Acute Mechanical Unloading Prior to Reperfusion is Cardioprotective and Limits the Development of Heart Failure After Myocardial Infarction

Ischemic heart failure after acute myocardial infarction (AMI) is a major cause of morbidity and mortality worldwide. We recently reported that activation of a trans-valvular axial-flow pump in the LV and delaying myocardial reperfusion, known as Primary Unloading, limits infarct size by reducing LV wall stress and increasing expression of the cardioprotective cytokine, stromal derived factor 1 alpha (SDF1a). The mechanisms underlying the cardioprotective benefit and sustained effect of Primary Unloading remain poorly understood. We now tested the importance of delayed reperfusion, the functional significance of SDF1a, and the late-term impact on myocardial function and scar size associated with Primary Unloading. Adult male swine were subjected to Primary Reperfusion or Primary Unloading after 90 minutes of left anterior descending artery occlusion. Compared to Primary Reperfusion, 30 minutes of Primary Unloading was necessary and sufficient prior to reperfusion to limit infarct size. Primary Unloading was associated with a global shift in gene expression within the infarct zone favoring cardioprotection. Compared to Primary Reperfusion, Primary Unloading for 30 minutes preserved SDF1a protein levels without changing SDF1a mRNA levels within the infarct zone and further promoted a shift towards anti-apoptotic signaling within the infarct zone. Primary Unloading reduced activity levels of proteases known to degrade SDF1a and blocking the SDF1a receptor, CXCR4, attenuated the cardioprotective effect of Primary Unloading. Primary Unloading further reduced LV scar size, improved cardiac function, limited expression of markers associated with heart failure and maladaptive remodeling within the non-infarct zone 28 days after acute myocardial infarction. In conclusion, we introduce that Primary Unloading for 30 minutes before, not after reperfusion limits infarct size, increases SDF1a levels within the infarct zone, and results in a durable reduction in LV scar size, improved cardiac function, and limits markers of heart failure and maladaptive remodeling 28 days after AMI. To explore the optimal duration of mechanical unloading prior to reperfusion, adult, male Yorkshire swine were divided into 4 groups (n=4/group; Figure 1A). All groups underwent 90 minutes of LAD occlusion. In Group 1, LAD occlusion followed by 120 minutes of reperfusion served as the control group (Primary Reperfusion). In Groups 2 and 3, LAD occlusion was followed by insertion and activation of a TV-Pump (Impella CP; Abiomed Inc; Danvers, MA) via a 14Fr sheath in the left femoral artery and maintained at maximal support (44,000 rotations per minute). This was followed by an additional 15 (Group 2) or 30 (Group 3; Primary Unloading) minutes of occlusion respectively, then 120 minutes of reperfusion. In Group 4, LAD occlusion was followed by reperfusion, and after 30 minutes of reperfusion, a TVP was inserted and activated for the remaining 90 minutes of reperfusion (Post Reperfusion Unloading). At the end of each study, animals were euthanized for determination of myocardial infarct size. Three sham-operated animals were intubated, anesthetized, and mechanically ventilated without myocardial infarction or mechanical unloading. LV tissue samples obtained from sham controls were used for tissue analysis. RNA was collected from the center of the infarct zone of group 1 (Primary Reperfusion), group 3 (Primary Unloading) animals, or from the same region of the left ventricle in sham operated animals. RNA was processed and hybridized to Affymetrix Porcine Gene 1.0 ST expression microarrays (3 to 4 arrays per condition), using the standard Affymetrix workflow. Gene level expression values were determined using the Robust Multiarray Average (RMA) method (1) as part of the affy package (version 1.36.1) (2) in the Bioconductor R suite (version 2.12) (3), together with Entrez Gene-specific probeset maping (17.0.0) from the Molecular and Behavioral Neuroscience Institute (Brainarray) at the University of Michigan (4)). Array quality was measured by computing Relative Log Expression (RLE) and Normalized Unscaled Standard Error (NUSE) using the affyPLM package (version 1.34.0) (5). The three best samples from each group, by RLE value, were selected for further analysis. Data was quantile normalized using normalize.quantiles in the R preprocessCore package (version 1.32.0), and differential expression was determined using Limma (6) (version 3.26.9) for three contrasts: Primary Reperfusion vs Sham, Primary Unloading vs. Sham, and Primary Unloading vs. Primary Reperfusion. Genes showing at least a 2.0 fold change in expression and a raw p.value of less than 0.01.