Cardiomyocyte GTP Cyclohydrolase 1 and Tetrahydrobiopterin Increase NOS1 Activity and Accelerate Myocardial Relaxation

Abstract

Rationale: Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthases (NOS). Oral BH4 supplementation preserves cardiac function in animal models of cardiac disease; however, the mechanisms underlying these findings are not completely understood. Objective: To study the effect of myocardial transgenic overexpression of the rate-limiting enzyme in BH4 biosynthesis, GTP cyclohydrolase 1 (GCH1), on NOS activity, myocardial function, and Ca 2+ handling. Methods and Results: GCH1 overexpression significantly increased the biopterins level in left ventricular (LV) myocytes but not in the nonmyocyte component of the LV myocardium or in plasma. The ratio between BH4 and its oxidized products was lower in mGCH1-Tg, indicating that a large proportion of the myocardial biopterin pool was oxidized; nevertheless, myocardial NOS1 activity was increased in mGCH1-Tg, and superoxide release was significantly reduced. Isolated hearts and field-stimulated LV myocytes (3 Hz, 35°C) overexpressing GCH1 showed a faster relaxation and a PKA-mediated increase in the PLB Ser 16 phosphorylated fraction and in the rate of decay of the [Ca 2+ ] i transient. RyR2 S-nitrosylation and diastolic Ca 2+ leak were larger in mGCH1-Tg and I Ca density was lower; nevertheless the amplitude of the [Ca 2+ ] i transient and contraction did not differ between genotypes, because of an increase in the SR fractional release of Ca 2+ in mGCH1-Tg myocytes. Xanthine oxidoreductase inhibition abolished the difference in superoxide production but did not affect myocardial function in either group. By contrast, NOS1 inhibition abolished the differences in I Ca density, Ser 16 PLB phosphorylation, [Ca 2+ ] i decay, and myocardial relaxation between genotypes. Conclusions: Myocardial GCH1 activity and intracellular BH4 are a limiting factor for constitutive NOS1 and SERCA2A activity in the healthy myocardium. Our findings suggest that GCH1 may be a valuable target for the treatment of LV diastolic dysfunction.