Ca2+-dependent modulation of voltage-gated myocyte sodium channels

Daidzin Enhances the Anticonvulsion Effects of Carbamazepine and Diazepam, Possibly Through Voltage-Gated Sodium Channels and GABAA-Dependent Pathways

Epilepsy is a neurological disorder characterized by recurrent seizures, affecting approximately 50 million people globally. Daidzin (DZN), a naturally occurring isoflavone, has shown various pharmacological effects, including neuroprotective activities in animals. This study investigated the anticonvulsant effects of DZN with possible mechanisms of action using behavioral studies using experimental animals and in silico approaches. For this, a pentylenetetrazole (PTZ, 80 mg/kg, i.p.)-induced seizure model was applied in young broiler chicks. Treatment groups included DZN (5, 10, 20 mg/kg, p.o.), carbamazepine (CAR: 80 mg/kg, p.o.), and diazepam (DZP: 5 mg/kg, p.o.) alone and in combinations. After PTZ administration, convulsion onset, frequency, duration, and mortality rates were recorded. We also performed an in vitro study to check GABAergic activity of DZN and DZP. Additionally, molecular docking studies were performed against the GABAA receptor and voltage-gated sodium channel, along with pharmacokinetics and toxicity assessments of the test compound and the reference drugs. Results showed that DZN dose-dependently increased convulsion onset and significantly reduced convulsion frequency and duration compared to the control group (p < 0.05). The combination of DZN- 20 with CAR- 80 and DZP- 5 significantly enhanced convulsion onset and protection rates while reducing convulsion frequency and durations compared to their individual treatment groups. Both DZP and DZN also showed a concentration-dependent GABA activity inhibition capacity. DZN showed the highest binding affinities with GABAA receptor (- 7.8 kcal/mol) and voltage-gated sodium channel (- 9.1 kcal/mol) than the standard drugs. It also supported acceptable pharmacokinetic and toxicity profiles in in silico studies. Taken together, DZN exerted and enhanced the anticonvulsant effects of CAR and DZP, possibly through GABAA receptor and voltage-gated sodium channel interaction pathways.

Nernst-Planck-Gaussian finite element modelling of Ca2+ electrodiffusion in amphibian striated muscle transverse tubule-sarcoplasmic reticular triadic junctional domains

Introduction

Intracellular Ca2+ signalling regulates membrane permeabilities, enzyme activity, and gene transcription amongst other functions. Large transmembrane Ca2+ electrochemical gradients and low diffusibility between cell compartments potentially generate short-lived, localised, high-[Ca2+] microdomains. The highest concentration domains likely form between closely apposed membranes, as at amphibian skeletal muscle transverse tubule-sarcoplasmic reticular (T-SR, triad) junctions.

Materials and methods

Finite element computational analysis characterised the formation and steady state and kinetic properties of the Ca2+ microdomains using established empirical physiological and anatomical values. It progressively incorporated Fick diffusion and Nernst-Planck electrodiffusion gradients, K+, Cl-, and Donnan protein, and calmodulin (CaM)-mediated Ca2+ buffering. It solved for temporal-spatial patterns of free and buffered Ca2+, Gaussian charge differences, and membrane potential changes, following Ca2+ release into the T-SR junction.

Results

Computational runs using established low and high Ca2+ diffusibility (D Ca2+) limits both showed that voltages arising from intracytosolic total [Ca2+] gradients and the counterions little affected microdomain formation, although elevated D Ca2+ reduced attained [Ca2+] and facilitated its kinetics. Contrastingly, adopting known cytosolic CaM concentrations and CaM-Ca2+ affinities markedly increased steady-state free ([Ca2+]free) and total ([Ca2+]), albeit slowing microdomain formation, all to extents reduced by high D Ca2+. However, both low and high D Ca2+ yielded predictions of similar, physiologically effective, [Ca2+-CaM]. This Ca2+ trapping by the relatively immobile CaM particularly increased [Ca2+] at the junction centre. [Ca2+]free, [Ca2+-CaM], [Ca2+], and microdomain kinetics all depended on both CaM-Ca2+ affinity and D Ca2+. These changes accompanied only small Gaussian (∼6 mV) and surface charge (∼1 mV) effects on tubular transmembrane potential at either D Ca2+.

