Angiotensin II preconditioning (APC) has been shown to reproduce the cardioprotective effects of ischemic preconditioning (IPC). However, the molecular mechanisms mediating the effects of APC remain unknown. In this study, Langendorff-perfused rat hearts were subjected to IPC, APC, or their combination (IPC/APC) followed by ischemia/reperfusion (IR). The following variables were determined: left-ventricular developed pressure (LVDP), the first derivative of developed pressure (dP/dt), the rate pressure product (RPP), and lactate dehydrogenase (LDH). Infarct size (IS) was determined in a subset of hearts at the end of the perfusion protocols. IPC, and less potently APC, significantly increased the percent recoveries of LVDP, the dP/dt, and RPP compared with control. Furthermore, the post-ischemic recovery of the heart was significantly higher for IPC/APC compared with IPC or APC. The improvements in cardiac function by IPC, APC, and IPC/APC were associated with similar reductions in LDH release and IS. These results indicate that APC and IPC interact through mechanisms that protect the heart and improve cardiac function after IR.

To determine whether the actions of APC and IPC on cardiac function depend on PKC-dependent mechanisms, the translocation of PKCepsilon, PKCdelta, Akt, ERK1/2, JNK, p38 MAPK and GSK-3beta to mitochondria was determined. Significant increases in mitochondrial PKCepsilon/PKCdelta ratio, Akt, ERK1/2, JNK, and inhibition of permeability transition pore (mPTP) opening were observed. Chelerythrine, a pan-PKC inhibitor, abolished the enhancements of PKCdelta but increased PKCdelta expression, and inhibited Akt, ERK1/2, and JNK protein levels. The drug did not affect on the APC- and IPC/APC-induced cardioprotection but enhanced the post-ischemic LVDP in controls. Losartan, an angiotensin II type 1 receptor (AT1-R) blocker, abolished the APC-stimulated increase of LVDP and reduced PKCepsilon, Akt, ERK1/2, JNK, and p38. Both drugs reduced ischemic contracture and LDH release and abolished the inhibition of mPTP by the preconditioning. Chelerythrine also prevented the reduction of IS by APC and IPC/APC. These results suggest that the cardioprotection induced by APC and IPC/APC involves an AT1-R-dependent translocation of PKCepsilon and survival kinases to the mitochondria leading to mPTP inhibition. In chelerythrine-treated hearts, however, alternate mechanisms appear to maintain cardiac function.

To evaluate the role that mitochondrial AT1-Rs and angiotensin II type 2 receptors (AT2-Rs) play in APC, losartan (L, AT1-R blocker), PD 123,319 (PD, AT2-R blocker) or their combination (L+PD), were infused before the preconditioning protocols. The following variables were evaluated: mitochondrial AT1-R, AT2-R, PKCepsilon, PKCdelta, Akt, PKG-1, MAPKs (ERK1/2, JNK, p38), mitochondrial respiration, cardiac function, and IS. The results indicate that expression of mitochondrial AT1-Rs and AT2-Rs were enhanced by APC 1.91-fold and 2.32-fold, respectively. Expression of AT2-R was abolished by PD but not by L, whereas the AT1-R levels were abrogated by both blockers. The AT1-R response profile to L and PD was also shared by PKCepsilon, Akt, MAPKs, and PKG-1, but not by PKCdelta. A marked increase in state 3 (1.84-fold) and respiratory control index (1.86-fold) of mitochondria was observed with PD regardless of L treatment. PD also enhanced the post-ischemic recovery of RPP by 74% (p < 0.05) compared with APC alone. Losartan, however, inhibited the rate pressure product (RPP) by 44% (p < 0.05) before IR and reduced the APC-induced increase of post-ischemic cardiac recovery by 73% (p < 0.05). Finally, L enhanced the reduction of IS by APC through a PD-sensitive mechanism. These findings suggest that APC upregulates angiotensin II receptors in mitochondria and that AT2-Rs are cardioprotective through their permissive action on AT1-R signaling and the suppression of cardiac function.

To examine whether mitochondria-generated reactive oxygen species (ROS) play a crucial role in the loss of cardioprotection in aging, the effects of XJB-5-131 (XJB), a mitochondria-targeted ROS and electron scavenger, on cardiac resistance to ischemia-reperfusion (IR)-induced oxidative stress in aged rats was determined. In this study male adult (5-month old) and aged (29-month old) Fischer Brown Norway (F344/BN) rats were randomly assigned to the following groups: adult (A), adult+XJB (AX), aged (O), and aged+XJB (OX). XJB was administered 3 times per week (3mg/kg body weight, IP) for four weeks. At the end of the treatment period, LVDP and RPP were determined prior, during and after IR. Ca2+-induced swelling, an indicator of permeability transition pore (mPTP) opening, was determined in isolated mitochondria at the end of the reperfusion period. XJB improved post-ischemic recovery of aged hearts, as evidenced by greater LVDP and RPP than the untreated, aged-matched group. The state 3 respiration rates at complexes I, II and IV of mitochondria isolated from XJB-treated aged hearts were 57% (P < 0.05), 25% (P < 0.05) and 28% (P < 0.05), respectively, higher than controls. (Abstract shortened by ProQuest.).