Myocardial ischemia results in tetrahydrobiopterin (BH₄) oxidation with impaired endothelial function ameliorated by BHIN₄

Coronary vasodilation is impaired in the postischemic heart with a loss of endothelial nitric oxide synthase (eNOS) activity, but the mechanisms underlying ischemia-induced eNOS dysfunction are not understood. For nitric oxide (NO) synthesis, eNOS requires the redox-sensitive cofactor tetrahydrobiopterin (BH₄); however, the role of BH₄ in ischemia-induced endothelial dysfunction remains unknown. Therefore, isolated rat hearts were subjected to varying durations of ischemia, and the alterations in NOS-dependent vasodilation were measured and correlated with assays of eNOS activity and cardiac BH₄ concentrations. Ischemia time-dependently decreased cardiac BH₄ content with 85, 95, or 97% irreversible degradation after 30, 45, or 60 min of ischemia, respectively. Paralleling the decreases in BH₄, reductions of eNOS activity were seen of 58, 86, or 92%, and NOS-derived superoxide production was greatly increased. Addition of 10 μM BH₄ enhanced eNOS activity in nonischemic hearts and partially restored activity after ischemia. It also suppressed NOS-derived superoxide production. Impaired coronary flow during postischemic reperfusion was improved by BH₄ infusion. Thus, BH₄ depletion contributes to postischemic eNOS dysfunction, and BH₄ treatment is effective in partial restoration of endothelium-dependent coronary flow. Supplementation of BH₄ may therefore be an important therapeutic approach to reverse endothelial dysfunction in postischemic tissues.