Pharmacological modulation of the hyperpolarization-activated current (If) in human atrial myocytes: focus on G protein-coupled receptors

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

The Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: from Biophysics to Pharmacology of a Unique Family of Ion Channels

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are important members of the voltage-gated pore loop channels family. They show unique features: they open at hyperpolarizing potential, carry a mixed Na/K current, and are regulated by cyclic nucleotides. Four different isoforms have been cloned (HCN1-4) that can assemble to form homo- or heterotetramers, characterized by different biophysical properties. These proteins are widely distributed throughout the body and involved in different physiologic processes, the most important being the generation of spontaneous electrical activity in the heart and the regulation of synaptic transmission in the brain. Their role in heart rate, neuronal pacemaking, dendritic integration, learning and memory, and visual and pain perceptions has been extensively studied; these channels have been found also in some peripheral tissues, where their functions still need to be fully elucidated. Genetic defects and altered expression of HCN channels are linked to several pathologies, which makes these proteins attractive targets for translational research; at the moment only one drug (ivabradine), which specifically blocks the hyperpolarization-activated current, is clinically available. This review discusses current knowledge about HCN channels, starting from their biophysical properties, origin, and developmental features, to (patho)physiologic role in different tissues and pharmacological modulation, ending with their present and future relevance as drug targets.

HCN Channels Modulators: The Need for Selectivity

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, the molecular correlate of the hyperpolarization-activated current (If/Ih), are membrane proteins which play an important role in several physiological processes and various pathological conditions. In the Sino Atrial Node (SAN) HCN4 is the target of ivabradine, a bradycardic agent that is, at the moment, the only drug which specifically blocks If. Nevertheless, several other pharmacological agents have been shown to modulate HCN channels, a property that may contribute to their therapeutic activity and/or to their side effects. HCN channels are considered potential targets for developing drugs to treat several important pathologies, but a major issue in this field is the discovery of isoform-selective compounds, owing to the wide distribution of these proteins into the central and peripheral nervous systems, heart and other peripheral tissues. This survey is focused on the compounds that have been shown, or have been designed, to interact with HCN channels and on their binding sites, with the aim to summarize current knowledge and possibly to unveil useful information to design new potent and selective modulators.

β-adrenergic receptor responsiveness in aging heart and clinical implications

Elderly healthy individuals have a reduced exercise tolerance and a decreased left ventricle inotropic reserve related to increased vascular afterload, arterial-ventricular load mismatching, physical deconditioning and impaired autonomic regulation (the so called "β-adrenergic desensitization"). Adrenergic responsiveness is altered with aging and the age-related changes are limited to the β-adrenergic receptor density reduction and to the β-adrenoceptor-G-protein(s)-adenylyl cyclase system abnormalities, while the type and level of abnormalities change with species and tissues. Epidemiological studies have shown an high incidence and prevalence of heart failure in the elderly and a great body of evidence correlate the changes of β-adrenergic system with heart failure pathogenesis. In particular it is well known that: (a) levels of cathecolamines are directly correlated with mortality and functional status in heart failure, (b) β1-adrenergic receptor subtype is down-regulated in heart failure, (c) heart failure-dependent cardiac adrenergic responsiveness reduction is related to changes in G proteins activity. In this review we focus on the cardiovascular β-adrenergic changes involvement in the aging process and on similarities and differences between aging heart and heart failure.

Chronic atrial fibrillation alters the functional properties of If in the human atrium

INTRODUCTION

Despite the evidence that the hyperpolarization-activated current (If) is highly modulated in human cardiomyopathies, no definite data exist in chronic atrial fibrillation (cAF). We investigated the expression, function, and modulation of If in human cAF.

METHODS AND RESULTS

Right atrial samples were obtained from sinus rhythm (SR, n = 49) or cAF (duration >1 year, n = 31) patients undergoing corrective cardiac surgery. Among f-channel isoforms expressed in the human atrium (HCN1, 2 and 4), HCN4 mRNA levels measured by RT-PCR were significantly reduced. However, protein expression was preserved in cAF compared to SR (+85% for HCN4); concurrently, miR-1 expression was significantly reduced. In patch-clamped atrial myocytes, current-specific conductance (gf) was significantly increased in cAF at voltages around the threshold for If activation (-60 to -80 mV); accordingly, a 10-mV rightward shift of the activation curve occurred (P < 0.01). β-Adrenergic and 5-HT4 receptor stimulation exerted similar effects on If in cAF and SR cells, while the ANP-mediated effect was significantly reduced (P < 0.02), suggesting downregulation of natriuretic peptide signaling.

