Project Summary The current application proposes to investigate the novel mechanisms through which cGMP and the cGMP- dependent protein kinase I (PKGI) maintain Ca homeostasis and oppose ventricular tachycardia (VT) in the diabetic heart in the setting of acute MI. Diabetes mellitus (DM) is associated with an increase in sudden cardiac death (SCD), but the molecular and cellular mechanisms of arrhythmogenesis in DM remain incompletely understood. T-Wave alternans (TWA) is a rate dependent beat-to-beat fluctuation on the surface EKG that has been associated with life threatening ventricular arrhythmias. T-wave alternans has been described in patients with Type II DM, and hyperactivity of GSK3β has been implicated in secondary effects of DMII, but little is known regarding the mechanism of alternans and its specific role in the pathogenesis of VT in patients, or in mouse models for Type II DM. Inducibility of VT in response to programmed ventricular stimulation has been used to identify patients whose hearts are predisposed to the development of VT. Preliminary data in the current application identify significant inducibility of VT and of TWA in two separate models: the Db/Db mouse model of DMII, and in the High Fat High Sucrose (HFHS) fed mouse model of insulin insensitivity. More importantly, we have demonstrated that these mice develop severe spontaneous arrhythmias following myocardial infarction (MI). We have observed that the intracellular signaling molecule cGMP is decreased in both of these above models. Further, genetic disruption of the primary cardiovascular cGMP effector, Protein Kinase G Iα (PKGIα) leads to T-wave alternans and highly inducible VT, providing direct evidence that this kinase plays a role in the pathogenesis of VT in vivo. Rescue experiments demonstrate that augmentation of cGMP with the cGMP- selective phosphodiesterase 5 inhibitor sildenafil abolishes VT and TWA in the Db/Db and HFHS mice, while the GSK3β inhibitor TWS-119 attenuates VT. Based on these and published data implicating GSK3β as a PKG1α substrate, the current application proposes to test the hypothesis that PKGIα opposes both inducible and spontaneous post MI VT in the diabetic heart through the regulation of GSK3β activity. We propose 3 Specific Aims: 1) Determine the anti-arrhythmic effects of PKGI augmentation in the DMII heart by PDE inhibitors and Guanylate cyclase activators; 2) Determine the physiologic mechanisms by which PKGIα dysfunction predisposes to inducible and post MI VT and alternans via an effect on Ca homeostasis; 3) Determine the downstream signaling mechanisms through which disruption of cGMP-PKG signaling promotes VT in the DMII heart. The data generated from these studies will determine the role of PKGIα signaling in the regulation of VT in the DM II heart and has the potential to identify pharmacologic modulation of PKGI as a novel anti-arrhythmic strategy in patients with DM.