SERCA2a overexpression decreases the incidence of aftercontractions in adult rabbit ventricular myocytes

Slow relaxation and poor contractile response to increasing stimulation frequency in failing human heart have been strongly linked to a decrease in the activity of the sarcoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA2a). Restoration of SERCA2a levels using gene transfer has beneficial effects on contractile function but, like β-adrenoceptor stimulation, could potentially produce excess SR Ca²⁺, arrhythmias and cell death. We have examined the effects of SERCA2a overexpression in adult rabbit cardiac myocytes, and compared changes in relaxation with those following β-adrenoceptor stimulation. Myocytes were infected with an adenovirus carrying both SERCA2a and green fluorescent protein (GFP) for positive identification of infected cells. Myocyte survival was significantly enhanced in the infected cultures. There was a reduction in both time-to-peak contraction and time-to-50% relaxation (R50) 48 h after infection. Time-to-90% relaxation (R90) was particularly improved (non-infected 516 ± 41 ms, AD.SERCA2a-GFP 230 ± 23 ms, n = 7 preparations, P<0.001). There was also a decreased incidence of aftercontractions in Ad.SERCA2a-GFP infected myocytes (21 ± 5% v 41 ± 4% in controls, P<0.01). This contrasts with β-adrenoceptor stimulation, which reduced R50 but prolonged R90 by 158 ± 76 ms (P<0.02, n = 16). At higher stimulation frequencies (2-3 Hz) contraction amplitude and SR calcium content were increased and diastolic contracture was reduced following SERCA2a overexpression. Overall, increasing levels of SERCA2a resulted in an improvement in systolic and diastolic function and a reduction in cell death and arrhythmic aftercontractions. SERCA2a overexpression therefore lacks the detrimental effects associated with some other inotropic interventions.