Studies on heart. XIX. Isolation of an atrial peptide that improves the rhythmicity of cultured myocardial cell clusters

Mechanisms of Connexin Regulating Peptides

Gap junctions (GJ) and connexins play integral roles in cellular physiology and have been found to be involved in multiple pathophysiological states from cancer to cardiovascular disease. Studies over the last 60 years have demonstrated the utility of altering GJ signaling pathways in experimental models, which has led to them being attractive targets for therapeutic intervention. A number of different mechanisms have been proposed to regulate GJ signaling, including channel blocking, enhancing channel open state, and disrupting protein-protein interactions. The primary mechanism for this has been through the design of numerous peptides as therapeutics, that are either currently in early development or are in various stages of clinical trials. Despite over 25 years of research into connexin targeting peptides, the overall mechanisms of action are still poorly understood. In this overview, we discuss published connexin targeting peptides, their reported mechanisms of action, and the potential for these molecules in the treatment of disease.

New Strategies for the Treatment of Atrial Fibrillation

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia in the clinical practice. It significantly contributes to the morbidity and mortality of the elderly population. Over the past 25-30 years intense effort in basic research has advanced the understanding of the relationship between the pathophysiology of AF and atrial remodelling. Nowadays it is clear that the various forms of atrial remodelling (electrical, contractile and structural) play crucial role in initiating and maintaining the persistent and permanent types of AF. Unlike in ventricular fibrillation, in AF rapid ectopic firing originating from pulmonary veins and re-entry mechanism may induce and maintain (due to atrial remodelling) this complex cardiac arrhythmia. The present review presents and discusses in detail the latest knowledge on the role of remodelling in AF. Special attention is paid to novel concepts and pharmacological targets presumably relevant to the drug treatment of atrial fibrillation.

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Cardiac to cancer: connecting connexins to clinical opportunity

Gap junctions and their connexin components are indispensable in mediating the cellular coordination required for tissue and organ homeostasis. The critical nature of their existence mandates a connection to disease while at the same time offering therapeutic potential. Therapeutic intervention may be offered through the pharmacological and molecular disruption of the pathways involved in connexin biosynthesis, gap junction assembly, stabilization, or degradation. Chemical inhibitors aimed at closing connexin channels, peptide mimetics corresponding to short connexin sequences, and gene therapy approaches have been incredibly useful molecular tools in deciphering the complexities associated with connexin biology. Recently, therapeutic potential in targeting connexins has evolved from basic research in cell-based models to clinical opportunity in the form of human trials. Clinical promise is particularly evident with regards to targeting connexin43 in the context of wound healing. The following review is aimed at highlighting novel advances where the pharmacological manipulation of connexin biology has proven beneficial in animals or humans.

Increasing gap junction coupling suppresses ibutilide-induced torsades de pointes

Drug-induced torsades de pointes (TdP) is common with class III antiarrhythmic drugs. Increased transmural dispersion of repolarization (TDR) contributes significantly to the development of TdP. Gap junctions play an important role in maintaining TDR in long QT syndrome. The present study examined the effect of a gap junction enhancer, antiarrhythmic peptide 10 (AAP10), on ibutilide-induced TdP. Coronary-perfused rabbit ventricular wedge preparations were used to evaluate the effect of AAP10 on ibutilide-induced arrhythmia. Transmural electrocardiograms and action potentials were recorded simultaneously. Early afterdepolarizations (EADs), R-on-T extrasystole, TdP and changes in Tpeak-end (Tp-e) and the Tp-e/QT ratio were observed. Changes in the levels of non-phosphorylated connexin 43 (Cx43) were measured by immunoblotting. Compared with those in the control group, the QT interval, Tp-e/QT and incidence rates of EAD and TdP increased with augmented dephosphorylation in the ventricular wedge preparations perfused with ibutilide under conditions of hypokalemia and hypomagnesemia. In the presence of AAP10, the incidence rates of EAD and TdP were reduced and the Tp-e/QT ratio decreased, with a parallel reduction in the level of non-phosphorylated Cx43. The results indicate that AAP10 suppressed ibutilide-induced TdP under conditions of hypokalemia and hypomagnesemia by decreasing TDR. AAP10 reduced TDR, possibly by preventing the dephosphorylation of Cx43 and thereby increasing myocardial cell gap junction coupling.

