Tetrahydrobiopterin and dysfunction of endothelial nitric oxide synthase in coronary arteries

The L-arginine/nitric oxide pathway plays a key role in the regulation of arterial tone. Biosynthesis of nitric oxide requires activation of nitric oxide synthase in the presence of tetrahydrobiopterin as a cofactor. Biochemical studies demonstrated that activation of purified nitric oxide synthase at suboptimal concentrations of tetrahydrobiopterin leads to production of hydrogen peroxide. The present experiments were designed to determine whether in coronary arteries inhibition of tetrahydrobiopterin synthesis may favor nitric oxide synthase-catalyzed production of hydrogen peroxide.

Primary branches of canine left anterior descending artery were incubated for 6 hours in minimum essential medium in the presence or in the absence of the tetrahydrobiopterin synthesis inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP; 10(-2) mol/L). Arterial rings were suspended for isometric tension recording. Production of cGMP was measured by radioimmunoassay. Experiments were performed in the presence of indomethacin (10(-5) mol/L). During contractions to the thromboxane A2/prostaglandin H2 receptor agonist U46619 (10(-7) mol/L), calcium ionophore A23187 (10(-9) to 10(-6) mol/L) caused endothelium-dependent relaxations. A nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (3 x 10(-4) mol/L), significantly inhibited these relaxations. In DAHP-treated arteries, relaxations to A23187 and its stimulating effect on cGMP production were significantly reduced in the presence of catalase (1200 U/mL). By contrast, catalase did not exert any effect in rings incubated in the absence of DAHP. Furthermore, the inhibitory effect of catalase on A23187-induced relaxations was abolished when coronary arteries were incubated in the presence of DAHP plus a liposoluble analogue of tetrahydrobiopterin, 6-methyltetrahydropterin (10(-4) mol/L).

The present study suggests that hydrogen peroxide may be a mediator of endothelium-dependent relaxations in coronary arteries depleted of tetrahydrobiopterin. This initially compensatory response, triggered by a dysfunctional nitric oxide synthase, may represent an important mechanism underlying oxidative vascular injury.