In the presence of diabetes (DM), myocardial glucose uptake and glycolysis are impaired and the heart rapidly adapts to use exclusively fatty acids (FA) for ATP generation. This maladaptation is believed to play a key role in the development of a cardiomyopathy over time. Here, we show that stimulating myocardial nitric oxide synthase (NOS) activity is sufficient to alleviate myocardial metabolic inflexibility, improve energy metabolism and prevent LV dysfunction in DM by increasing myocardial insulin-independent glucose transport.Methods
Myocardial-specific overexpression of GTP cyclohydrolase I (mGCH1) was used to increase both tetrahydrobiopterin (BH4) and NOS activity in cardiomyocytes. Diabetes mellitus (DM) was induced by multiple low-dose streptozotocin injections (vs sham). PCr/ATP ratio was measured in perfused hearts using 31P-MRS, glucose transport estimated by deoxy-glucose uptake, and oxygen consumption rate (OCR) of intact cardiomyocytes using a phosphorescent probe.Results
As expected, sham-injected mGCH1 transgenic hearts had higher BH4 levels and constitutive NOS activity compared with WT. 12 weeks after DM induction, LV dysfunction developed in WT mice but not in mGCH1 mice, in the absence of changes in myocardial BH4 content and NOS activity in either group. WT diabetic hearts had a lower PCr/ATP ratio (1.32±0.1 vs 1.73±0.1, p<0.05, n=11 per group) and mitochondrial creatine kinase (CK) activity (1.56±0.1 AU vs 1.98±0.1 AU, p<0.005, n=10 per group) when compared with non-diabetic WT mice, consistent with impaired cardiac energetics. By contrast, PCr/ATP and CK activity were preserved in diabetic mGCH1 hearts in the absence of differences in myocardial mitochondrial content.Results
Myocardial GCH1 overexpression was associated with a higher protein levels of the insulin-independent glucose transporter, GLUT-1 (p<0.05, n=12 per group), but no changes in GLUT-4 protein. Myocardial glucose transport was 40% higher in LV myocytes from mGCH1 diabetic mice when compared with WT diabetic mice. This was accompanied by increased myocardial glucose oxidation, as determined by OCR. Pre-incubation of myocytes with inhibitors of NOS-PKG signalling (L-NAME, 1 mmol/L or Rp8pCPT PET cGMP 10 µmol/L) or GLUT-1 (STF-31, 10 µmol/L,) abolished all differences between mGCH and WT diabetic hearts.Conclusions
Our study reveals that a myocardial increase in BH4 and NOS activity is sufficient to maintain a favourable substrate utilisation and preserve cardiac mitochondrial function in the presence of DM. This work provides new insight into the potential metabolic triggers of diabetic cardiomyopathy and suggests exciting new targets for BH4-based therapeutics.