Physiological modulation of voltage-dependent inactivation in the cardiac muscle L-type calcium channel: A modelling study

Applications of model simulation in pharmacological fields and the problems of theoretical reliability

The use of mathematical models has become increasingly prevalent in pharmacological fields, particularly in drug development processes. These models are instrumental in tasks such as designing clinical trials and assessing factors like efficacy, toxicity, and clinical practice. Various types of models have been developed and documented. Nevertheless, emphasizing the reliability of parameter values is crucial, as they play a pivotal role in shaping the behavior of the system. In some instances, parameter values reported previously are treated as fixed values, which can lead to convergence towards values that deviate substantially from those found in actual biological systems. This is especially true when parameter values are determined through fitting to limited observations. To mitigate this risk, the reuse of parameter values from previous reports should be approached with a critical evaluation of their validity. Currently, there is a proposal for a simultaneous search for plausible values for all parameters using comprehensive search algorithms in both pharmacokinetic and pharmacodynamic or systems pharmacological models. Implementing these methodologies can help address issues related to parameter determination. Furthermore, integrating these approaches with methods developed in the field of machine-learning field has the potential to enhance the reliability of parameter values and the resulting model outputs.

Puerarin inhibits NHE1 activity by interfering with the p38 pathway and attenuates mitochondrial damage induced by myocardial calcium overload in heart failure rats

Previous studies have shown that puerarin plays a key role in protecting humans and animals from cardiovascular diseases. The exact mechanism of the therapeutic effect of puerarin on various cardiovascular diseases (protective effect on cardiomyocytes) is still unclear. In the present study, we identify the role of puerarin in an animal model of experimental heart failure (HF) and explore its underlying mechanisms. The HF rat model is induced by intraperitoneal injection of adriamycin (ADR), and puerarin is administered intragastrically at low, medium, and high concentrations. We demonstrate that puerarin significantly improves myocardial fibrosis and inflammatory infiltration and, as a result, improves cardiac function in ADR-induced HF rats. Mechanistically, we find for the first time that puerarin inhibits overactivated Na +/H + exchange isoform 1 (NHE1) in HF, which may improve HF by decreasing Na + and Ca 2+ ion concentrations and attenuating mitochondrial damage caused by calcium overload; on the other hand, puerarin inhibits the activation of the p38 pathway in HF, reduces the expressions of TGF-β and proinflammatory cytokines, and suppresses myocardial fibrosis. In conclusion, our results suggest that Puerarin is an effective drug against HF and may play a protective role in the myocardium by inhibiting the activation of p38 and its downstream NHE1.

Effects of Puerarin on the Prevention and Treatment of Cardiovascular Diseases

Puerarin, an isoflavone glycoside derived from Pueraria lobata (Willd.) Ohwi, has been identified as a pharmacologically active component with diverse benefits. A large number of experimental and clinical studies have demonstrated that puerarin is widely used in the treatment of a variety of diseases. Among them, cardiovascular diseases (CVDs) are the leading cause of death in the world, and therefore remain one of the most prominent global public health concerns. In this review, we systematically analyze the preclinical investigations of puerarin in CVDs, such as atherosclerosis, cardiac hypertrophy, heart failure, diabetic cardiovascular complications, myocardial infarction, stroke and hypertension. In addition, the potential molecular targets of puerarin are also discussed. Furthermore, we summarize the clinical trails of puerarin in the treatment of CVDs. Finally, the therapeutic effects of puerarin derivatives and its drug delivery systems are overviewed.