Conclusion

These physical predictions of T-SR Ca2+ microdomain formation and properties are compatible with the microdomain roles in Ca2+ and Ca2+-CaM-mediated signalling but limited the effects on tubular transmembrane potentials. CaM emerges as a potential major regulator of both the kinetics and the extent of microdomain formation. These possible cellular Ca2+ signalling roles are discussed in relation to possible feedback modulation processes sensitive to the μM domain but not nM bulk cytosolic, [Ca2+]free, and [Ca2+-CaM], including ryanodine receptor-mediated SR Ca2+ release; Na+, K+, and Cl- channel-mediated membrane excitation and stabilisation; and Na+/Ca2+ exchange transport.

Honeybee CaV4 has distinct permeation, inactivation, and pharmacology from homologous NaV channels

DSC1, a Drosophila channel with sequence similarity to the voltage-gated sodium channel (NaV), was identified over 20 years ago. This channel was suspected to function as a non-specific cation channel with the ability to facilitate the permeation of calcium ions (Ca2+). A honeybee channel homologous to DSC1 was recently cloned and shown to exhibit strict selectivity for Ca2+, while excluding sodium ions (Na+), thus defining a new family of Ca2+ channels, known as CaV4. In this study, we characterize CaV4, showing that it exhibits an unprecedented type of inactivation, which depends on both an IFM motif and on the permeating divalent cation, like NaV and CaV1 channels, respectively. CaV4 displays a specific pharmacology with an unusual response to the alkaloid veratrine. It also possesses an inactivation mechanism that uses the same structural domains as NaV but permeates Ca2+ ions instead. This distinctive feature may provide valuable insights into how voltage- and calcium-dependent modulation of voltage-gated Ca2+ and Na+ channels occur under conditions involving local changes in intracellular calcium concentrations. Our study underscores the unique profile of CaV4 and defines this channel as a novel class of voltage-gated Ca2+ channels.

Loose-patch clamp analysis applied to voltage-gated ionic currents following pharmacological ryanodine receptor modulation in murine hippocampal cornu ammonis-1 pyramidal neurons

Introduction

The loose-patch clamp technique was first developed and used in native amphibian skeletal muscle (SkM), offering useful features complementing conventional sharp micro-electrode, gap, or conventional patch voltage clamping. It demonstrated the feedback effects of pharmacological modification of ryanodine receptor (RyR)-mediated Ca2+ release on the Na+ channel (Nav1.4) currents, initiating excitation-contraction coupling in native murine SkM. The effects of the further RyR and Ca2+-ATPase (SERCA) antagonists, dantrolene and cyclopiazonic acid (CPA), additionally implicated background tubular-sarcoplasmic Ca2+ domains in these actions.

Materials and methods

We extend the loose-patch clamp approach to ion current measurements in murine hippocampal brain slice cornu ammonis-1 (CA1) pyramidal neurons. We explored the effects on Na+ currents of pharmacologically manipulating RyR and SERCA-mediated intracellular store Ca2+ release and reuptake. We adopted protocols previously applied to native skeletal muscle. These demonstrated Ca2+-mediated feedback effects on the Na+ channel function.

Results

Experiments applying depolarizing 15 ms duration loose-patch clamp steps to test voltages ranging from -40 to 120 mV positive to the resting membrane potential demonstrated that 0.5 mM caffeine decreased inward current amplitudes, agreeing with the previous SkM findings. It also decreased transient but not prolonged outward current amplitudes. However, 2 mM caffeine affected neither inward nor transient outward but increased prolonged outward currents, in contrast to its increasing inward currents in SkM. Furthermore, similarly and in contrast to previous SkM findings, both dantrolene (10 μM) and CPA (1 μM) pre-administration left both inward and outward currents unchanged. Nevertheless, dantrolene pretreatment still abrogated the effects of subsequent 0.5- and 2-mM caffeine challenges on both inward and outward currents. Finally, CPA abrogated the effects of 0.5 mM caffeine on both inward and outward currents, but with 2 mM caffeine, inward and transient outward currents were unchanged, but sustained outward currents increased.

Conclusion

We, thus, extend loose-patch clamping to establish pharmacological properties of murine CA1 pyramidal neurons and their similarities and contrasts with SkM. Here, evoked though not background Ca2+-store release influenced Nav and Kv excitation, consistent with smaller contributions of background store Ca2+ release to resting [Ca2+]. This potential non-canonical mechanism could modulate neuronal membrane excitability or cellular firing rates.

