INHIBITORY ACTIONS OF DIPHENYLENEIODONIUM ON ENDOTHELIUM-DEPENDENT VASODILATATIONS IN-VITRO AND IN-VIVO

1 This study examined the in vitro and in vivo inhibitory effects of d iphenyleneiodonium (DPI), a novel inhibitor of nitric oxide (NO) synth ase, on endothelium-dependent vasodilatations. 2 DP1 (3 x 10(-8) 3 x 1 0(-6) M ) concentration-dependently inhibited acetylcholine (ACh)-indu ced relaxation in preconstricted rat thoracic aortic rings, with an IC 50 of 1.8 x 10(-7) M and a maximal inhibition of nearly 100%. DPI (3 x 10(-6) m) also completely inhibited the relaxation induced by the cal cium ionophore, A23187 but not by sodium nitroprusside (SNP). The inhi bitory effect of DPI (3 x 10(-7) M) on ACh-induced relaxation was prev ented by pretreatment with NADPH (5 x 10(-3) M) and FAD (5 x 10(-4) M) but not L-arginine (L-Arg, 2 x 10(-3) M). Pretreatment with NADPH did not alter the inhibitory effect of N(G)-nitro-L-arginine on ACh-induc ed relaxation. 3 The inhibitory effect of DPI on ACh-induced relaxatio n in the aortae lasted >4 h after washout. In contrast to pretreatment , post-treatment (1 h later) with NADPH (5 x 10(-3) m) reversed only s lightly the inhibitory effect of DPI. 4 In conscious rats, DPI (10(-5) mol kg-1) inhibited the depressor response to i.v. infused ACh, but n ot SNP. However. it caused only a transient pressor response which was previously shown to be due completely to sympathetic activation. 5 Th us, DPI is an efficacious and 'irreversible' inhibitor of endothelium- dependent vasodilatation in vivo and in vitro. The mechanism of the in hibition may involve antagonism of the effects of FAD and NADPH, co-fa ctors of NO synthase. However, unlike the N(G)-substituted arginine an alogues (another class of NO synthase inhibitors), DPI-induced suppres sion of endothelium-dependent vasodilatation in vivo does not lead to a sustained rise in blood pressure.