Oxygenation state of hemoglobin defines dynamics of water molecules in its vicinity

This study focuses on assessing the possible impact of changes in hemoglobin (Hb) oxygenation on the state of water in its hydration shell as it contributes to red blood cell deformability. Microwave Dielectric Spectroscopy (MDS) was used to monitor the changes in interactions between water molecules and Hb, the number of water molecules in the protein hydration shell, and the dynamics of pre-protein water in response to the transition of Hb from the tense (T) to the relaxed (R) state, and vice versa. Measurements were performed for Hb solutions of different concentrations (5 g/dl-30 g/dl) in phosphate-buffered saline buffer. Cole-Cole parameters of the main water relaxation peak in terms of interactions of water molecules (dipole-dipole/ionic dipole) during the oxygenation-deoxygenation cycle were used to analyze the obtained data. The water mobility-represented by α as a function of ln τ-differed dramatically between the R (oxygenated) state and the T (deoxygenated) state of Hb at physiologically relevant concentrations (30 g/dl-35 g/dl or 4.5 mM-5.5 mM). At these concentrations, oxygenated hemoglobin was characterized by substantially lower mobility of water in the hydration shell, measured as an increase in relaxation time, compared to deoxyhemoglobin. This change indicated an increase in red blood cell cytosolic viscosity when cells were oxygenated and a decrease in viscosity upon deoxygenation. Information provided by MDS on the intraerythrocytic water state of intact red blood cells reflects its interaction with all of the cytosolic components, making these measurements powerful predictors of the changes in the rheological properties of red blood cells, regardless of the cause.

A bilobal model of Ca2+-dependent inactivation to probe the physiology of L-type Ca2+ channels

L-type calcium channels undergo Ca²⁺-dependent inactivation (CDI) in order to precisely control the entry of Ca²⁺ into cells such as cardiomyocytes. Limpitikul et al. develop a bilobal model of CDI and use it to understand the pathogenesis of arrhythmias associated with mutations in CaM.

L-type calcium channels (LTCCs) are critical elements of normal cardiac function, playing a major role in orchestrating cardiac electrical activity and initiating downstream signaling processes. LTCCs thus use feedback mechanisms to precisely control calcium (Ca²⁺) entry into cells. Of these, Ca²⁺-dependent inactivation (CDI) is significant because it shapes cardiac action potential duration and is essential for normal cardiac rhythm. This important form of regulation is mediated by a resident Ca²⁺ sensor, calmodulin (CaM), which is comprised of two lobes that are each capable of responding to spatially distinct Ca²⁺ sources. Disruption of CaM-mediated CDI leads to severe forms of long-QT syndrome (LQTS) and life-threatening arrhythmias. Thus, a model capable of capturing the nuances of CaM-mediated CDI would facilitate increased understanding of cardiac (patho)physiology. However, one critical barrier to achieving a detailed kinetic model of CDI has been the lack of quantitative data characterizing CDI as a function of Ca²⁺. This data deficit stems from the experimental challenge of uncoupling the effect of channel gating on Ca²⁺ entry. To overcome this obstacle, we use photo-uncaging of Ca²⁺ to deliver a measurable Ca²⁺ input to CaM/LTCCs, while simultaneously recording CDI. Moreover, we use engineered CaMs with Ca²⁺ binding restricted to a single lobe, to isolate the kinetic response of each lobe. These high-resolution measurements enable us to build mathematical models for each lobe of CaM, which we use as building blocks for a full-scale bilobal model of CDI. Finally, we use this model to probe the pathogenesis of LQTS associated with mutations in CaM (calmodulinopathies). Each of these models accurately recapitulates the kinetics and steady-state properties of CDI in both physiological and pathological states, thus offering powerful new insights into the mechanistic alterations underlying cardiac arrhythmias.

Cardiac voltage gated calcium channels and their regulation by β-adrenergic signaling

Voltage-gated calcium channels (VGCCs) are the predominant source of calcium influx in the heart leading to calcium-induced calcium release and ultimately excitation-contraction coupling. In the heart, VGCCs are modulated by the β-adrenergic signaling. Signaling through β-adrenergic receptors (βARs) and modulation of VGCCs by β-adrenergic signaling in the heart are critical signaling and changes to these have been significantly implicated in heart failure. However, data related to calcium channel dysfunction in heart failure is divergent and contradictory ranging from reduced function to no change in the calcium current. Many recent studies have highlighted the importance of functional and spatial microdomains in the heart and that may be the key to answer several puzzling questions. In this review, we have briefly discussed the types of VGCCs found in heart tissues, their structure, and significance in the normal and pathological condition of the heart. More importantly, we have reviewed the modulation of VGCCs by βARs in normal and pathological conditions incorporating functional and structural aspects. There are different types of βARs, each having their own significance in the functioning of the heart. Finally, we emphasize the importance of location of proteins as it relates to their function and modulation by co-signaling molecules. Its implication on the studies of heart failure is speculated.

