Abstract-Hearts of wild-type and insulin-like growth factor-1 overexpressing (Igf-1 ϩ/Ϫ ) transgenic mice were subjected to Langendorff perfusions and progressive periods of ischemia followed by reperfusion. Apoptosis was measured by DNA nucleosomal cleavage and a hairpin probe labeling assay to detect single-base overhang. Transgenic hearts subjected to 20 minutes of ischemia and 4 hours of reperfusion (I/R) sustained a rate of apoptosis of 1.8Ϯ0.3% compared with 4.6Ϯ1.1% for wild-type controls (nϭ4; PϽ0.03). Phosphorylation of the protein kinase Akt/protein kinase B was elevated 6.2-fold in transgenic hearts at baseline and increased another 4.4-fold within 10 minutes of reperfusion, remaining elevated for up to 2 hours. I/R activated Akt in wild-type hearts but to a lesser extent (1.6Ϯ0.3-fold). Pretreatment of transgenic hearts with wortmannin immediately before and during ischemia eliminated reperfusionmediated activation of Akt and neutralized the resistance to apoptosis. The stress-activated kinase p38 was also activated during ischemia and reperfusion in both wild-type and transgenic hearts. Perfusion with the p38 inhibitor SB203580 (10 mol/L) blocked both p38 activation and phosphorylation of Akt and differentially modulated apoptosis in wild-type and transgenic hearts. Pretreatment with SB203580 reduced apoptosis in wild-type hearts but increased apoptosis in transgenic hearts. These results demonstrate that Akt phosphorylation during I/R is modulated by IGF-1 and prevents apoptosis in hearts that overexpress the IGF-1 transgene. (Circ Res. 2001;88:609-614.) Key Words: ischemia Ⅲ hypoxia Ⅲ phosphoinositol-3Ј-kinase Ⅲ p38 mitogen-activated protein kinase Ⅲ SB203580 P rotective and antiapoptotic properties of insulin-like growth factor-1 (IGF-1) have been demonstrated in different models of myocardial ischemia and infarction 1-4 as well as in isolated cardiac myocytes subjected to ischemic or oxidative stress. 5,6 IGF-1 stimulates the phosphoinositol-3Ј (PI3)-kinase pathway, producing phosphoinositides that promote activation of the kinase Akt. 7,8 Activated Akt kinase plays a central role in suppressing apoptosis by modulating the activities of Bcl-2 family proteins, 9 caspase 9, 10 and Fas ligand. 11 Transfer of mutationally activated PI3-kinase and Akt genes has been shown to prevent apoptosis of cardiac myocytes in vitro, 6 and activated Akt delivered by an adenovirus vector reduced apoptosis in the intact heart subjected to ischemia and reperfusion (I/R). 4 We previously reported that Igf-1 ϩ/Ϫ transgenic mice overexpressing IGF-1 had reduced rates of necrosis and apoptosis after myocardial infarction caused by coronary artery ligation. 3 The same hearts were also resistant to necrosis but not apoptosis caused by nonocclusive coronary artery constriction. 12 Other work has shown that the endogenous PI3-kinase pathway is activated in the postinfarcted myocardium, where it may contribute to cell survival and hypertrophy. [13][14][15] Regulation of the endogenous PI3-kinase pathway during I/R has not b...