Atrial selectivity of antiarrhythmic drugs

New antiarrhythmic drugs for treatment of atrial fibrillation should ideally be atrial selective in order to avoid pro-arrhythmic effects in the ventricles. Currently recognized atrial selective targets include atrial Nav1.5 channels, Kv1.5 channels and constitutively active Kir3.1/3.4 channels, each of which confers atrial selectivity by different mechanisms. Na(+) channel blockers with potential- and frequency-dependent action preferentially suppress atrial fibrillation because of the high excitation rate and less negative atrial resting potential, which promote drug binding in atria. Kv1.5 channels are truly atrial selective because they do not conduct repolarizing current IKur in ventricles. Constitutively active IK,ACh is predominantly observed in remodelled atria from patients in permanent atrial fibrillation (AF). A lot of effort has been invested to detect compounds which will selectively block Kir3.1/Kir3.4 in their remodelled constitutively active form. Novel drugs which have been and are being developed aim at atrial-selective targets. Vernakalant and ranolazine which mainly block atrial Na(+) channels are clinically effective. Newly designed selective IKur blockers and IK,ACh blockers are effective in animal models; however, clinical benefit in converting AF into sinus rhythm (SR) or reducing AF burden remains to be demonstrated. In conclusion, atrial-selective antiarrhythmic agents have a lot of potential, but a long way to go.

Pharmacological modulation of connexin-formed channels in cardiac pathophysiology

Coordinated electrical activity in the heart is supported by gap junction channels located at the intercalated discs of cardiomyocytes. Impaired gap junctional communication between neighbouring cardiomyocytes contributes to the development of re-entry arrhythmias after myocardial ischaemia. Current antiarrhythmic therapy is hampered by a lack of efficiency and side effects, creating the need for a new generation of drugs. In this review, we focus on compounds that increase gap junctional communication, thereby increasing the conduction velocity and decreasing the risk of arrhythmias. Some of these compounds also inhibit connexin 43 (Cx43) hemichannels, thereby limiting adenosine triphosphate loss and volume overload following ischaemia/reperfusion, thus potentially increasing the survival of cardiomyocytes. The compounds discussed in this review are: (i) antiarrythmic peptide (AAP), AAP10, ZP123; (ii) GAP-134; (iii) RXP-E; and (vi) the Cx mimetic peptides Gap 26 and Gap 27. None of these compounds have effects on Na(+) , Ca(2+) and K(+) channels, and therefore have no proarrhythmic activity associated with currently available antiarrhythmic drugs. GAP-134, RXP-E, Gap 26 and Gap 27 are pharmalogical agents with a favorable clinical safety profile, as already confirmed in phase I clinical trials for GAP-134. These agents show an excellent promise for treatment of arrhythmias in patients with ischaemic cardiomyopathy.

Effect of ZP123, a gap junction modifier, on prolonged ventricular fibrillation in swine

OBJECTIVES

It was the aim of this study to investigate the effect of ZP123 on prolonged ventricular fibrillation (VF) in swine.

METHODS

VF was electrically induced in 20 pigs. The animals randomly received either ZP123 or saline control infusion before VF. After 8 min of untreated VF, cardiopulmonary resuscitation and biphasic defibrillation shocks were applied. VF mean frequency (VF(mf)) and mean amplitude (VF(ma)), hemodynamics, outcome of defibrillation and the rate of return of spontaneous circulation (ROSC) were analyzed.

RESULTS

Compared with the control group, VF(mf) was higher but VF(ma) lower during the 8 min of VF in the drug group (11.8 ± 2.1 vs. 10.4 ± 2.0 Hz and 0.24 ± 0.10 vs. 0.31 ± 0.16 mV, respectively; p < 0.05). Hemodynamic variables in the 2 groups were comparable (p > 0.05). The defibrillation threshold was lower and the rate of successful defibrillation was higher in the drug group compared with the control group (92.2 ± 26.4 vs. 133.3 ± 28.9 J and 90 vs. 30%, respectively; p < 0.05). The rate of ROSC was not different between the 2 groups (40 vs. 30%; p > 0.05).

CONCLUSION

In prolonged VF, ZP123 could decrease the defibrillation threshold and improve the rate of successful defibrillation. However, it could not improve the rate of ROSC - which may be due to its side effect of decreasing VF(ma).

Improving cardiac gap junction communication as a new antiarrhythmic mechanism: the action of antiarrhythmic peptides

