A novel benzothiazine Ca2+ channel antagonist, semotiadil, inhibits cardiac L-type Ca2+ currents

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

AIBN-Promoted Synthesis of Bibenzo[ b][1,4]thiazines by the Condensation of 2,2'-Dithiodianiline with Methyl Aryl Ketones

A series of bibenzo[ b][1,4]thiazines with various functional groups has been synthesized by a free-radical condensation reaction. Bibenzo[ b][1,4]thiazines were obtained in moderate to good yield (up to 85%) through a one-step reaction of readily available 2,2'-dithiodianiline and methyl aryl ketones with AIBN as radical initiator in HOAc. Bibenzo[ b][1,4]thiazines exhibit diversiform solid-state packing.

Pharmacological modulation of mitochondrial calcium homeostasis

Mitochondria are pivotal organelles in calcium (Ca²⁺ ) handling and signalling, constituting intracellular checkpoints for numerous processes that are vital for cell life. Alterations in mitochondrial Ca²⁺ homeostasis have been linked to a variety of pathological conditions and are critical in the aetiology of several human diseases. Efforts have been taken to harness mitochondrial Ca²⁺ transport mechanisms for therapeutic intervention, but pharmacological compounds that direct and selectively modulate mitochondrial Ca²⁺ homeostasis are currently lacking. New avenues have, however, emerged with the breakthrough discoveries on the genetic identification of the main players involved in mitochondrial Ca²⁺ influx and efflux pathways and with recent hints towards a deep understanding of the function of these molecular systems. Here, we review the current advances in the understanding of the mechanisms and regulation of mitochondrial Ca²⁺ homeostasis and its contribution to physiology and human disease. We also introduce and comment on the recent progress towards a systems-level pharmacological targeting of mitochondrial Ca²⁺ homeostasis.

Standing of giants shoulders the story of the mitochondrial Na(+)Ca(2+) exchanger

It is now the 40th anniversary of the Journal of Molecular and Cellular Cardiology paper by Ernesto Carafoli and colleagues. This seminal study described for the first time mitochondrial Ca(2+) extrusion and its coupling to Na(+). This short review will describe the profound impact that this work had on mitochondrial signaling and the cross talk between the mitochondria, the ER, and the plasma membrane. It will further tell how the functional identification and in particular its unique cation selectivity to both Li(+) and Na(+) eventually contributed to the identification of the mitochondrial Na(+)/Ca(2+) exchanger gene NCLX many years later. The last part will describe how molecular tools derived from NCLX identification are used to study the novel physiological aspects of Ca(2+) signaling.

The Inhibitory Effects of Ca2+ Channel Blocker Nifedipine on Rat Kv2.1 Potassium Channels

It is well documented that nifedipine, a commonly used dihydropyridine Ca²⁺ channel blocker, has also significant interactions with voltage-gated K⁺ (Kv) channels. But to date, little is known whether nifedipine exerted an action on Kv2.1 channels, a member of the Shab subfamily with slow inactivation. In the present study, we explored the effects of nifedipine on rat Kv2.1 channels expressed in HEK293 cells. Data from whole-cell recording showed that nifedipine substantially reduced Kv2.1 currents with the IC₅₀ value of 37.5 ± 5.7 μM and delayed the time course of activation without effects on the activation curve. Moreover, this drug also significantly shortened the duration of inactivation and deactivation of Kv2.1 currents in a voltage-dependent manner. Interestingly, the half-maximum inactivation potential (V_1/2) of Kv2.1 currents was -11.4 ± 0.9 mV in control and became -38.5 ± 0.4 mV after application of 50 μM nifedipine. The large hyperpolarizing shift (27 mV) of the inactivation curve has not been reported previously and may result in more inactivation for outward delayed rectifier K⁺ currents mediated by Kv2.1 channels at repolarization phases. The Y380R mutant significantly increased the binding affinity of nifedipine to Kv2.1 channels, suggesting an interaction of nifedipine with the outer mouth region of this channel. The data present here will be helpful to understand the diverse effects exerted by nifedipine on various Kv channels.

