Cardiac-specific overexpression of GTP cyclohydrolase 1 restores ischaemic preconditioning during hyperglycaemia.

AIMS: Hyperglycaemia (HG) decreases intracellular tetrahydrobiopterin (BH(4)) concentrations, and this action may contribute to injury during myocardial ischaemia and reperfusion. We investigated whether increased BH(4) by cardiomyocyte-specific overexpression of the GTP cyclohydrolase (GTPCH) 1 gene rescues myocardial and mitochondrial protection by ischaemic preconditioning (IPC) during HG through a nitric oxide (NO)-dependent pathway. METHODS AND
RESULTS: Mice underwent 30 min of myocardial ischaemia followed by 2 h of reperfusion with or without IPC elicited with four cycles of 5 min ischaemia/5 min of reperfusion in the presence or absence of HG produced by d-glucose. In C57BL/6 wild-type mice, IPC increased myocardial BH(4) and NO concentrations and decreased myocardial infarct size (30 ± 3% of risk area) compared with control (56 ± 5%) experiments. This protective effect was inhibited by HG (48 ± 3%) but not hyperosmolarity. GTPCH-1 overexpression increased myocardial BH(4) and NO concentrations and restored cardioprotection by IPC during HG (32 ± 4%). In contrast, a non-selective NO synthase inhibitor N(G)-nitro-l-arginine methyl ester attenuated the favourable effects of GTPCH-1 overexpression (52 ± 3%) during HG. Mitochondria isolated from myocardium subjected to IPC required significantly higher in vitro Ca(2+) concentrations (184 ± 14 µmol mg(-1) protein) to open the mitochondrial permeability transition pore when compared with mitochondria isolated from control experiments (142 ± 10 µmol mg(-1) protein). This beneficial effect of IPC was reversed by HG and rescued by GTPCH-1 overexpression.
CONCLUSION: Increased BH(4) by cardiomyocyte-specific overexpression of GTPCH-1 preserves the ability of IPC to elicit myocardial and mitochondrial protection that is impaired by HG, and this action appears to be dependent on NO.