Ischemia-related subcellular redistribution of sodium channels enhances the proarrhythmic effect of class I antiarrhythmic drugs: a simulation study

BACKGROUND

Cardiomyocytes located at the ischemic border zone of infarcted ventricle are accompanied by redistribution of gap junctions, which mediate electrical transmission between cardiomyocytes. This ischemic border zone provides an arrhythmogenic substrate. It was also shown that sodium (Na+) channels are redistributed within myocytes located in the ischemic border zone. However, the roles of the subcellular redistribution of Na+ channels in the arrhythmogenicity under ischemia remain unclear.

METHODS

Computer simulations of excitation conduction were performed in a myofiber model incorporating both subcellular Na+ channel redistribution and the electric field mechanism, taking into account the intercellular cleft potentials.

RESULTS

We found in the myofiber model that the subcellular redistribution of the Na+ channels under myocardial ischemia, decreasing in Na+ channel expression of the lateral cell membrane of each myocyte, decreased the tissue excitability, resulting in conduction slowing even without any ischemia-related electrophysiological change. The conventional model (i.e., without the electric field mechanism) did not reproduce the conduction slowing caused by the subcellular Na+ channel redistribution. Furthermore, Na+ channel blockade with the coexistence of a non-ischemic zone with an ischemic border zone expanded the vulnerable period for reentrant tachyarrhythmias compared to the model without the ischemic border zone. Na+ channel blockade tended to cause unidirectional conduction block at sites near the ischemic border zone. Thus, such a unidirectional conduction block induced by a premature stimulus at sites near the ischemic border zone is associated with the initiation of reentrant tachyarrhythmias.

CONCLUSIONS

Proarrhythmia of Na+ channel blockade in patients with old myocardial infarction might be partly attributable to the ischemia-related subcellular Na+ channel redistribution.

Use of mutant-specific ion channel characteristics for risk stratification of long QT syndrome patients

Inherited long QT syndrome (LQTS) is caused by mutations in ion channels that delay cardiac repolarization, increasing the risk of sudden death from ventricular arrhythmias. Currently, the risk of sudden death in individuals with LQTS is estimated from clinical parameters such as age, gender, and the QT interval, measured from the electrocardiogram. Even though a number of different mutations can cause LQTS, mutation-specific information is rarely used clinically. LQTS type 1 (LQT1), one of the most common forms of LQTS, is caused by mutations in the slow potassium current (I(Ks)) channel α subunit KCNQ1. We investigated whether mutation-specific changes in I(Ks) function can predict cardiac risk in LQT1. By correlating the clinical phenotype of 387 LQT1 patients with the cellular electrophysiological characteristics caused by an array of mutations in KCNQ1, we found that channels with a decreased rate of current activation are associated with increased risk of cardiac events (hazard ratio=2.02), independent of the clinical parameters usually used for risk stratification. In patients with moderate QT prolongation (a QT interval less than 500 ms), slower activation was an independent predictor for cardiac events (syncope, aborted cardiac arrest, and sudden death) (hazard ratio = 2.10), whereas the length of the QT interval itself was not. Our results indicate that genotype and biophysical phenotype analysis may be useful for risk stratification of LQT1 patients and suggest that slow channel activation is associated with an increased risk of cardiac events.

