Myocardial Protection by Insulin at Reperfusion Requires Early Administration and Is Mediated via Akt and p70s6 Kinase Cell-Survival Signaling

Abstract

Abstract-The "metabolic cocktail" comprising glucose-insulin-potassium administrated at reperfusion reduces infarct size in the in vivo rat heart. We propose that insulin is the major component mediating this protection and acts via Akt prosurvival signaling. This hypothesis was studied in isolated perfused rat hearts (measuring infarct size to area of risk [%]) subjected to 35 minutes regional myocardial ischemia and 2 hours reperfusion. Insulin administered at the onset of reperfusion attenuated infarct size by Ն45% versus control hearts (PϽ0.001). Insulin-mediated cardioprotection was found to be independent of the presence of glucose at reperfusion. Moreover, the cell survival benefit of insulin is temporally dependent, in that insulin administration from the onset of reperfusion and maintained for either 15 minutes or for the duration of reperfusion reduced infarct size. In contrast, protection was abrogated if insulin administration was delayed until 15 minutes into reperfusion. Key Words: cardioprotection Ⅲ insulin Ⅲ Akt Ⅲ p70s6 kinase Ⅲ BAD T he management of patients with acute myocardial infarction has improved dramatically with the restoration of arterial perfusion with thrombolytic and antiplatelet therapy. Attention has turned to adjunctive pharmacological treatments to enhance myocardial tolerance to ischemia/reperfusion injury. This strategy is being pursued in an attempt to further reduce mortality in conjunction with reperfusion therapy. 1 Ideally, as this cytoprotective therapy would usually be administered after the onset of ischemia, candidate agents would need to be effective when administered during reperfusion. In a pilot randomized, controlled clinical study, administration of the "metabolic cocktail" comprising glucose, insulin, and potassium (GIK) has been shown to reduce mortality in patients with acute myocardial infarction undergoing reperfusion. 2 This was supported in experimental studies where we demonstrated that GIK infusion at reperfusion reduces myocardial infarct size in the in vivo rat. 3 Interestingly, in this in vivo rat study, we observed that the early reperfusion free fatty acid and glucose levels were similar in the GIK-treated and vehicle-control-treated rats. This experimental observation questioned the exclusivity of the previously hypothesized glucose/fatty acid hypothesis concerning the cardioprotective effect of GIK. 4 Moreover, as insulin itself is a mitogen and is known to promote cell survival, 5 we began to investigate whether insulin, when administered at reperfusion, could enhance tolerance to ischemia. In our initial study, we utilized rat neonatal cardiocytes to study the effects of insulin in response to simulated ischemia and reoxygenation. In that study, we demonstrated that insulin administration at reoxygenation reduced cardiomyocyte injury and attenuated the incidence of apoptosis during the reoxygenation period. 6 Collectively, these data support a role of insulin in the promotion of cell survival in the context of the postischemic reperfusion peri...

Activation of p38 mitogen-activated protein kinase abolishes insulin-mediated myocardial protection against ischemia-reperfusion injury

“…However, 30 min of ischemia before giving insulin significantly blunted insulin's effect, and an additional 30 min of reperfusion prior to giving insulin completely abolished its effect. This is consistent with a previous study in isolated, perfused rat hearts subjected to 35 min of regional myocardial ischemia and 2 h of reperfusion, in that insulin administered at the onset of reperfusion attenuated infarct size by 45%, but the protection was abrogated if insulin administration was delayed until 15 min into reperfusion (13). Thus, it is not surprising that in a large, randomized controlled trial conducted among patients with ST-segment elevation myocardial infarction who presented within 12 h of symptom onset intravenous GIK infusion for 24 h had a neutral effect on mortality, cardiac arrest, cardiogenic shock, and reinfarction (32).…”

“…Preischemic treatment with insulin triggers an infarctlimiting cardioprotective response in rabbit myocardium (3). This insulin-mediated cardioprotection is independent of the presence of glucose and involves the phosphatidylinositol 3-kinase (PI 3-kinase)/protein kinase B (Akt)/p70 S6K signaling mechanism, as pharmacological inhibition of PI 3-kinase with wortmannin and the mammalian target of rapamycin (mTOR) with rapamycin abolishes insulin-mediated cardioprotection (3,13).…”

“…In ex vivo experiments, insulin is capable of diminishing myocardial infarct size only when given before the onset of reperfusion (12,13). The mechanisms underlying this phenomenon remain unclear.…”

Akt mediates mitochondrial protection in cardiomyocytes through phosphorylation of mitochondrial hexokinase-II

“…For example, Akt activation by insulin or S1P decreases ischemia/reperfusion damage in isolated perfused heart or in vivo. 9,11 The acute cardioprotective effects of Akt activation, observed within hours, are unlikely to be mediated through transcriptional events. Thus post translational effects, as might result from Akt-mediated phosphorylation, should be considered.…”

Letter by Opie and Selker Regarding Article, “Reperfusion Starts in the Ambulance”

“…An even more cogent reason for giving insulin at reperfusion is that it stimulates the protective signaling paths that activate the key signaling molecule Akt to promote cell survival and powerfully to lessen apoptosis. Jonassen et al 5 have shown that insulin must be given at the time of reperfusion, or within 15 minutes thereof, to protect and decrease experimental infarct size. Testing this hypothesis in the ambulance would require a trial specifically designed to give reperfusion covered by insulin with glucose added to prevent hypoglycemia, also given in the ambulance.…”