Pharmacological characterisation of sodium channels in sinoatrial node pacemaking in the rat heart

KM-408, a novel phenoxyalkyl derivative as a potential anticonvulsant and analgesic compound for the treatment of neuropathic pain

BACKGROUND

Epilepsy frequently coexists with neuropathic pain. Our approach is based on the search for active compounds with multitarget profiles beneficial in terms of potential side effects and on the implementation of screening for potential multidirectional central activity.

METHODS

Compounds were synthesized by means of chemical synthesis. After antiseizure and neurotoxicity screening in vivo, KM-408 and its enantiomers were chosen for analgesic activity evaluations. Further safety studies included acute toxicity in mice, the effect on normal electrocardiogram and on blood pressure in rats, whole body plethysmography in rats, and in vitro and biochemical assays. Pharmacokinetics has been studied in rats after iv and po administration. Metabolism has been studied in vivo in rat serum and urine. Radioligand binding studies were performed as part of the mechanism of action investigation.

RESULTS

Selected results for KM-408: K_i sigma = 7.2*10^-8; K_i 5-HT_1A = 8.0*10^-7; ED₅₀ MES (mice, ip) = 13.3 mg/kg; formalin test (I phase, mice, ip)-active at 30 mg/kg; SNL (rats, ip)-active at 6 mg/kg; STZ-induced pain (mice, ip)-active at 1 mg/kg (von Frey) and 10 mg/kg (hot plate); hot plate test (mice, ip)-active at 30 mg/kg; ED₅₀ capsaicin test (mice, ip) = 18.99 mg/kg; tail immersion test (mice)-active at 0.5%; corneal anesthesia (guinea pigs)-active at 0.125%; infiltration anesthesia (guinea pigs)-active at 0.125%.

CONCLUSIONS

Within the presented study a novel compound, R,S-2-((2-(2-chloro-6-methylphenoxy)ethyl)amino)butan-1-ol hydrochloride (KM-408) with dual antiseizure and analgesic activity has been developed for potential use in neuropathic pain treatment.

The Bradycardic Agent Ivabradine Acts as an Atypical Inhibitor of Voltage-Gated Sodium Channels

Background and purpose: Ivabradine is clinically administered to lower the heart rate, proposedly by inhibiting hyperpolarization-activated cyclic nucleotide-gated cation channels in the sinoatrial node. Recent evidence suggests that voltage-gated sodium channels (VGSC) are inhibited within the same concentration range. VGSCs are expressed within the sinoatrial node and throughout the conduction system of the heart. A block of these channels thus likely contributes to the established and newly raised clinical indications of ivabradine. We, therefore, investigated the pharmacological action of ivabradine on VGSCs in sufficient detail in order to gain a better understanding of the pro- and anti-arrhythmic effects associated with the administration of this drug. Experimental Approach: Ivabradine was tested on VGSCs in native cardiomyocytes isolated from mouse ventricles and the His-Purkinje system and on human Nav1.5 in a heterologous expression system. We investigated the mechanism of channel inhibition by determining its voltage-, frequency-, state-, and temperature-dependence, complemented by a molecular drug docking to the recent Nav1.5 cryoEM structure. Automated patch-clamp experiments were used to investigate ivabradine-mediated changes in Nav1.5 inactivation parameters and inhibition of different VGSC isoforms. Key results: Ivabradine inhibited VGSCs in a voltage- and frequency-dependent manner, but did not alter voltage-dependence of activation and fast inactivation, nor recovery from fast inactivation. Cardiac (Nav1.5), neuronal (Nav1.2), and skeletal muscle (Nav1.4) VGSC isoforms were inhibited by ivabradine within the same concentration range, as were sodium currents in native cardiomyocytes isolated from the ventricles and the His-Purkinje system. Molecular drug docking suggested an interaction of ivabradine with the classical local anesthetic binding site. Conclusion and Implications: Ivabradine acts as an atypical inhibitor of VGSCs. Inhibition of VGSCs likely contributes to the heart rate lowering effect of ivabradine, in particular at higher stimulation frequencies and depolarized membrane potentials, and to the observed slowing of intra-cardiac conduction. Inhibition of VGSCs in native cardiomyocytes and across channel isoforms may provide a potential basis for the anti-arrhythmic potential as observed upon administration of ivabradine.

Linked biaromatic compounds as cardioprotective agents

Cardiovascular diseases (CVDs) are widespread in the modern world, and their number is constantly growing. For a long time, CVDs have been the leading cause of morbidity and mortality worldwide. Drugs for the treatment of CVD have been developed almost since the beginning of the 20th century, and a large number of effective cardioprotective agents of various classes have been created. Nevertheless, the need for the design and development of new safe drugs for the treatment of CVD remains. Literature data indicate that a huge number of cardioprotective agents of various generations and mechanisms correspond to a single generalized pharmacophore model containing two aromatic nuclei linked by a linear linker. In this regard, we put forward a concept for the design of a new generation of cardioprotective agents with a multitarget mechanism of action within the indicated pharmacophore model. This review is devoted to a generalization of the currently known compounds with cardioprotective properties and corresponding to the pharmacophore model of biaromatic compounds linked by a linear linker. Particular attention is paid to the history of the creation of these drugs, approaches to their design, and analysis of the structure-action relationship within each class.