Co-ordinated electrical activation of the heart is maintained by intercellular coupling of cardiomyocytes via gap junctional channels located in the intercalated disks. These channels consist of two hexameric hemichannels, docked to each other, provided by either of the adjacent cells. Thus, a complete gap junction channel is made from 12 protein subunits, the connexins. While 21 isoforms of connexins are presently known, cardiomyocytes typically are coupled by Cx43 (most abundant), Cx40 or Cx45. Some years ago, antiarrhythmic peptides were discovered and synthesised, which were shown to increase macroscopic gap junction conductance (electrical coupling) and enhance dye transfer (metabolic coupling). The lead substance of these peptides is AAP10 (H-Gly-Ala-Gly-Hyp-Pro-Tyr-CONH(2)), a peptide with a horseshoe-like spatial structure as became evident from two-dimensional nuclear magnetic resonance studies. A stable D: -amino-acid derivative of AAP10, rotigaptide, as well as a non-peptide analogue, gap-134, has been developed in recent years. Antiarrhythmic peptides act on Cx43 and Cx45 gap junctions but not on Cx40 channels. AAP10 has been shown to enhance intercellular communication in rat, rabbit and human cardiomyocytes. Antiarrhythmic peptides are effective against ventricular tachyarrhythmias, such as late ischaemic (type IB) ventricular fibrillation, CaCl(2) or aconitine-induced arrhythmia. Interestingly, the effect of antiarrhythmic peptides is higher in partially uncoupled cells and was shown to be related to maintained Cx43 phosphorylation, while arrhythmogenic conditions like ischaemia result in Cx43 dephosphorylation and intercellular decoupling. It is still a matter of debate whether these drugs also act against atrial fibrillation. The present review outlines the development of this group of peptides and derivatives, their mode of action and molecular mechanisms, and discusses their possible therapeutic potential.

Discovery of (2S,4R)-1-(2-aminoacetyl)-4-benzamidopyrrolidine-2-carboxylic acid hydrochloride (GAP-134)13, an orally active small molecule gap-junction modifier for the treatment of atrial fibrillation

Rotigaptide (3) is an antiarrhythmic peptide that improves cardiac conduction by modifying gap-junction communication. Small molecule gap-junction modifiers with improved physical properties were identified from a Zealand Pharma peptide library using pharmaceutical profiling, established SAR around 3, and a putative pharmacophore model for rotigaptide. Activity of the compounds was confirmed in a mouse cardiac conduction block model of arrhythmia. Dipeptide 9f (GAP-134) was identified as a potent, orally active gap-junction modifier for clinical development.

Discovery of a class of potent gap-junction modifiers as novel antiarrhythmic agents

In an effort to discover potent, orally bioavailable compounds for the treatment of atrial fibrillation (AF) and ventricular tachycardia (VT), we developed a class of gap-junction modifiers typified by GAP-134 (1, R(1)=OH, R(2)=NH(2)), a compound currently under clinical evaluation. Selected compounds with the desired in-vitro profile demonstrated positive in vivo results in the mouse CaCl(2) arrhythmia model upon oral administration.

Pharmacological modulation of gap junction function with the novel compound rotigaptide: a promising new principle for prevention of arrhythmias

Existing anti-arrhythmic therapy is hampered by lack of efficacy and unacceptable side effects. Thus, ventricular tachycardia and fibrillation remains the strongest predictor of in-hospital mortality in patients with myocardial infarction. In atrial fibrillation, rhythm control with conventional ion channel blockers provide no therapeutic benefit relative to rate control. Several lines of research indicate that impaired gap junctional cell-to-cell coupling between neighbouring cardiomyocytes is critical for the development of cardiac re-entry arrhythmias. Rotigaptide is the first drug that has been developed to prevent arrhythmias by re-establishing gap junctional intercellular communication. During conditions with acute cardiac ischaemia, rotigaptide effectively prevents induction of both ventricular and atrial tachyarrhythmia. Moreover, rotigaptide effectively prevents ischaemia reperfusion arrhythmias. At the cellular level, rotigaptide inhibits ischaemia-induced dephosphorylation of Ser297 and Ser368, which is considered important for the gating of connexin43 gap junction channels. No drug-related toxicity has been demonstrated at plasma concentrations 77,000 times above therapeutic concentrations. In rats and dogs, rotigaptide reduces infarct size following myocardial infarction. A series of phase I trials has been completed in which rotigaptide has been administered intravenously to ~200 healthy persons. No drug-related side effects have been demonstrated in healthy human beings. Clinical safety, tolerability and efficacy in patients with heart disease are being evaluated in ongoing clinical trials. Rotigaptide represents a pioneering pharmacological principle with a highly favourable preclinical and clinical safety profile, which makes this molecule a promising drug candidate for the prevention of cardiac arrhythmias.

Increasing Gap Junctional Coupling: A Tool for Dissecting the Role of Gap Junctions

Much of our current knowledge about the physiological and pathophysiological role of gap junctions is based on experiments where coupling has been reduced by either chemical agents or genetic modification. This has brought evidence that gap junctions are important in many physiological processes. In a number of cases, gap junctions have been implicated in the initiation and progress of disease, and experimental uncoupling has been used to investigate the exact role of coupling. The inverse approach, i.e., to increase coupling, has become possible in recent years and represents a new way of testing the role of gap junctions. The aim of this review is to summarize the current knowledge obtained with agents that selectively increase gap junctional intercellular coupling. Two approaches will be reviewed: increasing coupling by the use of antiarrhythmic peptide and its synthetic analogs and by interfering with the gating of gap junctional channels.

