P683The role of nitric oxide synthase (NOS) and its essential cofactor tetrahydrobiopterin (BH4) in diabetic cardiomyopathy.

PURPOSE: Diabetes can impact on cardiovascular health by causing a distinct condition termed "diabetic cardiomyopathy". Its characteristic left ventricular (LV) diastolic dysfunction has been associated with interstitial fibrosis, reduced NO availability, and abnormal calcium handling. However, the early triggers and the underlying cellular mechanisms remain unknown. Here, we investigate changes in vascular and myocardial reactive oxygen species (ROS) and NO availability in a murine model of type 1 diabetes, and evaluate potential beneficial effects of inducing a myocardial-specific increase in the NOS cofactor tetrahydrobiopterin (BH4) on the development of LV dysfunction. Methods: Diabetes was induced in male mice by daily intraperitoneal streptozotocin (STZ) injection (43mg/kg, 5 consecutive days). To augment myocardial BH4 and increase NOS activity, transgenic mice were generated with myocardial-specific overexpression of the rate-limiting enzyme for BH4 synthesis, GTP cyclohydrolase 1 (mGCH1 Tg). Vascular function in isolated aortas was evaluated by isometric tension studies (myograph), NOS activity and biopterins by HPLC, and superoxide production by lucigenin-enhanced chemiluminescence. High-resolution echocardiography was used to assess LV function. Results: After 12 weeks of diabetes, WT and mGCH1 Tg mice showed impaired aortic endothelium-dependent vasodilatation, in association with increased superoxide production and reduced BH4 bioavailability (n=6-10 per group). By contrast, diabetic LV homogenates showed no increase in superoxide generation or reduced BH4:BH2 ratio and no reduction in NOS activity (n=9-12 per group). Nevertheless, in vivo echocardiography revealed significant LV diastolic dysfunction in WT diabetic mice, which was prevented in mGCH1 Tg mice (E'/A' diabetic vs control: 1.52±0.08 vs 1.53±0.08 in mGCH1 Tg; 0.89±0.07 vs 1.35±0.06 in WT, n=9 per group, P<0.01 for the interaction between genotype and diabetes). In line with these results, isolated LV myocytes from WT diabetic mice displayed prolonged relaxation, which was prevented in diabetic mGCH1 Tg (t50 relaxation, diabetic vs control: 105.3±2.8 vs 95.6±2.4 in WT and 77.3±3 vs 73.3±2 in mGCH1 Tg; n=25 cells from 2-5 hearts per group, P<0.05 for the interaction between genotype and diabetes). Conclusions: Impaired LV diastolic function in diabetic mice can be prevented by myocardial GCH1 overexpression in the absence of NOS dysfunction or increased oxidative stress, suggesting that GCH1/BH4 protect the diabetic myocardium by mechanisms other than redressing the local nitroso-redox balance

Tetrahydrobiopterin protects against hypertrophic heart disease independent of myocardial nitric oxide synthase coupling.

Background Nitric oxide synthase uncoupling occurs under conditions of oxidative stress modifying the enzyme’s function so it generates superoxide rather than nitric oxide. Nitric oxide synthase uncoupling occurs with chronic pressure overload, and both are ameliorated by exogenous tetrahydrobiopterin (BH4)—a cofactor required for normal nitric oxide synthase function—supporting a pathophysiological link. Genetically augmenting BH4 synthesis in endothelial cells fails to replicate this benefit, indicating that other cell types dominate the effects of exogenous BH4 administration. We tested whether the primary cellular target of BH4 is the cardiomyocyte or whether other novel mechanisms are invoked. Methods and Results Mice with cardiomyocyte-specific overexpression of GTP cyclohydrolase 1 (mGCH1) and wild-type littermates underwent transverse aortic constriction. The mGCH1 mice had markedly increased myocardial BH4 and, unlike wild type, maintained nitric oxide synthase coupling after transverse aortic constriction; however, the transverse aortic constriction– induced abnormalities in cardiac morphology and function were similar in both groups. In contrast, exogenous BH4 supplementation improved transverse aortic constricted hearts in both groups, suppressed multiple inflammatory cytokines, and attenuated infiltration of inflammatory macrophages into the heart early after transverse aortic constriction. Conclusions BH4 protection against adverse remodeling in hypertrophic cardiac disease is not driven by its prevention of myocardial nitric oxide synthase uncoupling, as presumed previously. Instead, benefits from exogenous BH4 are mediated by a protective effect coupled to suppression of inflammatory pathways and myocardial macrophage infiltration.</p