Acute hyperglycemia abolishes ischemic preconditioning by inhibiting Akt phosphorylation: normalizing blood glucose before ischemia restores ischemic preconditioning

Abstract

This study examined the hypothesis that acute hyperglycemia (HG) blocks ischemic preconditioning (IPC) by inhibiting Akt phosphorylation. Brief HG of approximately 400 mg/dL was induced in C57BL/6 mice via intraperitoneal injection of 20% dextrose (2 g/kg). All mice underwent 40 min LAD occlusion and 60 min reperfusion. The IPC protocol was 2 cycles of 5 min ischemia and 5 min reperfusion prior to index ischemia.

Results: In control mice, infarct size (IF) was 51.7 ± 2.0 (% risk region). Preconditioning reduced IF by 50% to 25.8 ± 3.2 (P < 0.05 versus control). In HG mice, IF was significantly exacerbated to 58.1 ± 2.3. However, the effect of IPC completely disappeared in HG mice. Normalization of blood glucose with insulin 5 min before IPC recovered the cardioprotective effect. Administration of CCPA before index ischemia mimicked IPC effect. The cardioprotective effect of CCPA, not its chronotropic effect, completely disappeared in HG mice. Phosphorylation of cardiac tissue Akt before index ischemia was enhanced by IPC or CCPA but was significantly inhibited by HG in both groups. Normalization of glucose with insulin reversed the inhibition of Akt phosphorylation by HG.

Conclusion: HG abolishes the cardioprotective effect of preconditioning by inhibiting Akt phosphorylation. Normalization of blood glucose with insulin suffices to recover the cardioprotective effect of preconditioning.

Figure 1

Animal groups and experimental protocols.

Figure 2

Myocardial infarct size after 40 minutes of LAD occlusion and 60 minutes of reperfusion. Acute hyperglycemia exacerbates infarct size. Normalization of blood glucose levels before LAD occlusion failed to counteract the hyperglycemic effect.

Figure 3

TTC and Phthalo blue staining of representative hearts from groups corresponding to Figure 2.

Figure 4

The cardioprotective effect of ischemic preconditioning disappears in HG mice, but can be recovered by normalizing blood glucose levels with insulin prior to ischemia.

Figure 5

Myocardial phospho-Akt S473 to pan-Akt ratios in the indicated experimental groups. The ratio of phospho-Akt S473 to pan-Akt (the bar graph) was measured by densitometry, where the pan-AKT inputs were normalized to 1.

Figure 6

The cardioprotective effect of CCPA mimics the effect of ischemic preconditioning but disappears in HG mice.

Figure 7

Myocardial phospho-Akt S473 to pan-Akt ratios in mice treated with CCPA.