Isocyanide-based multicomponent reactions towards cyclic constrained peptidomimetics

In the recent past, the design and synthesis of peptide mimics (peptidomimetics) has received much attention. This because they have shown in many cases enhanced pharmacological properties over their natural peptide analogues. In particular, the incorporation of cyclic constructs into peptides is of high interest as they reduce the flexibility of the peptide enhancing often affinity for a certain receptor. Moreover, these cyclic mimics force the molecule into a well-defined secondary structure. Constraint structural and conformational features are often found in biological active peptides. For the synthesis of cyclic constrained peptidomimetics usually a sequence of multiple reactions has been applied, which makes it difficult to easily introduce structural diversity necessary for fine tuning the biological activity. A promising approach to tackle this problem is the use of multicomponent reactions (MCRs), because they can introduce both structural diversity and molecular complexity in only one step. Among the MCRs, the isocyanide-based multicomponent reactions (IMCRs) are most relevant for the synthesis of peptidomimetics because they provide peptide-like products. However, these IMCRs usually give linear products and in order to obtain cyclic constrained peptidomimetics, the acyclic products have to be cyclized via additional cyclization strategies. This is possible via incorporation of bifunctional substrates into the initial IMCR. Examples of such bifunctional groups are N-protected amino acids, convertible isocyanides or MCR-components that bear an additional alkene, alkyne or azide moiety and can be cyclized via either a deprotection-cyclization strategy, a ring-closing metathesis, a 1,3-dipolar cycloaddition or even via a sequence of multiple multicomponent reactions. The sequential IMCR-cyclization reactions can afford small cyclic peptide mimics (ranging from four- to seven-membered rings), medium-sized cyclic constructs or peptidic macrocycles (>12 membered rings). This review describes the developments since 2002 of IMCRs-cyclization strategies towards a wide variety of small cyclic mimics, medium sized cyclic constructs and macrocyclic peptidomimetics.

The mitochondrial Na(+)/Ca(2+) exchanger

Powered by the steep mitochondrial membrane potential Ca(2+) permeates into the mitochondria via the Ca(2+) uniporter and is then extruded by a mitochondrial Na(+)/Ca(2+) exchanger. This mitochondrial Ca(2+) shuttling regulates the rate of ATP production and participates in cellular Ca(2+) signaling. Despite the fact that the exchanger was functionally identified 40 years ago its molecular identity remained a mystery. Early studies on isolated mitochondria and intact cells characterized the functional properties of a mitochondrial Na(+)/Ca(2+) exchanger, and showed that it possess unique functional fingerprints such as Li(+)/Ca(2+) exchange and that it is displaying selective sensitivity to inhibitors. Purification of mitochondria proteins combined with functional reconstitution led to the isolation of a polypeptide candidate of the exchanger but failed to molecularly identify it. A turning point in the search for the exchanger molecule came with the recent cloning of the last member of the Na(+)/Ca(2+) exchanger superfamily termed NCLX (Na(+)/Ca(2+)/Li(+) exchanger). NCLX is localized in the inner mitochondria membrane and its expression is linked to mitochondria Na(+)/Ca(2+) exchange matching the functional fingerprints of the putative mitochondrial Na(+)/Ca(2+) exchanger. Thus NCLX emerges as the long sought mitochondria Na(+)/Ca(2+) exchanger and provide a critical molecular handle to study mitochondrial Ca(2+) signaling and transport. Here we summarize some of the main topics related to the molecular properties of the Na(+)/Ca(2+) exchanger, beginning with the early days of its functional identification, its kinetic properties and regulation, and culminating in its molecular identification.

A step towards characterisation of electrophysiological profile of torsadogenic drugs

INTRODUCTION

In a previous study, two electrophysiological patterns for torsadogenic drugs were characterised in the model of isolated canine Purkinje fibres from their respective effects on action potential. This study was designed to elucidate the possible mechanisms underlying these two electrophysiological profiles.

METHODS

Effects of representative torsadogenic agents and non torsadogenic drugs on I(Kr), I(Ks), I(K1), I(Na) and I(CaL) were studied in transfected HEK 293 cells using the path-clamp method as well as in conscious beagle dogs and cynomolgus monkeys by telemetry.

RESULTS

Patch-clamp studies confirmed that torsadogenic molecules could be discriminated into at least two subgroups. The first subgroup can be defined as apparently pure I(Kr) blockers. The second subgroup can be defined as I(Kr) blockers with ancillary properties on sodium and/or calcium channels which counterbalance the I(Kr) prolongation component. This discrimination is transposable to the telemetered cynomolgus monkey model in terms of QT prolongation but not to the telemetered beagle dog model. This latter inter-species difference could be related to the sympathetic/parasympathetic balance and could involve reserve repolarisation dependent mechanisms.

