Is reduced SERCA2a expression detrimental or beneficial to postischemic cardiac function and injury? Evidence from heterozygous SERCA2a knockout mice

Recent studies have demonstrated that increased expression of sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) 2a improves myocardial contractility and Ca²⁺ handling at baseline and in disease conditions, including myocardial ischemia-reperfusion (I/R). Conversely, it has also been reported that pharmacological inhibition of SERCA might improve postischemic function in stunned hearts or in isolated myocardium following I/R. The goal of this study was to test how decreases in SERCA pump level/activity affect cardiac function following I/R. To address this question, we used a heterozygous SERCA2a knockout (SERCA2a^+/-) mouse model with decreased SERCA pump levels and studied the effect of myocardial stunning (20-min ischemia followed by reperfusion) and infarction (30-min ischemia followed by reperfusion) following 60-min reperfusion. Our results demonstrate that postischemic myocardial relaxation was significantly impaired in SERCA2a ^+/- hearts with both stunning and infarction protocols. Interestingly, postischemic recovery of contractile function was comparable in SERCA2a^+/- and wild-type hearts subjected to stunning. In contrast, following 30-min ischemia, postischemic contractile function was reduced in SERCA2a^+/- hearts with significantly larger infarction. Rhod-2 spectrofluorometry revealed significantly higher diastolic intracellular Ca ²⁺ in SERCA2a^+/- hearts compared with wild-type hearts. Both at 30-min ischemia and 2-min reperfusion, intracellular Ca²⁺ levels were significantly higher in SERCA2a^+/- hearts. Electron paramagnetic resonance spin trapping showed a similar extent of postischemic free-radical generation in both strains. These data provide direct evidence that functional SERCA2a level, independent of oxidative stress, is crucial for postischemic myocardial function and salvage during I/R.