Abstract 1610: Repetitive Ischemia Elicits a Third Window of Ischemic Preconditioning

Abstract

Ischemic preconditioning (IPC), following 1 or 2 episodes of coronary occlusion (CO) is the most powerful mechanism of cardioprotection, but it remains unclear whether IPC occurs clinically. Since patients with coronary disease experience episodes of repetitive ischemia (RI), we hypothesized that a pattern of RI would provide more clinically relevant cardioprotection. Conscious, chronically instrumented pigs (6/group) were submitted to RI induced by 6 episodes, 12 hrs apart, of coronary stenosis for 90 min (RCS) and compared to pigs submitted to second window IPC (SWIPC) induced by 2 episodes of 10 min CO / 24 hr reperfusion (CR) and to shams. Lethal ischemia, induced by 60 min CO, resulted in an infarct size/area at risk (IS/AAR) of 42±4% in shams, and significantly less, p<0.05, after RCS (6±3%) and after SWIPC (16±4%). SWIPC showed 2–3-fold increases, p<0.05, in translocation of PKCepsilon and the inducible isoform of nitric oxide synthase (NOS), which were not found in RCS. Pretreatment with the NOS inhibitor L-NNA (35 mg/kg) prevented the protection by SWIPC but not by RCS. Microarrays were performed to further elucidate the differences in molecular mechanisms between the SWIPC and RCS models. The number of genes significantly regulated was greater in RCS (5739) than in SWIPC (2394). Of the 5739 genes regulated in RCS, only 31% were also regulated in SWIPC. Since another difference between the two models is that one involves CS and the other complete CO, we examined another group of RI pigs (n=5) with 6 episodes, 12 hrs apart, of 2–10 min periods of CO, which showed reduced IS/AAR with 60 min CO after RI (14±4%); similar to RCS the cardioprotection was still observed after L-NNA. Both RI models, in further contrast to SWIPC, demonstrated downregulation of genes involved in mitochondrial function, and upregulation of genes involving autophagy (such as cathepsins B and D), endoplasmic reticulum stress (Grp78) and cell cycle (E2F1). These results were confirmed at the protein level by western blotting. Therefore, RI, over a time course of several days, provides a NOS-independent “third window” of protection, which is as effective as traditional SWIPC, but is mechanistically distinct. This research has received full or partial funding support from the American Heart Association, AHA National Center.