Abstract 1005: Rac1 Deletion Impairs Ischemic Preconditioning In Mouse Hearts
The small GTPase Rac1 plays an important role in activation of NADPH oxidase, the major source of reactive oxygen species (ROS) in the heart following ischemia-reperfusion. The production of ROS is thought to be required for the induction of ischemic preconditioning (IPC). In this study, we examined whether Rac1 itself is essential for IPC cardioprotection. Temporally regulated cardiac-specific Rac1 deficient mice were generated by crossing Rac1loxP/loxP mice with MHC-MerCreMer+/+ mice, expressing a tamoxifen-inducible Cre recombinase protein fused to two mutant estrogen-receptor ligand-binding domains (MerCreMer) under the control of the α-myosin heavy chain (MHC) promoter. Rac1 deletion was induced by intra-peritoneal injection of tamoxifen in the experimental mice (Rac1loxP/+;MHC-MerCreMer+/− ) and confirmed by Western blot analysis. Littermates with the same genotype were treated with vehicle as controls. No change in Rac1 levels occurred in the control mice. Wildtype mice were subjected to 30 min of ischemia (I-30) and 120 min of reperfusion (R-120) with or without IPC, which consisted of I-10/R-5. The conditional Rac1 deficient mice and their littermate controls were exposed to I-30/R-120 after IPC. IPC significantly reduced infarct size (IS) in the wildtype mouse hearts, expressed as a percent of the area at risk (RA) (IS/RA: no IPC/IPC = 46.0 ± 2.7/16.7 ± 3.8, p < 0.01). However, the protective effect of IPC was completely eliminated in the Rac 1 deficient hearts (RACD), but preserved in their littermate controls (CON) (IS/RA: IPC + RACD/IPC + CON + 46.2 ± 4.0/10.3 ± 3.8, p < 0.01). There was no significant difference in the area at risk among these groups. Furthermore, IPC significantly increased active Rac1-GTP levels (IPC/no IPC = 108 ± 22/30 ± 16, p < 0.05) and phosphorylation of the survival protein kinase Akt in isolated perfused wildtype hearts. Blocking the assembly of NADPH oxidase subunits with apocynin (AP) inhibited Akt phosphorylation in the IPC hearts (IPC/IPC + AP = 129 ± 4/62 ± 10, p < 0.01). Therefore, Rac1 is required for IPC cardioprotection, and its action may be through downstream activation of Akt.