Contribution of Akt and endothelial nitric oxide synthase to diazoxide-induced late preconditioning

The opening of mitochondrial ATP-sensitive K⁺ (mitoK _ATP) channels has a significant role in delayed ischemic preconditioning, and nitric oxide (NO) is a well-known trigger for its activation. However, the source of NO remains unknown. Phosphorylation of endothelial NO synthase (eNOS) increases NO production and reduces apoptosis through the Akt signaling pathway. To elucidate the Akt signaling pathway involved in the opening and antiapoptotic effect of mitoK_ATP channel during delayed pharmacological preconditioning, the mitoK_ATP channel opener diazoxide (DE, 7 μg/kg ip) alone or DE plus N^ω-nitro- L-arginine methyl ester (L-NAME, 30 μg/kg iv), an inhibitor of NOS, or wortmannin (WTN, 15 μg/kg iv), an inhibitor of phosphatidylinositol 3′-kinase (PI3 kinase), was administered to wild-type (WT) or eNOS ^-/- mice during DE treatment. Twenty-four hours later, hearts were isolated and subjected to 40 min ischemia and 30 min reperfusion (I/R). The effect of DE and other interventions on hemodynamic, terminal dUTP nick-end labeling staining and biochemical changes during I/R was assessed in mouse hearts. Treatment with DE resulted in a 2.2-fold increase in phosphorylation of Akt and a significant increase in eNOS and inducible NOS (iNOS) proteins. Akt is upstream of NOS and the mitoK_ATP channel as simultaneous pretreatment of WTN with DE abolished phosphorylation of Akt, which was not affected by L-NAME and 5-hydroxydecanoate. In hearts treated with DE, cardiac function was significantly improved after I/R, and apoptosis was also significantly decreased. WTN abolished the antiapoptotic effect of DE. Similarly, S-methylisothiourea, a specific iNOS inhibitor, when given to eNOS^-/- mice that were pretreated with DE completely abolished the beneficial effects of DE on reduction of apoptotic death. DE was partially effective in eNOS^-/- mice against the ischemic injury. It is concluded that DE activates Akt through the PI3 kinase signaling pathway and iNOS and eNOS is downstream of Akt.