Roles of subcellular Na+ channel distributions in the mechanism of cardiac conduction

The gap junction and voltage-gated Na(+) channel play an important role in the action potential propagation. The purpose of this study was to elucidate the roles of subcellular Na(+) channel distribution in action potential propagation. To achieve this, we constructed the myocardial strand model, which can calculate the current via intercellular cleft (electric-field mechanism) together with gap-junctional current (gap-junctional mechanism). We conducted simulations of action potential propagation in a myofiber model where cardiomyocytes were electrically coupled with gap junctions alone or with both the gap junctions and the electric field mechanism. Then we found that the action potential propagation was greatly affected by the subcellular distribution of Na(+) channels in the presence of the electric field mechanism. The presence of Na(+) channels in the lateral membrane was important to ensure the stability of propagation under conditions of reduced gap-junctional coupling. In the poorly coupled tissue with sufficient Na(+) channels in the lateral membrane, the slowing of action potential propagation resulted from the periodic and intermittent dysfunction of the electric field mechanism. The changes in the subcellular Na(+) channel distribution might be in part responsible for the homeostatic excitation propagation in the diseased heart.

Kudzu root: traditional uses and potential medicinal benefits in diabetes and cardiovascular diseases

Kudzu root (Gegen in Chinese) is the dried root of Pueraria lobata (Willd.) Ohwi, a semi-woody, perennial and leguminous vine native to South East Asia. It is often used interchangeably in traditional Chinese medicine with thomson kudzu root (Fengen in Chinese), the dried root of P. thomsonii, although the Chinese Pharmacopoeia has separated them into two monographs since the 2005 edition. For more than 2000 years, kudzu root has been used as a herbal medicine for the treatment of fever, acute dysentery, diarrhoea, diabetes and cardiovascular diseases. Both English and Chinese literatures on the traditional applications, phytochemistry, pharmacological activities, toxicology, quality control and potential interactions with conventional drugs of both species have been included in the present review. Over seventy phytochemicals have been identified in kudzu root, with isoflavonoids and triterpenoids as the major constituents. Isoflavonoids, in particular puerarin, have been used in most of the pharmacological studies. Animal and cellular studies have provided support for the traditional uses of kudzu root on cardiovascular, cerebrovascular and endocrine systems, including diabetes and its complications. Further studies to define the active phytochemical compositions, quality standards and clinical efficacy are warranted. Strong interdisciplinary collaboration to bridge the gap between traditional medicine and modern biomedical medicine is therefore needed for the development of kudzu root as an effective medicine for the management of diabetes and cardiovascular diseases.

The mechanisms underlying ICa heterogeneity across murine left ventricle

L-type calcium current (I(Ca)) plays a critical role in excitation-contraction coupling (ECC). Unlike transient outward K(+) current (I(to)), it is controversial whether I(Ca) transmural gradient exists in left ventricle. Although previous studies have shown some evidences for I(Ca) heterogeneity, the mechanism is still unknown. In this study, the authors recorded I(Ca) from epicardial (EPI) and endocardial (ENDO) myocytes isolated from murine left ventricle using patch-clamp technique. It was found that I(Ca) density was obviously larger in EPI than in ENDO (7.3 ± 0.3 pA/pF vs. 6.2 ± 0.2 pA/pF, at test potential of +10 mV, P < 0.05). The characteristics of I(Ca) showed no difference between these two regions except for the fast inactivation time constants (9.9 ± 0.9 ms in EPI vs. 13.5 ± 0.9 ms in ENDO, at test potential of +10 mV, P < 0.05). In addition, it was explored the molecular mechanism underlying I(Ca) transmural gradient by Western blot. The authors demonstrated that a higher activity of CaMKII in ENDO cells induced more nuclear translocation of p65, a component of nuclear factor-kappa B (NF-kB). Consequently, p65 in ENDO inhibited more transcription of Cav1.2, the main encoding gene for L-type calcium channels (LTCCs). These results reveal a difference in CaMKII/p65 signal pathway between EPI and ENDO that underlies this mechanism of I(Ca) heterogeneity in murine left ventricle.

Transmural dispersion of repolarization determines scroll wave behavior during ventricular tachyarrhythmias

BACKGROUND

Ventricular tachyarrhythmia is the leading cause of sudden cardiac death, and scroll wave re-entry is known to underlie this condition. Class III antiarrhythmic drugs are commonly used worldwide to treat ventricular tachyarrhythmias; however, these drugs have a proarrhythmic adverse effect and can cause Torsade de Pointes or ventricular fibrillation. Transmural dispersion of repolarization (TDR) has been suggested to be a strong indicator of ventricular tachyarrhythmia induction. However, the role of TDR during sustained scroll wave re-entry is poorly understood. The purpose of the present study was to investigate how TDR affects scroll wave behavior and to provide a novel analysis of the mechanisms that sustain tachyarrhythmias, using computer simulations.

