Tetrahydrobiopterin analogues with NO-dependent pulmonary vasodilator properties

Reduced NO levels due to the deficiency of tetrahydrobiopterin (BH4) contribute to impaired vasodilation in pulmonary hypertension. Due to the chemically unstable nature of BH4, it was hypothesised that oxidatively stable analogues of BH4 would be able to support NO synthesis to improve endothelial dysfunction in pulmonary hypertension. Two analogues of BH4, namely 6-hydroxymethyl pterin (HMP) and 6-acetyl-7,7-dimethyl-7,8-dihydropterin (ADDP), were evaluated for vasodilator activity on precontracted rat pulmonary artery rings. ADDP was administered to pulmonary hypertensive rats, followed by measurement of pulmonary vascular resistance in perfused lungs and eNOS expression by immunohistochemistry. ADDP and HMP caused significant relaxation in vitro in rat pulmonary arteries depleted of BH4 with a maximum relaxation at 0.3 μM (both P < 0.05). Vasodilator activity of ADDP and HMP was completely abolished following preincubation with the NO synthase inhibitor, L-NAME. ADDP and HMP did not alter relaxation induced by carbachol or spermine NONOate. BH4 itself did not produce relaxation. In rats receiving ADDP 14.1 mg/kg/day, pulmonary vasodilation induced by calcium ionophore A23187 was augmented and eNOS immunoreactivity was increased. In conclusion, ADDP and HMP are two analogues of BH4, which can act as oxidatively stable alternatives to BH4 in causing NO-mediated vasorelaxation. Chronic treatment with ADDP resulted in improvement of NO-mediated pulmonary artery dilation and enhanced expression of eNOS in the pulmonary vascular endothelium. Chemically stable analogues of BH4 may be able to limit endothelial dysfunction in the pulmonary vasculature.