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

Abstract

Wang, Yigang, Nauman Ahmad, Mitsuhiro Kudo, and Muhammad Ashraf. Contribution of Akt and endothelial nitric oxide synthase to diazoxide-induced late preconditioning. Am J Physiol Heart Circ Physiol 287: H1125-H1131, 2004. First published May 13, 2004; 10.1152/ajpheart.00183.2004.-The opening of mitochondrial ATP-sensitive K ϩ (mitoKATP) channels has a significant role in delayed ischemic preconditioning, and nitric oxide (NO) is a wellknown 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 g/kg ip) alone or DE plus N -nitro-L-arginine methyl ester (L-NAME, 30 g/kg iv), an inhibitor of NOS, or wortmannin (WTN, 15 g/kg iv), an inhibitor of phosphatidylinositol 3Ј-kinase (PI3 kinase), was administered to wild-type (WT) or eNOS Ϫ/Ϫ 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 Ϫ/Ϫ mice that were pretreated with DE completely abolished the beneficial effects of DE on reduction of apoptotic death. DE was partially effective in eNOS Ϫ/Ϫ 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. diazoxide; apoptosis NITRIC OXIDE (NO) is formed from L-arginine and oxygen by NO synthase (NOS), which is present in the coronary endothelium, endocardial endothelium, and cardiac myocytes (5). There are three known isoforms of NOS (13). Among them endothelial (eNOS), and neuronal cells (nNOS) are Ca 2ϩ dependent and remain inactive until intracellular calcium levels increase, resulting in enhanced calcium-calmodulin binding and subsequent activation (19), whereas the Ca 2ϩ -independent isoenzyme [inducible NOS (iNOS)] can be induced by cytokines (27) and other stimuli. Although NO is involved in late preconditioning (1, 24), it is still not clear which isoform is responsible for NO production, which serves as a trigger or mediator of preconditioning. Because the mit...

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“…The first study to implicate the PI3K-Akt-p70S6K pathway as an early mediator of delayed IPC, was by Kis and co-workers [89] who demonstrated that administering pharmacological inhibitors of either PI3K or p70S6K prior to IPC, abrogated both the infarct-limiting effects and Akt and p70S6K phosphorylation induced by IPC 24 h later in the rabbit heart. Subsequent studies have confirmed the early mediator role of the PI3K-Akt pathway in delayed preconditioning induced by diazoxide (a mK ATP channel opener) [90] and a δ1 opioid receptor agonist [40], and in the case of the former the Akt downstream target, eNOS, was implicated in the cardioprotective effect [90]. Again, the PIK3-Akt pathway has a bifunctional role by acting as a distal mediator as part of the reperfusion injury salvage kinase pathway [87].…”

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