Cardiac arrhythmogenesis: roles of ion channels and their functional modification

Cardiac arrhythmias cause significant morbidity and mortality and pose a major public health problem. They arise from disruptions in the normally orderly propagation of cardiac electrophysiological activation and recovery through successive cardiomyocytes in the heart. They reflect abnormalities in automaticity, initiation, conduction, or recovery in cardiomyocyte excitation. The latter properties are dependent on surface membrane electrophysiological mechanisms underlying the cardiac action potential. Their disruption results from spatial or temporal instabilities and heterogeneities in the generation and propagation of cellular excitation. These arise from abnormal function in their underlying surface membrane, ion channels, and transporters, as well as the interactions between them. The latter, in turn, form common regulatory targets for the hierarchical network of diverse signaling mechanisms reviewed here. In addition to direct molecular-level pharmacological or physiological actions on these surface membrane biomolecules, accessory, adhesion, signal transduction, and cytoskeletal anchoring proteins modify both their properties and localization. At the cellular level of excitation-contraction coupling processes, Ca2+ homeostatic and phosphorylation processes affect channel activity and membrane excitability directly or through intermediate signaling. Systems-level autonomic cellular signaling exerts both acute channel and longer-term actions on channel expression. Further upstream intermediaries from metabolic changes modulate the channels both themselves and through modifying Ca2+ homeostasis. Finally, longer-term organ-level inflammatory and structural changes, such as fibrotic and hypertrophic remodeling, similarly can influence all these physiological processes with potential pro-arrhythmic consequences. These normal physiological processes may target either individual or groups of ionic channel species and alter with particular pathological conditions. They are also potentially alterable by direct pharmacological action, or effects on longer-term targets modifying protein or cofactor structure, expression, or localization. Their participating specific biomolecules, often clarified in experimental genetically modified models, thus constitute potential therapeutic targets. The insights clarified by the physiological and pharmacological framework outlined here provide a basis for a recent modernized drug classification. Together, they offer a translational framework for current drug understanding. This would facilitate future mechanistically directed therapeutic advances, for which a number of examples are considered here. The latter are potentially useful for treating cardiac, in particular arrhythmic, disease.

Cardiomyocyte electrophysiology and its modulation: current views and future prospects

Normal and abnormal cardiac rhythms are of key physiological and clinical interest. This introductory article begins from Sylvio Weidmann's key historic 1950s microelectrode measurements of cardiac electrophysiological activity and Singh & Vaughan Williams's classification of cardiotropic targets. It then proceeds to introduce the insights into cardiomyocyte function and its regulation that subsequently emerged and their therapeutic implications. We recapitulate the resulting view that surface membrane electrophysiological events underlying cardiac excitation and its initiation, conduction and recovery constitute the final common path for the cellular mechanisms that impinge upon this normal or abnormal cardiac electrophysiological activity. We then consider progress in the more recently characterized successive regulatory hierarchies involving Ca2+ homeostasis, excitation-contraction coupling and autonomic G-protein signalling and their often reciprocal interactions with the surface membrane events, and their circadian rhythms. Then follow accounts of longer-term upstream modulation processes involving altered channel expression, cardiomyocyte energetics and hypertrophic and fibrotic cardiac remodelling. Consideration of these developments introduces each of the articles in this Phil. Trans. B theme issue. The findings contained in these articles translate naturally into recent classifications of cardiac electrophysiological targets and drug actions, thereby encouraging future iterations of experimental cardiac electrophysiological discovery, and testing directed towards clinical management. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Emergent activity, heterogeneity, and robustness in a calcium feedback model of the sinoatrial node

The sinoatrial node (SAN) is the primary pacemaker of the heart. SAN activity emerges at an early point in life and maintains a steady rhythm for the lifetime of the organism. The ion channel composition and currents of SAN cells can be influenced by a variety of factors. Therefore, the emergent activity and long-term stability imply some form of dynamical feedback control of SAN activity. We adapt a recent feedback model-previously utilized to describe control of ion conductances in neurons-to a model of SAN cells and tissue. The model describes a minimal regulatory mechanism of ion channel conductances via feedback between intracellular calcium and an intrinsic target calcium level. By coupling a SAN cell to the calcium feedback model, we show that spontaneous electrical activity emerges from quiescence and is maintained at steady state. In a 2D SAN tissue model, spatial variability in intracellular calcium targets lead to significant, self-organized heterogeneous ion channel expression and calcium transients throughout the tissue. Furthermore, multiple pacemaking regions appear, which interact and lead to time-varying cycle length, demonstrating that variability in heart rate is an emergent property of the feedback model. Finally, we demonstrate that the SAN tissue is robust to the silencing of leading cells or ion channel knockouts. Thus, the calcium feedback model can reproduce and explain many fundamental emergent properties of activity in the SAN that have been observed experimentally based on a minimal description of intracellular calcium and ion channel regulatory networks.

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.