Rotigaptide (ZP123) reverts established atrial conduction velocity slowing

Rotigaptide (ZP123) increases gap junction intercellular communication (GJIC) and prevents stress-induced cardiac conduction velocity (CV) slowing. However, the effect of rotigaptide on established cardiac conduction slowing and the duration of effect on rotigaptide during washout is unknown. Metabolic stress (induced by superfusion with nonoxygenated glucose-free Tyrodes buffer) was associated with a 30% decrease in atrial CV in vehicle-treated rat atria. Rotigaptide treatment initiated after a period of 30 minutes of metabolic stress produced a rapid and significant increase in CV compared to vehicle-treated time controls. During washout of rotigaptide for 30 min (while subjected to metabolic stress), there was a minor decrease in atrial CV; however, this was not significantly different from atrial CV in a rotigaptide-treated time control group. Rotigaptide treatment rapidly normalizes established conduction slowing in atria subjected to metabolic stress. However, the cessation of effect was considerably slower than the onset of action.

Model-Dependent Effects of the Gap Junction Conduction–Enhancing Antiarrhythmic Peptide Rotigaptide (ZP123) on Experimental Atrial Fibrillation in Dogs

Background— Abnormal intercellular communication caused by connexin dysfunction may be involved in atrial fibrillation (AF). The present study assessed the effect of the gap junctional conduction–enhancing peptide rotigaptide on AF maintenance in substrates that result from congestive heart failure induced by 2-week ventricular tachypacing (240 bpm), atrial tachypacing (ATP; 400 bpm for 3 to 6 weeks), and isolated atrial myocardial ischemia.

Methods and Results— Electrophysiological study and epicardial mapping were performed before and after rotigaptide administration in dogs with ATP and congestive heart failure, as well as in similarly instrumented sham dogs that were not tachypaced. For atrial myocardial ischemia, dogs administered rotigaptide before myocardial ischemia were compared with no-drug myocardial ischemia controls. ATP significantly shortened the atrial effective refractory period ( P =0.003) and increased AF duration ( P =0.008), with AF lasting >3 hours in all 6-week ATP animals. Rotigaptide increased conduction velocity in ATP dogs slightly but significantly ( P =0.04) and did not affect the effective refractory period, AF duration, or atrial vulnerability. In dogs with congestive heart failure, rotigaptide also slightly increased conduction velocity ( P =0.046) but failed to prevent AF promotion. Rotigaptide had no statistically significant effects in sham dogs. Myocardial ischemia alone increased AF duration and impaired conduction (based on conduction velocity across the ischemic border and indices of conduction heterogeneity). Rotigaptide prevented myocardial ischemia–induced conduction slowing and AF duration increases.

Conclusions— Rotigaptide improves conduction in various AF models but suppresses AF only for the acute ischemia substrate. These results define the atrial antiarrhythmic profile of a mechanistically novel antiarrhythmic drug and suggest that gap junction dysfunction may be more important in ischemic AF than in ATP remodeling or congestive heart failure substrates.

Innovative approaches to anti-arrhythmic drug therapy

Normal cardiac function requires an appropriate and regular beating rate (cardiac rhythm). When the heart rhythm is too fast or too slow, cardiac function can be impaired, with derangements that vary from mild symptoms to life-threatening complications. Irregularities, particularly those involving excessively fast or slow rates, constitute cardiac 'arrhythmias'. In the past, drug treatment of cardiac arrhythmias has proven difficult, both because of inadequate effectiveness and a risk of serious complications. However, a variety of recent advances have opened up exciting possibilities for the development of novel and superior approaches to arrhythmia therapy. This article will review recent progress and future prospects for treating two particularly important cardiac arrhythmias: atrial fibrillation and ventricular fibrillation.

Rotigaptide (ZP123) improves atrial conduction slowing in chronic volume overload-induced dilated atria

Chronic atrial dilation is associated with atrial conduction velocity slowing and an increased risk of developing atrial tachyarrhythmias. Rotigaptide (ZP123) is a selective gap junction modifier that increases cardiac gap junctional intercellular communication. We hypothesised that rotigaptide treatment would increase atrial conduction velocity and reduce the inducibility to atrial tachyarrhythmias in a model of chronic volume overload induced chronic atrial dilatation characterized by atrial conduction velocity slowing. Chronic volume overload was created in Japanese white rabbits by arterio-venous shunt formation. Atrial conduction velocity and atrial tachyarrhythmias inducibility were examined in Langendorff-perfused chronic volume overload hearts (n=12) using high-resolution optical mapping before and after treatment with rotigaptide. Moreover, expression levels of atrial gap junction proteins (connexin40 and connexin43) were examined in chronic volume overload hearts (n=6) and compared to sham-operated controls (n=6). Rotigaptide treatment significantly increased atrial conduction velocity in chronic volume overload hearts, however, rotigaptide did not decrease susceptibility to the induction of atrial tachyarrhythmias. Protein expressions of Cx40 and Cx43 were decreased by 32% and 72% (P<0.01), respectively, in chromic volume overload atria compared to control. To conclude, rotigaptide increased atrial conduction velocity in a rabbit model of chromic volume overload induced atrial conduction velocity slowing. The demonstrated effect of rotigaptide on atrial conduction velocity did not prevent atrial tachyarrhythmias inducibility. Whether rotigaptide may possess antiarrhythmic efficacy in other models of atrial fibrillation remains to be determined.

