Distinct Occurrence of Proarrhythmic Afterdepolarizations in Atrial Versus Ventricular Cardiomyocytes: Implications for Translational Research on Atrial Arrhythmia

Abstract

Background: Principal mechanisms of arrhythmia have been derived from ventricular but not atrial cardiomyocytes of animal models despite higher prevalence of atrial arrhythmia (e.g., atrial fibrillation). Due to significant ultrastructural and functional differences, a simple transfer of ventricular proneness toward arrhythmia to atrial arrhythmia is critical. The use of murine models in arrhythmia research is widespread, despite known translational limitations. We here directly compare atrial and ventricular mechanisms of arrhythmia to identify critical differences that should be considered in murine models for development of antiarrhythmic strategies for atrial arrhythmia. Methods and Results: Isolated murine atrial and ventricular myocytes were analyzed by wide field microscopy and subjected to a proarrhythmic protocol during patch-clamp experiments. As expected, the spindle shaped atrial myocytes showed decreased cell area and membrane capacitance compared to the rectangular shaped ventricular myocytes. Though delayed afterdepolarizations (DADs) could be evoked in a similar fraction of both cell types (80% of cells each), these led significantly more often to the occurrence of spontaneous action potentials (sAPs) in ventricular myocytes. Interestingly, numerous early afterdepolarizations (EADs) were observed in the majority of ventricular myocytes, but there was no EAD in any atrial myocyte (EADs per cell; atrial myocytes: 0 ± 0; n = 25/12 animals; ventricular myocytes: 1.5 [0-43]; n = 20/12 animals; p < 0.05). At the same time, the action potential duration to 90% decay (APD₉₀) was unaltered and the APD₅₀ even increased in atrial versus ventricular myocytes. However, the depolarizing L-type Ca²⁺ current (I_Ca) and Na⁺/Ca²⁺-exchanger inward current (I_NCX) were significantly smaller in atrial versus ventricular myocytes. Conclusion: In mice, atrial myocytes exhibit a substantially distinct occurrence of proarrhythmic afterdepolarizations compared to ventricular myocytes, since they are in a similar manner susceptible to DADs but interestingly seem to be protected against EADs and show less sAPs. Key factors in the generation of EADs like I_Ca and I_NCX were significantly reduced in atrial versus ventricular myocytes, which may offer a mechanistic explanation for the observed protection against EADs. These findings may be of relevance for current studies on atrial level in murine models to develop targeted strategies for the treatment of atrial arrhythmia.

Keywords: afterdepolarizations; atrial arrhythmia; atrial cardiomyocytes; atrioventricular differences; translational research.

FIGURE 1

Basic differences in morphometry between atrial and ventricular myocytes. (A) Exemplary microscopic images demonstrate morphological differences between murine atrial and ventricular myocytes. Atrial myocytes exhibit a smaller 2D cell area compared to ventricular myocytes (B) and consistently a reduced electrical cell capacitance (C). ^∗p < 0.05 atrial vs. ventricular.

FIGURE 2

Distinct action potential kinetics in atrial versus ventricular myocytes. (A) Representative action potentials in atrial (left) and ventricular cardiomyocytes without (mid) and with (right) an EAD. Atrial myocytes exhibited a depolarized resting membrane potential (RMP) (B), a decreased maximal action potential (AP) upstroke velocity (V_max) (C), a decreased AP amplitude (D), and prolonged action potential duration to 50% decay (APD₅₀) (E). (F) Though a fraction of ventricular cells exhibited incessant EADs leading to AP prolongation (raw values within the dashed box), the average APD₉₀ was unaltered between both cell types. ^∗p < 0.05 atrial vs. ventricular; n.s., non-significant.

FIGURE 3

DADs and spontaneous action potentials (sAPs) in atrial and ventricular cardiomyocytes. (A) Representative tracings with a stimulated action potential (stim) of atrial (upper panel) and ventricular (lower panel) myocytes exhibiting DADs in the resting period. Ventricular cells did also exhibit translation of DADs into sAPs. While the fraction of cells with DADs was unaltered (B), the fraction of cells with sAPs (C) and the average number of sAPs per cell (D) was increased in ventricular compared to atrial myocytes. ^∗p < 0.05 atrial vs. ventricular.

FIGURE 4

Occurrence of EADs in ventricular but not atrial cardiomyocytes. (A) Representative tracings show various shapes of EADs in ventricular myocytes (red tracing) when the membrane potential of the plateau first falls short of and then exceeds –40 mV threshold. The EAD waveforms ranged between a spike-like shape (left) and a prolonged slew rate (right). No EADs occurred in any atrial myocyte (black tracing). The majority of ventricular cells (B) exhibited multiple EADs (C). ^∗p < 0.05 atrial vs. ventricular.

FIGURE 5

Reduced L-type Ca²⁺ current (I_Ca) in atrial versus ventricular myocytes. (A) Original tracings of I_Ca in atrial (upper panel) and ventricular (lower panel) myocytes. The applied stimulation protocol is depicted by the inset scheme. I_Ca was significantly reduced in atrial compared to ventricular myocytes over a voltage range between –10 mV and +40 mV as shown in the I–V-curve representing the mean values (B). (C) Median values of I_Ca peak amplitude in response to a voltage step to +10mV. (D) I_Ca inactivation kinetics were unaltered between both cell types. ^∗p < 0.05 atrial vs. ventricular [two-way repeated measures ANOVA for (B)].

FIGURE 6

Caffeine-induced NCX inward current (I_NCX) in atrial versus ventricular myocytes. (A) Original tracings of caffeine-induced NCX inward current (I_NCX) in atrial (upper panel) and ventricular (lower panel) cells. (B) I_NCX amplitude in response to caffeine was significantly reduced in atrial versus ventricular myocytes. (C) Integrated I_NCX from the area above the current curve (pink colored area). ^∗p < 0.05 atrial vs. ventricular.