Age-dependent alterations of voltage-gated Na(+) channel isoforms in rat sinoatrial node

Multiple isoforms of voltage-gated Na(+) channels (NaChs) have been identified in sinoatrial node (SAN) and contribute to a rapid intrinsic heart rate. However, their roles in aging remain unclear. Here, we sought to clarify whether the age-related expression of NaChs contributes to the impaired SAN function during aging. Blockade of the tetrodotoxin (TTX)-sensitive Na(+) current with nanomolar concentrations of TTX prolonged the cycle length (CL) in both the rat intact heart and SAN. The effect of nanomolar concentrations of TTX on SAN pacemaking was lessened in adulthood compared with that in youth. Interestingly, the pacemaking became more sensitive to TTX and TTX-induced sinus arrhythmias occurred more frequently in the senescent group. The presences of NaCh α subunit isoforms Nav1.1, Nav1.6 as well as β subunit isoforms Navβ1 and Navβ3 in SAN were confirmed by immunohistochemistry. Western blot revealed a declination of Nav1.1, Nav1.6, Navβ1 and Navβ3 proteins during aging. Furthermore, laser captured SAN cells were used for further real-time quantitative RT-PCR analysis, which also confirmed the presences of Nav1.1, Nav1.6, Navβ1 and Navβ3 mRNA and their reduced levels in rat SAN during aging. These results indicated an age-dependent alterations in expression and relative function of NaCh in rat SAN.

The anti-addiction drug ibogaine and the heart: a delicate relation

The plant indole alkaloid ibogaine has shown promising anti-addictive properties in animal studies. Ibogaine is also anti-addictive in humans as the drug alleviates drug craving and impedes relapse of drug use. Although not licensed as therapeutic drug and despite safety concerns, ibogaine is currently used as an anti-addiction medication in alternative medicine in dozens of clinics worldwide. In recent years, alarming reports of life-threatening complications and sudden death cases, temporally associated with the administration of ibogaine, have been accumulating. These adverse reactions were hypothesised to be associated with ibogaine’s propensity to induce cardiac arrhythmias. The aim of this review is to recapitulate the current knowledge about ibogaine’s effects on the heart and the cardiovascular system, and to assess the cardiac risks associated with the use of this drug in anti- addiction therapy. The actions of 18-methoxycoronaridine (18-MC), a less toxic ibogaine congener with anti-addictive properties, are also considered.

In vivo reversal of general anesthesia by cucurbit[7]uril with zebrafish models

We demonstrate for the first time that cucurbit[7]uril effectively reversed general anesthesia induced by tricaine in zebrafish models.

Esmolol cardioplegia: the cellular mechanism of diastolic arrest

AIMS

Esmolol, an ultra-short-acting beta-blocker, acts as a cardioplegic agent at millimolar concentrations. We investigated the mechanism by which esmolol induces diastolic ventricular arrest.

METHODS AND RESULTS

In unpaced Langendorff-perfused rat hearts, esmolol (0.03-3 mmol/L) had a profound negative inotropic effect resulting in diastolic arrest at 1 mmol/L and above. This inhibition of contraction was maintained during ventricular pacing. At 3 mmol/L, esmolol also abolished action potential conduction. To determine the cellular mechanism for the negative inotropism, we measured contraction (sarcomere shortening) and the calcium transient (fura-2 fluorescence ratio; Ca(tr)) in electrically-stimulated rat ventricular myocytes at 23 and 34 degrees C. The decrease in contraction (by 72% at 23 degrees C, from 0.16 +/- 0.01 to 0.04 +/- 0.01 microm, P < 0.001) was similar to that of isolated hearts and was caused by a large decrease in Ca(tr) (from 0.13 +/- 0.02 to 0.07 +/- 0.02, P < 0.001). There was no additional effect on myofilament Ca(2+) sensitivity. Esmolol's effects on contraction and Ca(tr) were not shared or altered by the beta-blocker, atenolol (1 mmol/L). Sarcoplasmic reticulum inhibition with thapsigargin did not alter the inhibitory effects of esmolol. Whole-cell voltage-clamp experiments revealed that esmolol inhibited the L-type calcium current (I(Ca,L)) and the fast sodium current (I(Na)), with IC(50) values of 0.45 +/- 0.05 and 0.17 +/- 0.025 mmol/L, respectively.

CONCLUSION

Esmolol at millimolar concentrations causes diastolic ventricular arrest by two mechanisms: at 1 mmol/L (and below), the pronounced negative inotropic effect is due largely to inhibition of L-type Ca(2+) channels; additionally, higher concentrations prevent action potential conduction, probably due to the inhibition of fast Na(+) channels.

The cardiac persistent sodium current: an appealing therapeutic target?

The sodium current in the heart is not a single current with a mono-exponential decay but rather a mixture of currents with different kinetics. It is not clear whether these arise from distinct populations of channels, or from modulation of a single population. A very slowly inactivating component, [(INa(P))] I(Na(P)) is usually about 1% of the size of the peak transient current [I(Na(T))], but is enhanced by hypoxia. It contributes to Na(+) loading and cellular damage in ischaemia and re-perfusion, and perhaps to ischaemic arrhythmias. Class I antiarrhythmic agents such as flecainide, lidocaine and mexiletine generally block I(NA(P)) more potently than block of I(Na(T)) and have been used clinically to treat LQT3 syndrome, which arises because mutations in SCN5A produce defective inactivation of the cardiac sodium channel. The same approach may be useful in some pathological situations, such as ischaemic arrhythmias or diastolic dysfunction, and newer agents are being developed with this goal. For example, ranolazine blocks I(Na(P)) about 10 times more potently than I(Na(T)) and has shown promise in the treatment of angina. Alternatively, the combination of I(Na(P)) block with K(+) channel block may provide protection from the induction of Torsades de Pointe when these agents are used to treat atrial arrhythmias (eg Vernakalant). In all of these scenarios, an understanding of the role of I(Na(P)) in cardiac pathophysiology, the mechanisms by which it may affect cardiac electrophysiology and the potential side effects of blocking I(Na(P)) in the heart and elsewhere will become increasingly important.