These results suggest that treatment selleck chemicals with exogenous SOD may drive overproduction of H2O2 and promote formation of HO• in the endothelium. Deferoxamine alone reversed impairment of flow-induced

vasodilation in coronary arterioles from old rats, but had no effect on arterioles from young rats [40], suggesting that flow stimulates production of HO• in arterioles from old but not young rats. Similarly, deferoxamine reversed Tempol-induced reduction of flow-induced vasodilation in skeletal muscle of old rats [78]. Together these data suggest that although H2O2 may function as an important endothelium-dependent vasodilator, production of H2O2 that exceeds the buffering capacity of the endothelium can impair endothelial function, and this is likely due to excess production of HO•. The age-related increase in production of HO• could result from (1) an age-associated selleck screening library decrease in the activities of catalase and/or peroxidases in the endothelium, (2) an age-induced increase

in the activity of SOD isoforms, or (3) increased accumulation of Fe2+ in the aged endothelium. It is also possible that accumulation of Fe2+ is accompanied by a relative imbalance in the activities of SOD and catalase. Several in vivo models have been used to study vascular aging in humans. Doppler methods for determination of cutaneous blood flow and blood flow in large/medium size upper body arteries are the most commonly employed models [1,11,28,36]. In general, these models have assessed the participation of NO• in vascular reactivity

using NOS inhibition (i.e., l-NAME or l-NNMA). Interestingly, these studies have shown conflicting results, which could be associated with differences Tyrosine-protein kinase BLK in the vascular beds being studied and differences in the stimuli employed to trigger vasodilation, e.g., acetylcholine vs. cuff occlusion methods. Both Green et al. [28] and Casey et al. [11] have shown an age-dependent decrease in NO•-mediated forearm blood flow during exercise. In contrast, Holowatz et al. [34,35] have shown an increase in NO•-dependent, cutaneous vasodilation in the elderly. Despite these conflicting results, all these studies concluded that reduced NO• bioavailability would be the principal cause of age-related impairment of vascular reactivity [11,34,35]. Compensatory vasodilation that occurs in response to a stressor such as hypoxic exercise is blunted in aged subjects [10,11]. Casey et al. [11] reported that eNOS inhibition reduced the vascular response to hypoxemic exercise in young but not in old subjects, suggesting that the age-related reduction of this vasodilatory response occurred as a result of impaired NO• signaling.