Dysregulation of cell adhesion proteins and cardiac arrhythmogenesis

Proper mechanical and electrical coupling of cardiomyocytes is crucial for normal propagation of the electrical impulse throughout the working myocardium. Various proteins on the surface of cardiomyocytes are responsible for the integration of structural information and cell-cell communication. Increasing evidence from diseased myocardium and animal models indicates that alteration in electrical coupling via gap junctions is a critical determinant in the development of an arrhythmogenic substrate. What is less clear is how gap junctions are maintained and regulated in the working myocardium. In this review, we present data from human disease and animal models that support the idea that cell adhesion proteins regulate the stability of the gap junction protein, connexin.

Treatment With the Gap Junction Modifier Rotigaptide (ZP123) Reduces Infarct Size in Rats With Chronic Myocardial Infarction

Treatment with non-selective drugs (eg, long-chain alcohols, halothane) that reduce gap junction intercellular communication (GJIC) is associated with reduced infarct size after myocardial infarction (MI). Therefore, it has been suggested that gap junction intercellular communication stimulating compounds may increase infarct size. The antiarrhythmic peptide analogue rotigaptide (ZP123) increases cardiac gap junction intercellular communication and the purpose of the present study was to examine the effects of rotigaptide treatment on infarct size. Myocardial infarction was induced in male rats by ligation of the left anterior descending artery (LAD). Rats (n = 156) were treated with rotigaptide at three dose levels or vehicle from the onset of ischemia and for 3 weeks following LAD occlusion. Infarct size was determined using histomorphometry after 3 weeks treatment. Rotigaptide treatment producing steady state plasma levels of 0.8 +/- 0.1, 5.5 +/- 0.5, and 86 +/- 8 nmol/L had no effect on mortality, but reduced infarct size to 90 +/- 10% (P = 0.41), 67 +/- 7% (P = 0.005), and 82 +/- 7% (P = 0.13), respectively relative to vehicle-treated myocardial infarction rats (100 +/- 12%). In contrast to what was predicted, our data demonstrates that rotigaptide treatment was associated with a significant infarct size reduction. We conclude that whereas treatment with non-selective inhibitors of gap junction intercellular communication cause a reduction in infarct size, this information cannot be extrapolated to the effects of compounds that selectively increase gap junction intercellular communication.

Rotigaptide (ZP123) prevents spontaneous ventricular arrhythmias and reduces infarct size during myocardial ischemia/reperfusion injury in open-chest dogs

The antiarrhythmic and cardioprotective effect of increasing gap junction intercellular communication during ischemia/reperfusion injury has not been studied. The antiarrhythmic peptide rotigaptide (previously ZP123), which maintains gap junction intercellular communication, was tested in dogs subjected to a 60-min coronary artery occlusion and 4 h of reperfusion. Rotigaptide was administered i.v. 10 min before reperfusion as a bolus + i.v. infusion at doses of 1 ng/kg bolus + 10 ng/kg/h infusion (n = 6), 10 ng/kg bolus + 100 ng/kg/h infusion (n = 5), 100 ng/kg bolus + 1000 ng/kg/h infusion (n = 8), 1000 ng/kg bolus + 10 mug/kg/h infusion (n = 6), and vehicle control (n = 5). Premature ventricular complexes (PVCs) were quantified during reperfusion. A series of four or more consecutive PVCs was defined as ventricular tachycardia (VT). The total incidence of VT was reduced significantly with the two highest doses of rotigaptide (20.3 +/- 10.9 and 4.3 +/- 4.1 events; p < 0.05) compared with controls (48.7 +/- 6.0). Total PVCs were reduced significantly from 25.1 +/- 4.2% in control animals to 11.0 +/- 4.4 and 1.7 +/- 1.3% after the two highest doses of rotigaptide. Infarct size, expressed as a percentage of the left ventricle, was reduced significantly from 13.2 +/- 1.9 in controls to 7.1 +/- 1.0 (p < 0.05) at the highest dose of rotigaptide. Ultrastructural evaluation revealed no differences in myocardial injury in the infarct area, area at risk, border zone, or normal zone in vehicle and rotigaptide-treated animals. However, rotigaptide did increase the presence of gap junctions in the area at risk (p = 0.022, Fisher's exact test). Rotigaptide had no effect on heart rate, blood pressure, heart rate-corrected QT interval, or left ventricular end-diastolic pressure. In conclusion, these results demonstrate that rotigaptide is a potent antiarrhythmic compound with cardioprotective effects and desirable safety.

