The endothelial nitric oxide synthase (eNOS) requires tetrahydrobiopterin (H4B) as a cofactor and, in its absence, produces superoxide (O2·−) rather than nitric oxide (NO·), a condition referred to as eNOS uncoupling. DOCA-salt-induced hypertension is associated with H4B oxidation and uncoupling of eNOS. The present study investigated whether administration of sepiapterin or H4B 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 H4B. 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 H4B (10 μmol/l, 30 min) partially restored vascular NO· production. Combined administration of H4B and the NADPH oxidase inhibitor apocynin (100 μmol/l, 30 min) fully restored NO· bioavailability while reducing O2·− 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 H4B and apocynin is highly efficient in recoupling eNOS. This is consistent with our previous observations that in angiotensin II hypertension, endothelial deficiency in dihydrofolate reductase is alternatively responsible for uncoupling of eNOS. Taken together, these data indicate that strategies specifically targeting at different H4B metabolic enzymes might be necessary in restoring eNOS function in different types of hypertension.
New inhibitors of sepiapterin reductase. Lack of an effect of intracellular tetrahydrobiopterin depletion upon in vitro proliferation of two human cell lines.
