Increasing tetrahydrobiopterin in cardiomyocytes adversely affects cardiac redox state and mitochondrial function independently of changes in NO production.

Tetrahydrobiopterin (BH 4 ) represents a potential strategy for the treatment of cardiac remodeling, fibrosis and/or diastolic dysfunction. The effects of oral treatment with BH 4 (Sapropterin™ or Kuvan™) are however dose-limiting with high dose negating functional improvements. Cardiomyocyte-specific overexpression of GTP cyclohydrolase I (mGCH) increases BH 4 several-fold in the heart. Using this model, we aimed to establish the cardiomyocyte-specific responses to high levels of BH 4 . Quantification of BH 4 and BH 2 in mGCH transgenic hearts showed age-based variations in BH 4 :BH 2 ratios. Hearts of mice (<6 months) have lower BH 4 :BH 2 ratios than hearts of older mice while both GTPCH activity and tissue ascorbate levels were higher in hearts of young than older mice. No evident changes in nitric oxide (NO) production assessed by nitrite and endogenous iron–nitrosyl complexes were detected in any of the age groups. Increased BH 4 production in cardiomyocytes resulted in a significant loss of mitochondrial function. Diminished oxygen consumption and reserve capacity was verified in mitochondria isolated from hearts of 12-month old compared to 3-month old mice, even though at 12 months an improved BH 4 :BH 2 ratio is established. Accumulation of 4-hydroxynonenal (4-HNE) and decreased glutathione levels were found in the mGCH hearts and isolated mitochondria. Taken together, our results indicate that the ratio of BH 4 :BH 2 does not predict changes in neither NO levels nor cellular redox state in the heart. The BH 4 oxidation essentially limits the capacity of cardiomyocytes to reduce oxidant stress. Cardiomyocyte with chronically high levels of BH 4 show a significant decline in redox state and mitochondrial function. [ABSTRACT FROM AUTHOR]