Simultaneous assessment of compound activity on cardiac Nav1.5 peak and late currents in an automated patch clamp platform

INTRODUCTION

High throughput in vitro profiling of the cardiac Nav1.5 peak sodium current (I_Na) is widely used in cardiac safety screening. However, there is no standardized high throughput method to measure late I_Na. This study assessed the pharmacological and biophysical properties of veratridine and ATX-II, as well as the channel mutation (Nav1.5-∆KPQ) on the late I_Na. We describe a method for simultaneous measurement of both peak and late I_Na.

METHODS

The planar patch clamp technique (QPatch) was applied to record the peak and late I_Na.

RESULTS

The Nav1.5-∆KPQ mutant produced a small late I_Na (41.9 ± 5.4 pA) not large enough to enable compound profiling. In contrast in wild type Nav1.5 expressing cells veratridine (100 μM) and ATX-II (100 nM) enhanced concentration-dependent increases in the late I_Na (maximum responses of 1162.2 ± 258.5 pA and 392.4 ± 71.3 pA, respectively). Veratridine inhibited, whereas, ATX-II had a minimal effect, on the peak I_Na and preserved the current-voltage curve. Peak and late I_Na inhibition was characterized for 25 clinical I_Na blockers in the presence of ATX-II. Compound IC₅₀ values for peak I_Na generated in the absence or presence of ATX-II correlated. The potency of the late I_Na block was found to be dependent on whether it was measured at the end of the depolarizing pulse or during the ramp.

DISCUSSION

In the presence of ATX-II, both peak and late I_Na could be assessed simultaneously. Late I_Na may be best assessed using the maximum response obtained during the ramp of the voltage protocol.

Intracellular Na+ overload causes oxidation of CaMKII and leads to Ca2+ mishandling in isolated ventricular myocytes

An increase of late Na(+) current (INaL) in cardiac myocytes can raise the cytosolic Na(+) concentration and is associated with activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and alterations of mitochondrial metabolism and Ca(2+) handling by sarcoplasmic reticulum (SR). We tested the hypothesis that augmentation of INaL can increase mitochondrial reactive oxygen species (ROS) production and oxidation of CaMKII, resulting in spontaneous SR Ca(2+) release and increased diastolic Ca(2+) in myocytes. Increases of INaL and/or of the cytosolic Na(+) concentration led to mitochondrial ROS production and oxidation of CaMKII to cause dysregulation of Ca(2+) handling in rabbit cardiac myocytes.

Drug-induced functional cardiotoxicity screening in stem cell-derived human and mouse cardiomyocytes: effects of reference compounds

INTRODUCTION

Early prediction of drug-induced functional cardiotoxicity requires robust in-vitro systems suitable for medium/high throughput and easily accessible cardiomyocytes with defined reproducible properties. The xCELLigence Cardio system uses 96-well plates with interdigitated electrodes that detect the impedance changes of rhythmic contractions of stem cell-derived cardiomyocyte (SC-CM) layers. Here, we report on our initial screening experience in comparison to established (multi)cellular and in-vivo models.

METHODS

Impedance signals from human iPSC-CM (iCells™) and mouse eSC-CM (Cor.At™) were analyzed for contraction amplitude (CA) and duration, rise/fall time, beating rate (BR) and irregularity.

RESULTS

Following solution exchange, impedance signals re-approximated steady-state conditions after about 2 (Cor.At™) and 3h (iCells™); these time points were used to analyze drug effects. The solvent DMSO (≤1%) hardly influenced contraction parameters in Cor.At™, whereas in iCells™ DMSO (>0.1%) reduced CA and enhanced BR. The selective hERG K⁺ channel blockers E-4031 and dofetilide reduced CA and accelerated BR (≥30 nM) according to the analysis software. The latter, however, was due to burst-like contractions (300 nM) that could be detected only by visual inspection of recordings, and were more pronounced in Cor.At™ as in iCells™. In cardiac myocytes and tissue preparations, however, E4031 and dofetilide have been reported to increase cell shortening and contractile force and to reduce BR. Compounds (pentamidine, HMR1556, ATX2, TTX, and verapamil) with other mechanisms of action were also investigated; their effects differed partially between cell lines (e.g. TTX) and compared to established (multi)cellular models (e.g. HMR1556, ouabain).

CONCLUSION

Mouse and human stem cell-derived cardiomyocytes respond differently to drugs and these responses occasionally also differ from those originating from established in-vitro and in-vivo models. Hence, drug-induced cardiotoxic effects may be detected with this system, however, the predictive or even translational value of results is considered limited and not yet firmly established.