Tachycardia may cause substantial molecular and ultrastructural alterations in cardiac tissue. The underlying pathophysiology has not been fully explored. The purpose of this study was (I) to validate a three-dimensional in vitro pacing model, (II) to examine the effect of rapid pacing on mitochondrial function in intact cells, and (III) to evaluate the involvement of L-type-channel-mediated
calcium influx in alterations of mitochondria in cardiomyocytes during rapid pacing. In vitro differentiated cardiomyocytes from P19 cells that formed embryoid bodies were paced for 24 h with 0.6 and 2.0 Hz. Pacing at 2.0 Hz increased
mRNA expression and phosphorylation of ERK1/2 and caused cellular
hypertrophy, indicated by increased
protein/
DNA ratio, and oxidative stress measured as loss of cellular
thiols. Rapid pacing additionally provoked structural alterations of mitochondria. All these changes are known to occur in vivo during
atrial fibrillation. The structural alterations of mitochondria were accompanied by limitation of
ATP production as evidenced by decreased endogenous respiration in combination with decreased
ATP levels in intact cells. Inhibition of
calcium inward current with
verapamil protected against hypertrophic response and oxidative stress.
Verapamil ameliorated morphological changes and dysfunction of mitochondria. In conclusion, rapid pacing-dependent changes in
calcium inward current via L-type channels mediate both oxidative stress and
mitochondrial dysfunction. The in vitro pacing model presented here reflects changes occurring during
tachycardia and, thus, allows functional analyses of the signaling pathways involved.