Superoxide-dependent cerebrovascular effects of homocysteine.

Recent evidence indicates that elevated plasma levels of homocysteine are a risk factor for ischemic cerebrovascular diseases. However, little is known about cerebrovascular effects of homocysteine. Homocysteine could impair cerebrovascular function by metal-catalyzed production of activated oxygen species. We studied whether homocysteine, in the presence of Cu2+, alters reactivity of cerebral circulation and, if so, whether this effect depends on[Formula: see text] generation. In halothane-anesthetized rats the parietal cortex was exposed and superfused with Ringer solution. Cerebrocortical blood flow (CBF) was monitored by a laser-Doppler probe. With Ringer solution superfusion, CBF increased with hypercapnia (+134 ± 7%;[Formula: see text] = 50-60 mmHg) and topical application of 10 μM ACh (+35 ± 3%), the NO donor S-nitroso- N-acetylpenicillamine (SNAP, 500 μM; +66 ± 6%), or 1 mM papaverine (+100 ± 6%; n = 5). Superfusion with 40 μM Cu2+ alone did not perturb resting CBF or responses to hypercapnia, ACh, SNAP, or papaverine ( P > 0.05, n = 5). However, superfusion of homocysteine-Cu2+ reduced resting CBF (-28 ± 4%) and attenuated ( P < 0.05) responses to hypercapnia (-31 ± 9%), ACh (-73 ± 6%), or SNAP (-48 ± 4%), but not papaverine. The effect was observed only at 1 mM homocysteine. Cerebrovascular effects of homocysteine-Cu2+ were prevented by coadministration of superoxide dismutase (SOD; 1,000 U/ml; n = 5). SOD alone did not affect resting CBF or CBF reactivity ( n = 5). The observation that homocysteine-Cu2+ attenuates the response to hypercapnia, ACh, and SNAP, but not the NO-independent vasodilator papaverine, suggests that homocysteine-Cu2+ selectively impairs NO-related cerebrovascular responses. The fact that SOD prevents such impairment indicates that the effect of homocysteine is[Formula: see text] dependent. The data support the conclusion that [Formula: see text], generated by the reaction of homocysteine with Cu2+, inhibits NO-related cerebrovascular responses by scavenging NO, perhaps through peroxynitrite formation. [Formula: see text]-mediated scavenging of NO might be one of the mechanisms by which hyperhomocysteinemia predisposes to cerebrovascular diseases.