METHODS AND RESULTS

Computer simulations were carried out to quantify the TDR and QT interval under a variety of I(Ks) and I(Kr) during transmural conduction. Simulated scroll wave re-entries were done under a variety of I(Ks) and I(Kr) in a ventricular wall slab model, and the scroll wave behavior and the filament dynamics (3-dimensional organizing center) were analyzed. A slight increase in TDR, but not in the QT interval, reflected antiarrhythmic properties resulting from the restraint of scroll wave breakup, whereas a marked increase in TDR was proarrhythmic, as a result of scroll wave breakup.

CONCLUSIONS

The TDR determines the sustainment of ventricular tachyarrhythmias, through control of the scroll wave filament dynamics.

Pharmacological preconditioning by diazoxide downregulates cardiac L-type Ca(2+) channels

BACKGROUND AND PURPOSE

Pharmacological preconditioning (PPC) with mitochondrial ATP-sensitive K(+) (mitoK(ATP) ) channel openers such as diazoxide, leads to cardioprotection against ischaemia. However, effects on Ca(2+) homeostasis during PPC, particularly changes in Ca(2+) channel activity, are poorly understood. We investigated the effects of PPC on cardiac L-type Ca(2+) channels.

EXPERIMENTAL APPROACH

PPC was induced in isolated hearts and enzymatically dissociated cardiomyocytes from adult rats by preincubation with diazoxide. We measured reactive oxygen species (ROS) production and Ca(2+) signals associated with action potentials using fluorescent probes, and L-type currents using a whole-cell patch-clamp technique. Levels of the α(1c) subunit of L-type channels in the cellular membrane were measured by Western blot.

KEY RESULTS

PPC was accompanied by a 50% reduction in α(1c) subunit levels, and by a reversible fall in L-type current amplitude and Ca(2+) transients. These effects were prevented by the ROS scavenger N-acetyl-L-cysteine (NAC), or by the mitoK(ATP) channel blocker 5-hydroxydecanoate (5-HD). PPC significantly reduced infarct size, an effect blocked by NAC and 5-HD. Nifedipine also conferred protection against infarction when applied during the reperfusion period. Downregulation of the α(1c) subunit and Ca(2+) channel function were prevented in part by the protease inhibitor leupeptin.

CONCLUSIONS AND IMPLICATIONS

PPC downregulated the α(1c) subunit, possibly through ROS. Downregulation involved increased degradation of the Ca(2+) channel, which in turn reduced Ca(2+) influx, which may attenuate Ca(2+) overload during reperfusion.

Cellular modelling: experiments and simulation to develop a physiological model of G-protein control of muscarinic K+ channels in mammalian atrial cells

The first model of G-protein-K(ACh) channel interaction was developed 14 years ago and then expanded to include both the receptor-G-protein cycle and G-protein-K(ACh) channel interaction. The G-protein-K(ACh) channel interaction used the Monod-Wyman-Changeux allosteric model with the idea that one K(ACh) channel is composed of four subunits, each of which binds one active G-protein subunit (G(betagamma)). The receptor-G-protein cycle used a previous model to account for the steady-state relationship between K(ACh) current and intracellular guanosine-5-triphosphate at various extracellular concentrations of acetylcholine (ACh). However, simulations of the activation and deactivation of K(ACh) current upon ACh application or removal were much slower than experimental results. This clearly indicates some essential elements were absent from the model. We recently found that regulators of G-protein signalling are involved in the control of K(ACh) channel activity. They are responsible for the voltage-dependent relaxation behaviour of K(ACh) channels. Based on this finding, we have improved the receptor-G-protein cycle model to reproduce the relaxation behaviour. With this modification, the activation and deactivation of K(ACh) current in the model are much faster and now fall within physiological ranges.