CONCLUSIONS

In human cAF modifications in transcriptional and posttranscriptional mechanisms of HCN channels occur, associated with a slight yet significant gain-of-function of If , which may contribute to enhanced atrial ectopy.

Pathophysiological relevance of the cardiac β2-adrenergic receptor and its potential as a therapeutic target to improve cardiac function

β-adrenoceptors are members of the G protein-coupled receptor superfamily which play a key role in the regulation of myocardial function. Their activation increases cardiac performance but can also induce deleterious effects such as cardiac arrhythmias or myocardial apoptosis. In fact, inhibition of β-adrenoceptors exerts a protective effect in patients with sympathetic over-stimulation during heart failure. Although β(2)-adrenoceptor is not the predominant subtype in the heart, it seems to importantly contribute to the cardiac effects of adrenergic stimulation; however, the mechanism by which this occurs is not fully understood. This review summarizes the current knowledge on the role of β(2)-adrenoceptors in the regulation of cardiac contractility, metabolism, cardiomyocyte survival and cardiac arrhythmias. In addition, therapeutic considerations relating to stimulation of the β(2)-adrenoceptor such as an increase in cardiac contractility with low arrythmogenic effect, protection of the myocardium again apoptosis or positive regulation of heart metabolism are discussed.

Altered Excitation-Contraction Coupling in Human Chronic Atrial Fibrillation

This review focuses on the (mal)adaptive processes in atrial excitation-contraction coupling occurring in patients with chronic atrial fibrillation. Cellular remodeling includes shortening of the atrial action potential duration and effective refractory period, depressed intracellular Ca²⁺ transient, and reduced myocyte contractility. Here we summarize the current knowledge of the ionic bases underlying these changes. Understanding the molecular mechanisms of excitation-contraction-coupling remodeling in the fibrillating human atria is important to identify new potential targets for AF therapy.

Cardiac adrenergic control and atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and it causes substantial mortality. The autonomic nervous system, and particularly the adrenergic/cholinergic balance, has a profound influence on the occurrence of AF. Adrenergic stimulation from catecholamines can cause AF in patients. In human atrium, catecholamines can affect each of the electrophysiological mechanisms of AF initiation and/or maintenance. Catecholamines may produce membrane potential oscillations characteristic of afterdepolarisations, by increasing Ca(2+) current, [Ca(2+)](i) and consequent Na(+)-Ca(2+) exchange, and may also enhance automaticity. Catecholamines might affect reentry, by altering excitability or conduction, rather than action potential terminal repolarisation or refractory period. However, which arrhythmia mechanisms predominate is unclear, and likely depends on cardiac pathology and adrenergic tone. Heart failure (HF), a major cause of AF, causes adrenergic activation and adaptational changes, remodelling, of atrial electrophysiology, Ca(2+) homeostasis, and adrenergic responses. Chronic AF also remodels these, but differently to HF. Myocardial infarction and AF cause neural remodelling that also may promote AF. beta-Adrenoceptor antagonists (beta-blockers) are used in the treatment of AF, mainly to control the ventricular rate, by slowing atrioventricular conduction. beta-Blockers also reduce the incidence of AF, particularly in HF or after cardiac surgery, when adrenergic tone is high. Furthermore, the chronic treatment of patients with beta-blockers remodels the atria, with a potentially antiarrhythmic increase in the refractory period. Therefore, the suppression of AF by beta-blocker treatment may involve an attenuation of arrhythmic activity that is caused by increased [Ca(2+)](i), coupled with effects of adaptation to the treatment. An improved understanding of the involvement of the adrenergic system and its control in basic mechanisms of AF under differing cardiac pathologies might lead to better treatments.

Electrophysiological characterization of isolated human atrial myocytes exposed to tegaserod

Tegaserod (Teg), a 5-hydroxytryptamine type-4 (5-HT(4)) receptor partial agonist, represents a novel treatment for irritable bowel syndrome with constipation and chronic constipation. Cardiovascular safety data from pooled clinical studies showed a signal suggestive of increased occurrence of ischaemic cardiovascular events in patients exposed to Teg versus placebo. Thereafter, marketing of Teg was suspended in the USA and other countries. The clinical data did not demonstrate a causative effect but raised questions of whether a non-recognized effect on the heart was present. Our aim was to evaluate for arrhythmogenic potential of Teg on human cardiomyocytes. Cells isolated from human atrial specimens during cardiac surgery were used to assess the effects of Teg (1, 10, 30 and 100 nM) on action potential and I(f) (funny current) by patch-clamp technique. Results showed that Teg (at all concentrations tested) did not significantly affect action potential characteristics of atrial myocytes when driven at different rates (0.2, 0.5 and 1 Hz). In contrast, 5HT significantly prolonged action potential duration (1 and 10 nM) and caused cell un-excitability (100 nM). Teg, at the highest concentration tested (100 nM, corresponding to 10 times C(max), produced by the recommended dose of 6 mg b.i.d.) increased the I(f) amplitude and caused a shift of its activation curve. This effect of a high concentration of Teg is not considered clinically relevant. When evaluated on single human atrial cells, Teg does not appear to exhibit arrhythmogenic properties, as it did not affect the action potential profile.

