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

1 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. 2 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 muM (pulmonary artery) and 200-250 muM 4-ABH(4) (coronary artery). 3 Pig coronary artery strips precontracted with 0.1 muM 9, 11-dideoxy-9, 11-methanoepoxy-prostaglandin F-2 alpha (U46619) showed a time-dependent relaxation (monitored for up to 18 h) upon incubation with 1 mug ml(-1) lipopolysaccharide (LPS). Addition of 10 muM 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 given greater than or equal to4 h after LPS. 4 Incubation of pulmonary and coronary artery strips with 1 mug ml(-1) LPS attenuated contractile responses to norepinephrine (1 muM) and U46619 (0.1 muM) This hyporeactivity of the blood vessels to vasoconstrictor agents was inhibited by 4-ABH(4) in a concentration-dependent manner [IC50 = 17.5+/-5.9 muM (pulmonary artery) and 20.7+/-3 muM (coronary artery)]. The effect of 0.1 mM 4-ABH(4) was antagonized by coincubation with 0.1 mM sepiapterin, which is known to supply intracellular BH4 via a salvage pathway. 5 These results demonstrate that 4-APH(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.