SERCA2 Knockout Mice Exhibit Impaired Control of Ca 2+ Current but not Ventricular Arrhythmias

Abstract

Impaired Ca2+ handling by the sarcoplasmic reticulum (SR) and Ca2+-dependent arrhythmias are hallmark features of human heart failure. We investigated the control of L-type Ca2+ current (ICa, L) when SR function is reduced and the consequences for arrhythmogenesis. Experiments were performed on cardiomyocytes isolated from conditional SERCA2 KO mice (KO) which had developed heart failure 7 weeks following gene disruption. SERCA2flox/flox (FF) mice served as controls. SR Ca2+ content was reduced to 4% (P<0.05) of FF values in KO cardiomyocytes, and SR Ca2+ release did not occur on a beat-to-beat basis. Marked up-regulation of the L-type Ca2+ channel in KO (α1C subunit= 178% FF, α2/δ1= 147% FF, P<0.05) was accompanied by a 40% increase in peak ICa, L (P<0.05). Loss of SR function resulted in slower Ca2+ current inactivation, prolonged duration of current activation, and loss of frequency-dependent facilitation. The larger magnitude and prolonged ICa,L in KO resulted in AP prolongation, which was not observed in the presence of nifedipine or upon removal of extracellular Ca2+. AP prolongation was associated with prolonged QT intervals corrected for heart rate in KO mice compared to FF (5.63 ms vs 4.90 ms, P<0.05). While AP prolongation is expected to be arrhythmogenic, incidence of early after-depolarizations in KO cardiomyocytes was not increased (FF= 2/13 cells, KO= 0/10 cells, P=NS). Telemetric ECG surveillance during pharmacological stress also revealed a similar incidence of ventricular arrhythmias in FF and KO mice. In conclusion, loss of SR function results in greater L-type Ca2+ entry, loss of Ca2+-dependent inactivation, and prolonged APs and QT interval. While such alterations would be expected to pro-arrhythmic in larger species, the relatively brief AP in failing mice may preclude occurrence of early after-depolarizations.