Abstract 16841: Cardiomyocyte-Targeted Overexpression of GTP Cyclohydrolase-1 Increases Myocyte Lusitropy and Reduces Superoxide Production
Hypothesis: By causing uncoupling of nitric oxide synthase (NOS) activity, myocardial depletion of the NOS co-factor BH4 has been associated with adverse LV remodelling and impaired relaxation in animal models of cardiac disease. Oral supplementation of BH4 has been shown to be beneficial under these conditions; however, whether this is due to a genuine increase in myocardial BH4 availability or to a systemic antioxidant effect of BH4 remains to be established. We evaluated the effect of increasing myocardial BH4 availability by transgenic overexpression of GTP cyclohydrolase-1 (GCH) under the control of the MHC promoter on cardiomyocyte function.
Methods: LV myocytes shortening was measured under basal conditions and in the presence of 10 nmol/L isoproterenol (ISO). The [Ca2+]i transient was evaluated in Fura-2-loaded LV myocytes. SR Ca2+ content and calcium buffering were assessed by measuring the caffeine-induced [Ca2+]i transient and NCX current. The RyR Ca2+ leak was evaluated by using the Shannon & Bers protocol. Myocardial superoxide production was assessed by lucigenin-enhanced chemiluminescence.
Results: There was no difference in body weight, cardiac mass or myocyte size between genotypes. As expected, BH4 concentration in myocytes from GCH-Tg was significantly increased (55.4 ± 10 vs. 2.6 ± 0.4 pmol/mg protein in WT, n=3–4) whereas BH4 levels in liver, brain, skeletal muscle, and plasma were unchanged. The increase in BH4 was associated with a significant reduction in basal myocardial superoxide production (45.7 ± 4.8 vs. 59.6 ± 5.8 RLU/mg protein in WT, P<0.05 n=6–8). Cell shortening, the amplitude of the [Ca2+]i transient, and SR Ca2+ content did not differ between genotypes, both under basal conditions and in the presence of ISO; however, GCH-Tg myocytes showed a faster rate of relaxation (dL/dt: 190 ± 18 vs. 241 ± 17 μm/s in WT, P<0.05 n=25 cells), which was associated with a more rapid decay of the [Ca2+]i transient (Tau: 95 ± 3.6 vs. 116 ± 4.3 s in WT, P<0.01 ). Ca2+ extrusion rate by the NCX, myocardial Ca2+ buffering and the RyR Ca2+ leak were not affected by the transgene.
Conclusions: A myocardial-specific increase in BH4 availability increases the rate of relaxation and Ca2+ decay and reduces superoxide production without affecting inotropy.
- © 2010 by American Heart Association, Inc.