The antiarrhythmic peptide analog rotigaptide (ZP123) stimulates gap junction intercellular communication in human osteoblasts and prevents decrease in femoral trabecular bone strength in ovariectomized rats

Gap junctions play an important role in bone development and function, but the lack of pharmacological tools has hampered the gap junction research. The antiarrhythmic peptides stimulate gap junction communication between cardiomyocytes, but effects in noncardiac tissue are unknown. The purpose of this study was to examine whether antiarrhythmic peptides, which are small peptides increasing gap junctional conductivity, show specific binding to osteoblasts and investigate the effect of the stable analog rotigaptide (ZP123) on gap junctional intercellular communication in vitro and on bone mass and strength in vivo. Cell coupling and calcium signaling were assessed in vitro on human, primary, osteoblastic cells. In vivo effects of rotigaptide on bone strength and density were determined 4 wk after ovariectomy in rats treated with either vehicle, sc injection twice daily (300 nmol per kilogram body weight) or by continuous ip infusion (158 nmol per kilogram body weight per day). During metabolic stress, a high affinity-binding site (KD=0.1 nM) with low density (15 fmol/mg protein) for [125I]di-I-AAP10 was demonstrated. During physiological conditions, specific binding sites for [125I]AAP10 could not be shown. Studies of the effects of rotigaptide on propagation of intercellular calcium waves and cell-to-cell coupling demonstrated that 10 nM rotigaptide produced a small increase in intercellular communication during physiological conditions (+4.5+/-1.6% vs. vehicle; P<0.05). During conditions with metabolic stress, 10 nM rotigaptide produced an increase in coupling measured by both methods. Four weeks after ovariectomy, bone strength of the femoral head was reduced by 20% in vehicle-treated ovariectomized rats, which was completely prevented in both rotigaptide-treated groups. Rotigaptide also prevented decreases in bone mineral. We conclude that the stable analog rotigaptide increases gap junctional communication in osteoblasts in vitro and preferably during conditions with metabolic stress. Rotigaptide further prevents ovariectomy-induced bone loss in vivo. Thus, gap junction modulation may be a promising new target for osteoporosis therapy.

Pharmacology of gap junctions. New pharmacological targets for treatment of arrhythmia, seizure and cancer?

Intercellular communication in many organs is maintained via intercellular gap junction channels composed of connexins, a large protein family with a number of isoforms. This gap junction intercellular communication (GJIC) allows the propagation of action potentials (e.g., in brain, heart), and the transfer of small molecules which may regulate cell growth, differentiation and function. The latter has been shown to be involved in cancer growth: reduced GJIC often is associated with increased tumor growth or with de-differentiation processes. Disturbances of GJIC in the heart can cause arrhythmia, while in brain electrical activity during seizures seems to be propagated via gap junction channels. Many diseases or pathophysiological conditions seem to be associated with alterations of gap junction protein expression. Thus, depending on the target disease opening or closure of gap junctions may be of interest, or alteration of connexin expression. GJIC can be affected acutely by changing gap junction conductance or--more chronic--by altering connexin expression and membrane localisation. This review gives an overview on drugs affecting GJIC.

The Antiarrhythmic Peptide Analog ZP123 Prevents Atrial Conduction Slowing During Metabolic Stress

OBJECTIVE

As atrial conduction slowing is important in the pathogenesis of atrial reentry arrhythmias, a drug that increases atrial conduction or prevents atrial conduction slowing could serve to prevent atrial reentry arrhythmias. In this study, we investigated whether the novel stable antiarrhythmic peptide analog, ZP123, was able to prevent atrial conduction slowing.

METHODS

We examined the effect of ZP123 on metabolic stress-induced changes in conduction velocity (CV) and on dynamic CV restitution in isolated left atria from male Sprague-Dawley rats. We performed binding of ZP123 to a broad panel of 80 different cardiac and noncardiac ion channels and receptors and examined the effect of ZP123 on HERG channel conductance.

RESULTS

ZP123 dose-dependently prevented metabolic stress-induced atrial CV slowing at doses ranging from 1 nM to 10 microM. ZP123 did not affect CV during physiological conditions nor did it affect dynamic CV restitution. ZP123 had no effect on atrial contractility. ZP123 showed no or low affinity binding to all ion channels and receptors examined. ZP123 had no effects on HERG channel activity in concentrations that affected atrial conduction. The concentration of ZP123 giving maximal effect on atrial conduction (100 nM) inhibited the outward K(+)-current by 2.7 +/- 0.1%.