I(f) channels as a therapeutic target in heart disease

In the normal heart, impulses are generated from the sinoatrial node. It is generally accepted that the pacemaker current, I(f), plays a major role in the spontaneous rhythmic activity. Recently, several electrophysiological and molecular data demonstrate that I(f) channels are present in embryonic and post-natal ventricular myocytes and undergo a downregulation during maturation. Interestingly, the I(f) current is re-expressed in some pathological conditions such as cardiac hypertrophy and heart failure. In these conditions, the overexpression of f-channels is a consequence of electrophysiological remodeling and may represent an arrhythmogenic mechanism in heart failure, a condition associated with high risk for sudden cardiac death. For its physiological and pathophysiological role and the availability of selective f-channel blockers, I(f) may be a suitable therapeutic target in heart failure.

NO underlies the muscarinic receptor-mediated inhibition of If in early embryonic heart cells

BACKGROUND/AIMS

Early embryonic cardiomyocytes beat spontaneously. The hyperpolarization-activated cyclic-nucleotide-modulated current (I(f)) appears to be involved in its modulation as it is highly expressed at this stage. The spontaneous beating of early embryonic heart cells is slowed by acetylcholine (ACh), and our earlier studies identified a key role for nitric oxide (NO) in the regulation of the voltage dependent L-type Ca(2+) current (I(Ca,L)). The aim of the present study was to clarify whether and via which signalling pathway(s) I(f) is regulated upon muscarinic receptor activation in early embryonic (E9.5 to E11.5) cardiomyocytes.

METHODS

The whole-cell patch clamp technique in combination with pharmacology and/or knock out mouse models was used to investigate the regulation of I(f).

RESULTS

We found that the ACh analogue carbachol (CCh, 10 micromol) led in the majority of cells (68%, n=50) to a significant depression of I(f) by 16.3+/-1.4% (n=34, p<0.01, voltage steps from -35 mV to -110 mV). This cholinergic inhibition was mediated by the NO/cGMP signalling pathway as it was largely reversed by superfusion with the non selective nitric oxide synthase (NOS) inhibitor N(G)-Methyl-L-arginine acetate salt (L-NMMA, 1 mmol), the inhibitor of the soluble guanylyl cyclase (sGC) 1H-[1, 2, 4]Oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ, 100 micromol) and a selective inhibitor of the phosphodiesterase (PDE) type 2 Erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, 30 micromol). Analysis of the muscarinic signalling in embryonic cardiomyocytes harvested from NOS2 (-/-) and NOS3 (-/-) mice revealed that the NOS3 isoform was entirely responsible for the muscarinic receptor-induced NO production.

CONCLUSIONS

Muscarinic receptor stimulation depresses I(f) by generating NO via the NOS3 and the cGMP/PDE type 2 signalling pathway in early embryonic cardiomyocytes. This suggests that NO is a key signalling molecule involved in the regulation of chronotropy of early embryonic heart cells.

Effects of pacemaker currents on creation and modulation of human ventricular pacemaker: theoretical study with application to biological pacemaker engineering

A cardiac biological pacemaker (BP) has been created by suppression of the inward rectifier K+ current ( IK1) or overexpression of the hyperpolarization-activated current ( Ih). We theoretically investigated the effects of incorporating Ih, T-type Ca2+ current ( ICa,T), sustained inward current ( Ist), and/or low-voltage-activated L-type Ca2+ channel current ( ICa,LD) on 1) creation of BP cells, 2) robustness of BP activity to electrotonic loads of nonpacemaking (NP) cells, and 3) BP cell ability to drive NP cells. We used a single-cell model for human ventricular myocytes (HVMs) and also coupled-cell models composed of BP and NP cells. Bifurcation structures of the model cells were explored during changes in conductance of the currents and gap junction. Incorporating the pacemaker currents did not yield BP activity in HVM with normal IK1 but increased the critical IK1 conductance for BP activity to emerge. Expressing Ih appeared to be most helpful in facilitating creation of BP cells via IK1 suppression. In the coupled-cell model, Ist significantly enlarged the gap conductance ( GC) region where stable BP cell pacemaking and NP cell driving occur, reducing the number of BP cells required for robust pacemaking and driving. In contrast, Ih enlarged the GC region of pacemaking and driving only when IK1 of the NP cell was relatively low. ICa,T or ICa,LD exerted effects similar to those of Ist but caused shrinkage or irregularity of BP oscillations. These findings suggest that expressing Ist most effectively improves the structural stability of BPs to electrotonic loads and the BP ability to drive the ventricle.

