Inhibition of endotoxin‐induced vascular hyporeactivity by 4‐amino‐tetrahydrobiopterin

The 4‐amino analogue of tetrahydrobiopterin (4‐ABH 4 ) is a potent pterin‐site inhibitor of nitric oxide synthases (NOS). Although 4‐ABH 4 does not exhibit selectivity between purified NOS isoforms, a pronounced selectivity of the drug towards inducible NOS (iNOS) is apparent in intact cells. This work was carried out to investigate the potential iNOS selectivity of 4‐ABH 4 in isolated pig pulmonary and coronary arteries. Endothelium‐dependent relaxations of pig pulmonary and coronary artery strips to bradykinin or calcium ionophore A23187 were inhibited by 4‐ABH 4 in a concentration‐dependent manner. Half‐maximal inhibition was observed at 60–65 μ M (pulmonary artery) and 200–250 μ M 4‐ABH 4 (coronary artery). Pig coronary artery strips precontracted with 0.1 μ M 9, 11‐dideoxy‐9, 11‐methanoepoxy‐prosta‐glandin F 2α (U46619) showed a time‐dependent relaxation (monitored for up to 18 h) upon incubation with 1 μg ml −1 lipopolysaccharide (LPS). Addition of 10 μ M 4‐ABH 4 1 h after LPS led to a pronounced inhibition of the LPS‐triggered relaxation, whereas the pterin antagonist had no effect when given4 h after LPS. Incubation of pulmonary and coronary artery strips with 1 μg ml −1 LPS attenuated contractile responses to norepinephrine (1 μ M ) and U46619 (0.1 μ M ). This hyporeactivity of the blood vessels to vasoconstrictor agents was inhibited by 4‐ABH 4 in a concentration‐dependent manner [IC 50 =17.5±5.9 μ M (pulmonary artery) and 20.7±3 μ M (coronary artery)]. The effect of 0.1 m M 4‐ABH 4 was antagonized by coincubation with 0.1 m M sepiapterin, which is known to supply intracellular BH 4 via a salvage pathway. These results demonstrate that 4‐ABH 4 is a fairly selective inhibitor of iNOS in an in vitro model of endotoxaemia, suggesting that this drug and/or related pterin‐site NOS inhibitors may be useful to increase blood pressure in severe infections associated with a loss of vascular responsiveness to constrictor agents caused by endotoxin‐triggered iNOS induction in the vasculature.British Journal of Pharmacology (2000) 131 , 1757–1765; doi: 10.1038/sj.bjp.0703752