CONCLUSION

ZP123 has no effects on atrial conduction during physiological conditions, but it selectively prevents atrial conduction slowing during metabolic stress.

Pharmacological characterization of the new stable antiarrhythmic peptide analog Ac-D-Tyr-D-Pro-D-Hyp-Gly-D-Ala-Gly-NH2 (ZP123): in vivo and in vitro studies

Antiarrhythmic peptides (AAPs) are a group of compounds with antiarrhythmic properties; however, their use has been hampered by very low plasma stability. The aim of this study was to compare the in vitro and in vivo stability of our new stable AAP analog Ac-d-Tyr-d-Pro-d-Hyp-Gly-d-Ala-Gly-NH2 (ZP123) with the previously described AAP analog AAP10. Moreover, the effect of the two compounds was examined in a murine in vivo model of ouabain-induced second degree AV-block, and the effect on dispersion of action potential duration (APD dispersion) was studied during hypokalemic-ischemia in isolated perfused rabbit hearts. The in vitro t1/2 of ZP123 in rat and human plasma was about 1,700 times longer than t1/2 of AAP10. Due to rapid elimination, it was not possible to obtain an in vivo pharmacokinetic characterization of AAP10; however, calculations suggested that the clearance of ZP123 was at least 140 times slower than for AAP10. AAP10 and ZP123 produced a dose-dependent delay in onset of ouabain-induced AV-block in mice at doses of 10-11 to 10-7 mol/kg i.v. ZP123 and 10-11 to 10-6 mol/kg i.v. AAP10. Maximal efficacy of ZP123 was reached at a 10-fold lower dose (10-8 mol/kg i.v.) than with AAP10. In the isolated rabbit hearts, ZP123 and AAP10 had no effect on dispersion during control conditions. The increased APD dispersion during hypokalemic ischemia is considered a major arrhythmic substrate and only ZP123 prevented the increase in APD dispersion. In conclusion, ZP123 is a new potent AAP analog with improved stability.

Protein kinase Calpha mediates the effect of antiarrhythmic peptide on gap junction conductance

We investigated the effects of the antiarrhythmic peptide AAP10 (GAG-4Hyp-PY-CONH2, 50 nM) on pairs of adult guinea pig cardiomyocytes and on pairs of HeLa-cells transfected with rat connexin43 (Cx43). Using double cell voltage clamp technique in cardiomyocytes under control conditions, gap junction conductance (Gj) steadily decreased (by -0.3 to -0.4 nS/min). In contrast, 50 nM AAP10 significantly enhanced Gj (by +0.22 to +0.29 nS/min). This effect of AAP10 could be significantly antagonized by bisindolylmaleimide I (BIM), and by the protein kinase C (PKC) subtype-specific inhibitors HBDDE (PKCgamma and -alpha) and CGP 54345 (PKCalpha). In HeLa-Cx43 cells we found similar electrophysiological effects of AAP10. For further analysis, we incubated HeLa-Cx43 cells with [32P]orthophosphate (0.05 mCi/ml) for 4 h at 37 degrees C followed by addition of 50 nM AAP10 for 15 min. We found that incorporation of 32P into Cx43 was significantly enhanced in the presence of AAP10, which was completely inhibited in presence of BIM. PKC enzyme-linked immunosorbent assay (ELISA) revealed significant activation of PKC by AAP10 in HeLa-Cx43 cells, which could be inhibited by HBDDE and CGP 54345. Finally, a binding study using [14C]-AAP10 as radioligand was performed. We found a saturable binding of [14C]-AAP10 with a KD of 0.88 nM to cardiac membrane preparations. For assessment of the antiarrhythmic activity in anesthetized rats, we infused aconitine until the occurrence of ventricular fibrillation (VF). The aconitine dose required for initiation of VF was significantly enhanced in the presence of AAP10. In conclusion; AAP10 increases Gj in both adult cardiomyocytes and transfected HeLa-Cx43 cells. AAP10 leads to enhanced phosphorylation of Cx43 via activation of PKCalpha. A membrane receptor exists for antiarrhythmic peptides.

Anti-arrhythmic peptide N-3-(4-hydroxyphenyl)propionyl Pro-Hyp-Gly-Ala-Gly-OH reduces dispersion of action potential duration during ischemia/reperfusion in rabbit hearts