Interplay of voltage and Ca-dependent inactivation of L-type Ca current

Inactivation of L-type Ca channels (LTCC) is regulated by both Ca and voltage-dependent processes (CDI and VDI). To differentiate VDI and CDI, several experimental and theoretical studies have considered the inactivation of Ba current through LTCC (I(Ba)) as a measure of VDI. However, there is evidence that Ba can weakly mimic Ca, such that I(Ba) inactivation is still a mixture of CDI and VDI. To avoid this complication, some have used the monovalent cation current through LTCC (I(NS)), which can be measured when divalent cation concentrations are very low. Notably, I(NS) inactivation rate does not depend on current amplitude, and hence may reflect purely VDI. However, based on analysis of existent and new data, and modeling, we find that I(NS) can inactivate more rapidly and completely than I(Ba), especially at physiological temperature. Thus VDI that occurs during I(Ba) (or I(Ca)) must differ intrinsically from VDI during I(NS). To account for this, we have extended a previously published LTCC mathematical model of VDI and CDI into an excitation-contraction coupling model, and assessed whether and how experimental I(Ba) inactivation results (traditionally used in VDI experiments and models) could be recapitulated by modifying CDI to account for Ba-dependent inactivation. Thus, the view of a slow and incomplete I(NS) inactivation should be revised, and I(NS) inactivation is a poor measure of VDI during I(Ca) or I(Ba). This complicates VDI analysis experimentally, but raises intriguing new questions about how the molecular mechanisms of VDI differ for divalent and monovalent currents through LTCCs.

L-type Ca(2+) current in ventricular cardiomyocytes

L-type Ca(2+) channels are mediators of Ca(2+) influx and the regulatory events accompanying it and are pivotal in the function and dysfunction of ventricular cardiac myocytes. L-type Ca(2+) channels are located in sarcolemma, including the T-tubules facing the sarcoplasmic reticulum junction, and are activated by membrane depolarization, but intracellular Ca(2+)-dependent inactivation limits Ca(2+) influx during action potential. I(CaL) is important in heart function because it triggers excitation-contraction coupling, modulates action potential shape and is involved in cardiac arrhythmia. L-type Ca(2+) channels are multi-subunit complexes that interact with several molecules involved in their regulations, notably by beta-adrenergic signaling. The present review highlights some of the recent findings on L-type Ca(2+) channel function, regulation, and alteration in acquired pathologies such as cardiac hypertrophy, heart failure and diabetic cardiomyopathy, as well as in inherited arrhythmic cardiac diseases such as Timothy and Brugada syndromes.

In silico risk assessment for drug-induction of cardiac arrhythmia

The main components of repolarization reserve for the ventricular action potential (AP) are the rapid (I(Kr)) and slow (I(Ks)) delayed outward K(+) currents. While many drugs block I(Kr) and cause life-threatening arrhythmias including torsades de pointes, the frequency of arrhythmias varies between different I(Kr)-blockers. Different types of block of I(Kr) cause distinct phenotypes of prolongation of action potential duration (APD), increase in transmural dispersion of repolarization (TDR) and, accordingly, occurrence of torsades de pointes. Therefore the assessment of a drug's proarrhythmic risk requires a method that provides quantitative and comprehensive comparison of the effects of different forms of I(Kr)-blockade upon APDs and TDR. However, most currently available methods are not adapted to such an extensive comparison. Here, we introduce I(Kr)-I(Ks) two-dimensional maps of APD and TDR as a novel risk-assessment method. Taking the kinetics of I(Kr)-blockade into account, APDs can be calculated upon a ventricular AP model which systematically alters the magnitudes of I(Kr) and I(Ks). The calculated APDs are then plotted on a map where the x axis represents the conductance of I(Kr) while the y axis represents that of I(Ks). TDR is simulated with models corresponding to APs in epicardial, midcardial and endocardial myocardium. These two-dimensional maps of APD and TDR successfully account for differences in the risk resulting from three distinct types of I(Kr)-blockade which correspond to the effects of dofetilide, quinidine and vesnarinone. This method may be of use to assess the arrhythmogenic risk of various I(Kr)-blockers.