGP12177-evoked increases of sinoatrial beating rate and force of paced left atria of the rat. Rolipram (n = 8) increased basal sinoatrial rate by 27 +/- 5 bpm but cilostamide (n = 8) had no effect. The chronotropic potency of (-)-CGP12177 (-logEC(50)M = 7.5) was not changed by rolipram and cilostamide or their combination. (-)-CGP12177 increased left atrial force with intrinsic activity 0.25 compared to (-)-isoprenaline. Rolipram (n = 8) and cilostamide (n = 8) did not change basal force of left atria but concurrent rolipram + cilostamide (n = 8) increased force by 52 +/- 9% of the effect of 200 microM (-)-isoprenaline. Neither rolipram nor cilostamide affected the inotropic potency of (-)-CGP12177 (-logEC(50)M = 7.4) but concurrent rolipram + cilostamide caused potentiation (-logEC(50)M = 8.2) and converted (-)-CGP12177 into a full agonist compared to (-)-isoprenaline. Cyclic AMP appears to maintain sinoatrial rate and PDE4 elicits bradycardia through hydrolysis of cAMP in a compartment distinct from the beta(1L)AR-induced cAMP compartment through which (-)-CGP12177 causes tachycardia. In contrast to the (-)-CGP12177-evoked tachycardia, not controlled by PDE3 and PDE4, these isoenzymes jointly reduce (-)-CGP12177-evoked increases of left atrial contractility through beta(1L)AR.