Endothelium-specific sepiapterin reductase deficiency in DOCA-salt hypertension

The endothelial nitric oxide synthase (eNOS) requires tetrahydrobiopterin (H ₄B) as a cofactor and, in its absence, produces superoxide (O ₂ ^-) rather than nitric oxide (NO), a condition referred to as eNOS uncoupling. DOCA-salt-induced hypertension is associated with H ₄B oxidation and uncoupling of eNOS. The present study investigated whether administration of sepiapterin or H ₄B recouples eNOS in DOCA-salt hypertension. Bioavailable NO detected by electron spin resonance was markedly reduced in aortas of DOCA-salt hypertensive mice. Preincubation with sepiapterin (10 μmol/l for 30 min) failed to improve NO bioavailability in hypertensive aortas while it augmented NO production from control vessels, implicating a hypertension-associated deficiency in sepiapterin reductase (SPR), the rate-limiting enzyme for sepiapterin conversion to H ₄B. Indeed, a decreased SPR expression was observed in aortic endothelial cells, but not in endothelium-denuded aortic remains, implicating an endothelium-specific SPR deficiency. Administration of hypertensive aortas with H ₄B (10 μmol/l, 30 min) partially restored vascular NO production. Combined administration of H ₄B and the NADPH oxidase inhibitor apocynin (100 μmol/l, 30 min) fully restored NO bioavailability while reducing O ₂ ^- production. In angiotensin II-induced hypertension, however, aortic endothelial SPR expression was not affected. In summary, administration of sepiapterin is not effective in recoupling eNOS in DOCA-salt hypertension, due to an endothelium-specific loss in SPR, whereas coadministration of H ₄B and apocynin is highly efficient in recoupling eNOS. This is consistent with our previous observations that in angiotensin II hypertension, endothelial deficiency in dihydro-folate reductase is alternatively responsible for uncoupling of eNOS. Taken together, these data indicate that strategies specifically targeting at different H ₄B metabolic enzymes might be necessary in restoring eNOS function in different types of hypertension.