Impairment of Endothelial-Myocardial Interaction Increases the Susceptibility of Cardiomyocytes to Ischemia/Reperfusion Injury

Endothelial-myocardial interactions may be critically important for ischemia/reperfusion injury. Tetrahydrobiopterin (BH(4)) is a required cofactor for nitric oxide (NO) production by endothelial NO synthase (eNOS). Hyperglycemia (HG) leads to significant increases in oxidative stress, oxidizing BH(4) to enzymatically incompetent dihydrobiopterin. How alterations in endothelial BH(4) content impact myocardial ischemia/reperfusion injury remains elusive. The aim of this study was to examine the effect of endothelial-myocardial interaction on ischemia/reperfusion injury, with an emphasis on the role of endothelial BH(4) content. Langendorff-perfused mouse hearts were treated by triton X-100 to produce endothelial dysfunction and subsequently subjected to 30 min of ischemia followed by 2 h of reperfusion. The recovery of left ventricular systolic and diastolic function during reperfusion was impaired in triton X-100 treated hearts compared with vehicle-treated hearts. Cardiomyocytes (CMs) were co-cultured with endothelial cells (ECs) and subsequently subjected to 2 h of hypoxia followed by 2 h of reoxygenation. Addition of ECs to CMs at a ratio of 1∶3 significantly increased NO production and decreased lactate dehydrogenase activity compared with CMs alone. This EC-derived protection was abolished by HG. The addition of 100 µM sepiapterin (a BH(4) precursor) or overexpression of GTP cyclohydrolase 1 (the rate-limiting enzyme for BH(4) biosynthesis) in ECs by gene trasfer enhanced endothelial BH(4) levels, the ratio of eNOS dimer/monomer, eNOS phosphorylation, and NO production and decreased lactate dehydrogenase activity in the presence of HG. These results demonstrate that increased BH(4) content in ECs by either pharmacological or genetic approaches reduces myocardial damage during hypoxia/reoxygenation in the presence of HG. Maintaining sufficient endothelial BH(4) is crucial for cardioprotection against hypoxia/reoxygenation injury.