During ischemia, cardiac gap junctions close and neighboring cells uncouple. This leads to slow conduction, increased dispersion of APD90 (duration from action potential beginning to 90% of repolarization), nonuniform anisotropy, and unidirectional conduction block, all of which favor the induction of reentry arrhythmias. It has been suggested that anti-arrhythmic peptides increase gap junction conductance during states of reduced coupling. The aim of this study was to test the effect of the anti-arrhythmic peptide N-3-(4-hydroxyphenyl)propionyl Pro-Hyp-Gly-Ala-Gly-OH (HP-5) (10(-10) ) on dispersion of epicardial APD90 during both normokalemic and hypokalemic ischemia/reperfusion in isolated perfused rabbit hearts. HP-5 did not affect average APD90, heart rate, left ventricular contractility (LVP dP/dtmax), or mean coronary flow. HP-5 significantly reduced the epicardial APD dispersion during hypokalemic ischemia (HP-5 treated: 24.1 +/- 3.4 ms, untreated: 33.9 +/- 3.1 ms, p < 0.05 versus untreated) and during normokalemic reperfusion but not during normokalemic ischemia or control conditions. In addition, among untreated hearts subjected to hypokalemic ischemia/reperfusion, seven of 10 developed ventricular fibrillation, whereas only three of nine hearts perfused with HP-5 developed ventricular fibrillation. These results show that HP-5 is able to reduce APD90 dispersion during hypokalemic ischemia in rabbit hearts.

Peptides acting at gap junctions

Gap junction channels are low resistance pathways allowing an action potential to propagate from one cell to the neighboring. Moreover, small molecules (<1000 Da) may pass the channel providing a possibility for metabolic coupling, growth and differentiation control of a cell by its surrounding. Antiarrhythmic peptides can enhance the conductivity of the channels while other peptides, angiotensin or extracellular loop peptides, reduce intercellular communication. On the other hand, peptides like angiotensin II or endothelin-1 can increase expression of certain gap junction channel proteins and, thereby, may affect intercellular coupling chronically. Thus, intercellular communication can be controlled using peptide drugs.

Gap junction channels in the cardiovascular system: pharmacological and physiological modulation

Intercellular communication provides the basis for the intact functioning of tissue and for various organs and tissue types in an organism to work together. It is the crucial difference between isolated cells and intact tissue. Cells communicate in various ways with each other; these include the release of chemical transmitters, hormones and mediators as well as direct electrical and chemical intercellular communication via gap junction channels. The gap junction coupling is important for the organization of the tissue as an electrical syncytium and for accurate development. Pharmacological modulation of these channels could be important in the fields of arrhythmogenesis, vasomotion and cell differentiation. In this review, Stefan Dhein outlines the structure, synthesis and function of gap junction channels. Since their physiology and pharmacology are best investigated in the cardiovascular system, the second part of the article focuses on the role of gap junctions in the heart and vasculature, with special emphasis on the regulation of the channels by physiological stimuli such as ions, pH mediators and transjunctional voltage as well as their pharmacological modulation.

Increase in gap junction conductance by an antiarrhythmic peptide

Impaired cellular coupling is thought to be a very important factor for the genesis of cardiac arrhythmia. Cellular coupling is mediated by gap junctions. However, there are no therapeutic agents or experimental substances yet that increase cellular coupling. In addition, it has been shown that most antiarrhythmic drugs available now possess serious adverse effects. Thus, there is an urgent need for new antiarrhythmic agents. Previous studies using epicardial mapping in isolated rabbit hearts provided indirect evidence supporting the hypothesis that a newly synthesised antiarrhythmic peptide (Gly-Ala-Gly-4Hyp-Pro-Tyr-CONH2 = AAP10) might act via an increase in cellular, i.e., gap junctional coupling. The aim of the present study was to test this hypothesis. Measurement of the stimulus-response interval in papillary muscle showed a decrease of about 10% after application of 1 microM AAP10. These results are compatible with the hypothesis of AAP10 acting on gap junctions. In order to prove this hypothesis, gap junction conductance was measured directly by performing double-cell voltage-clamp experiments in isolated pairs of guinea-pig myocytes. During a 10 min control period gap junction conductance slowly decreased with a rate of -2.5 +/- 2.0 nS/min. After application of 10 nM AAP10 this behaviour reversed and gap junction conductance now increased with +1.0 +/- 0.7 nS/min. Upon washout of AAP10 gap junction conductance again decreased with a rate similar to that under control conditions. Another important finding was that we could not detect any other actions of AAP10 on cardiac myocytes. All parameters of the transmembrane action potential remained unchanged and, similarly, no changes in the IV relationship of single cardiac myocytes treated with 10 nM AAP10 could be observed. We conclude that AAP10 increases gap junction conductance, i.e., cellular coupling in the heart. This finding might be the first step towards the development of a new class of antiarrhythmic agents.

Antiarrhythmic peptide has no direct cardiac actions

The electrophysiologic, inotropic, and muscarinic effects of antiarrhythmic peptide (AAP) were examined in canine cardiac Purkinje fibers, ferret papillary muscle, and canine cardiac membranes, respectively. Aside from a prolongation of time to peak force in papillary muscle, no biologically significant effects of AAP could be determined in any preparation, suggesting that its antiarrhythmic effects are not mediated by direct membrane actions.