DISCUSSION

The confirmation that torsadogenic drugs might have at least two different electrophysiological profiles should be taken into consideration in preclinical safety pharmacology studies because it increases the value of the cynomolgus monkey model in two particular situations: firstly when an NCE causes sympathetic activation and secondly, when an NCE exhibits a pure I(Kr) blocker pattern independently of its potency to block HERG channels.

Recent advances in electrophysiology-based screening technology and the impact upon ion channel discovery research

Ion channels are recognised as an increasingly tractable class of targets for the discovery and development of new drugs, with a diverse range of ion channel proteins now implicated across a wide variety of disease states and potential therapeutic applications. Whilst the field now ranks as one of the most dynamic fields for drug discovery research, it has historically been regarded by many researchers as a class of proteins associated with numerous technical challenges. Recent advances in our understanding of molecular biology and the increasing acceptance of electrophysiology-based screening methodology mean that ion channels are rapidly progressing towards universal acceptance as worthy and approachable targets for drug discovery. This chapter will outline the commercially available electrophysiology-based screening technologies and give an overview of the range of options for progressing pharmaceutical research and development against this important target class.

Effects of diltiazem and nifedipine on transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

1. It is unknown whether the widely used L-type Ca(2+) channel antagonists diltiazem and nifedipine would block the repolarization K(+) currents, transient outward current (I(to1)) and ultra-rapid delayed rectifier K(+) current (I(Kur)), in human atrium. The present study was to determine the effects of diltiazem and nifedipine on I(to1) and I(Kur) in human atrial myocytes with whole-cell patch-clamp technique. 2. It was found that diltiazem substantially inhibited I(to1) in a concentration-dependent manner, with an IC(50) of 29.2+/-2.4 microM, and nifedipine showed a similar effect (IC(50)=26.8+/-2.1 muM). The two drugs had no effect on voltage-dependent kinetics of the current; however, they accelerated I(to1) inactivation significantly, suggesting an open channel block. 3. In addition, diltiazem and nifedipine suppressed I(Kur) in a concentration-dependent manner (at +50 mV, IC(50)=11.2+/-0.9 and 8.2+/-0.8 microM, respectively). These results indicate that the Ca(2+) channel blockers diltiazem and nifedipine substantially inhibit I(to1) and I(Kur) in human atrial myocytes.

Particular sensitivity to calcium channel blockers of the fast inward voltage-dependent sodium current involved in the invasive properties of a metastastic breast cancer cell line

1. A voltage-dependent sodium current has been described in the highly invasive breast cancer cell line MDA-MB-231. Its activity is associated with the invasive properties of the cells. The aim of our study was to test whether this current (I(Na)) is sensitive to three representative calcium channel blockers: verapamil, diltiazem and nifedipine. I(Na) was studied in patch-clamp conditions. 2. I(Na) was sensitive to verapamil (IC(50)=37.6+/-2.5 microM) and diltiazem (53.2+/-3.6 microM), while it was weakly sensitive to nifedipine. 3. The tetrodotoxin (TTX) concentration, which fully blocks I(Na) (30 microM), did not affect cell proliferation. Diltiazem and verapamil, at concentrations that do not fully block I(Na), strongly reduced cell proliferation, suggesting, regarding proliferation, that these molecules act on targets distinct from sodium channels. These targets are probably not other ionic channels, since the current measured at the end of a 500 ms long pulse in the voltage range between -60 and +40 mV was unaffected by verapamil and diltiazem. 4. We conclude that the sodium channel expressed in MDA-MB-231 cells is sensitive to several calcium channel blockers. The present study also underlines the danger of concluding to the possible involvement of membrane channel proteins in any phenomenon on the sole basis of pharmacology, and without an electrophysiological confirmation.