Electrophysiological and arrhythmogenic effects of 5-hydroxytryptamine on human atrial cells are reduced in atrial fibrillation

5-Hydroxytryptamine (5-HT) is proarrhythmic in atrial cells from patients in sinus rhythm (SR) via activation of 5-HT₄ receptors, but its effects in atrial cells from patients with atrial fibrillation (AF) are unknown. The whole-cell perforated patch-clamp technique was used to record L-type Ca²⁺ current (I_CaL), action potential duration (APD) and arrhythmic activity at 37 °C in enzymatically isolated atrial cells obtained from patients undergoing cardiac surgery, in SR or with chronic AF. In the AF group, 5-HT (10 μM) produced an increase in I_CaL of 115 ± 21% above control (n = 10 cells, 6 patients) that was significantly smaller than that in the SR group (232 ± 33%; p < 0.05; n = 27 cells, 12 patients). Subsequent co-application of isoproterenol (1 μM) caused a further increase in I_CaL in the AF group (by 256 ± 94%) that was greater than that in the SR group (22 ± 6%; p < 0.05). The APD at 50% repolarisation (APD₅₀) was prolonged by 14 ± 3 ms by 5-HT in the AF group (n = 37 cells, 14 patients). This was less than that in the SR group (27 ± 4 ms; p < 0.05; n = 58 cells, 24 patients). Arrhythmic activity in response to 5-HT was observed in 22% of cells in the SR group, but none was observed in the AF group (p < 0.05). Atrial fibrillation was associated with reduced effects of 5-HT, but not of isoproterenol, on I_CaL in human atrial cells. This reduced effect on I_CaL was associated with a reduced APD₅₀ and arrhythmic activity with 5-HT. Thus, the potentially arrhythmogenic influence of 5-HT may be suppressed in AF-remodelled human atrium.

If in non-pacemaker cells: Role and pharmacological implications

Pacemaker channels play a major role in the generation of sinoatrial rhythmic activity. However, their expression is not confined to specialized myocardial cells, such as primary and subsidiary pacemakers. Electrophysiological and molecular data collected over the last ten years have demonstrated that f-channels are also present in non-pacemaker cardiomyocytes, and become upregulated in cardiac hypertrophy and failure. Mislocalized expression and/or overexpression of f-channels are a consequence of electrophysiological remodeling and, from a clinical point of view, may represent an arrhythmogenic mechanism in heart failure, a condition associated with a high risk for sudden cardiac death. The potential arrhythmogenic role of I(f) and the availability of selective f-channel blockers cause I(f) to be a suitable therapeutic target in heart disease.

Signal transduction events involved in human epithelial cell invasion by Campylobacter jejuni 81-176

Analyses of invasive enteric bacteria (e.g. Shigella, Salmonella, Listeria, and Campylobacter) have shown that these pathogens initiate orchestrated signal transduction cascades in host cells leading to host cytoskeletal rearrangements that result in bacterial uptake. This current study was specifically aimed at examining the involvement of host membrane caveolae and certain protein kinases in epithelial cell invasion by C. jejuni strain 81-176, for which we have previously characterized the kinetics of entry and a unique microtubule-dependent mechanism of internalization. Utilizing in vitro cultured cell invasion assays with a gentamicin-kill step, disruption of membrane caveolae by pretreatment of INT407 cell monolayers with filipin III reduced C. jejuni 81-176 entry by >95%. Strain 81-176 uptake into INT407 cells was markedly inhibited by monolayer pretreatment with the protein kinase inhibitors genistein and staurosporine, or specific inhibitors of PI 3-kinase, wortmannin and LY294002. Western blot analysis using monoclonal anti-protein tyrosine phosphorylation antibody revealed distinctive changes during invasion in phosphorylation of at least nine proteins. Further inhibitor studies indicated that heterotrimeric G proteins, plus ERK and p38 MAP kinase activation are also involved in C. jejuni 81-176 invasion. These results suggest that C. jejuni 81-176 interact at host cell surface membrane caveolae with G protein-coupled receptors, which presumably trigger G-proteins and kinases to activate host proteins including PI 3-kinase and MAP kinases, that appear to be intimately involved in the events controlling 81-176 internalization.