Effects of tetrahydrobiopterin oral treatment in hypoxia-induced pulmonary hypertension in rat

Endothelial nitric oxide synthase (eNOS) plays a major role in maintaining pulmonary vascular homeostasis. Tetrahydrobiopterin (BH₄), an essential cofactor that stabilizes the dimerization of eNOS and balances nitric oxide (NO) and superoxide production, may have therapeutic potential in pulmonary hypertension. In the isolated perfused lung, we demonstrated a direct effect of exogenous administration of BH₄ on pulmonary NO production, leading to acute vasorelaxation during the plateau phase of hypoxia-induced pulmonary vasoconstriction. In the chronic hypoxia-induced pulmonary hypertension rat model, chronic BH₄ oral administration attenuated the pressor response to hypoxia (mean pulmonary artery pressure ± standard error of the mean, 31.8 ± 0.5 mmHg at 100 mg/kg/day; placebo group, 36.3 ± 0.6 mmHg; P < 0.05). During telemetric monitoring, right ventricular systolic pressure was reduced by approximately 50% after 1 week of BH4 treatment at 100 mg/kg/day. BH₄ at 100 mg/kg/day reduced right ventricular hypertrophy (from 0.55 ± 0.01 to 0.50 ± 0.01; P < 0.05) and pulmonary vascular muscularization (from 79.2% ± 2% to 65.2% ± 3%; P < 0.01). BH₄ treatment enhanced lung eNOS activity and reduced superoxide production, with a net increase in cyclic guanosine monophosphate levels. BH₄ is effective in attenuating pulmonary hypertension in the hypoxic rat model when given as a rescue therapy.