Combined sodium and calcium channel blockade in prevention of lethal arrhythmias

Anti-arrhythmic compounds with multiple actions reduce arrhythmic death risk in post-myocardial infarction (MI) patients. Sudden death prevention, however, may rely more on implantable defibrillators than anti-arrhythmic drugs due to ineffective pharmacologic intervention. Widespread use of implantable defibrillators should not obscure the need for development of new anti-arrhythmic drugs. This study tested the hypothesis that combined blockade of I(Na) and I(Ca(L)) prevents ischemia-dependent ventricular fibrillation (VF) in conscious dogs after MI. I(Na) and I(Ca(L)) blockade was accomplished with levosemotiadil in 11 dogs known to be at high risk for VF during 2 min of coronary occlusion during submaximal treadmill exercise 30 days after MI. Negative chronotropic effect of levosemotiadil was examined using the heart rate response to isoproterenol and comparing it with response to propranolol. Levosemotiadil prevented VF in 64% (7 of 11) of the high-risk animals. Heart rate responses to myocardial ischemia and to graded doses of isoproterenol were blunted by the high dose of levosemotiadil. Propranolol prevented VF in 73% (8 of 11) of the dogs. Levosemotiadil had approximately one half the beta-blocking activity of propranolol. The combination of I(Na) and I(Ca(L)) channel blockade coupled with partial beta-adrenergic blockade was equally effective in preventing VF as propranolol.

Selectivity of different calcium antagonists on T- and L-type calcium currents in guinea-pig ventricular myocytes

Both L- and T-type calcium channels are present in the heart. In cardiac myocytes L-type calcium channels are blocked by the classical calcium channel blockers, while T-type calcium channels are thought to be insensitive to these drugs and to be selectively blocked by mibefradil. We aimed to compare the T/L calcium channel blocking selectivity of several calcium channel blockers by evaluating their effects on both components evoked in the same cell from a holding potential corresponding to the normal physiological value (-90mV). Currents were recorded in single patch-clamped guinea-pig ventricular myocytes, superfused with a Na(+)- and K(+)-free solution to abolish overlapping currents. Two dihydropyridines (amlodipine and lacidipine), verapamil diltiazem and mibefradil were tested; for each compound concentrations equieffective on L-type Ca(2+) current were used. All calcium channel blockers, at concentrations blocking less than 30% of L-type Ca(2+) current, inhibited a significant amount of T-type Ca(2+) current, varying from 0.8% (diltiazem) to 28% (mibefradil). We calculated for each compound the T/L ratio. As expected, mibefradil showed the highest T selectivity; lacidipine and diltiazem resulted to be L selective. Verapamil and amlodipine were not selective. Thus, the calcium channel blockers can be differentiated on the basis of their T/L selectivity.

T-type and L-type calcium channel blockers exert opposite effects on renin secretion and renin gene expression in conscious rats

1. This study aimed to investigate and to compare the effects of pharmacological T-type calcium channel and of L-type calcium channel blockade on the renin system. To this end, male healthy Sprague-Dawley rats were treated with the T-channel blocker mibefradil or with the L-channel blocker amlodipine at doses of 5 mg kg(-1), 15 mg kg(-1) and 45 mg kg(-1) per day for four days and their effects on plasma renin activity (PRA) and kidney renin mRNA levels were determined. 2. Whilst amlodipine lowered basal systolic blood pressure at 5 mg kg(-1), mibefradil had no effect on basal blood pressure in the whole dose range examined. Amlodipine dose-dependently induced up to 7 fold elevation of PRA and renin mRNA levels. Mibefradil significantly lowered PRA and renin mRNA levels at 5 mg kg(-1) and moderately increased both parameters at a dose of 45 mg kg(-1), when PRA and renin mRNA levels were increased by 100% and 30%, respectively. In primary cultures of renal juxtaglomerular cells neither amlodipine nor mibefradil (0.1-10 microM) changed renin secretion. 3. In rats unilateral renal artery clips (2K-1C) mibefradil and amlodipine at doses of 15 mg kg(-1) day(-1) were equally effective in lowering blood pressure. In contrast mibefradil (5 mg kg(-1) and 15 mg kg(-1) day(-1)) significantly attenuated the rise of PRA and renin mRNA levels, whilst amlodipine (15 mg kg(-1)) additionally elevated the rise of PRA and renin mRNA levels in response to renal artery clipping. 4. These findings suggest that T-type calcium channel blockers can inhibit renin secretion and renin gene expression in vivo, whilst L-type calcium channel blockers act as stimulators of the renin system. Since the inhibitory effect of T-type antagonists is apparent in vivo but not in vitro, one may infer that the effect on the renin system is indirect rather than directly mediated at the level of renal juxtaglomerular cells.