Alternatively, because age-induced changes in vascular signaling occur over an extended time course, alterations in the relative activity of SOD and catalase could compensate for reduced eNOS-mediated production of authentic NO•. For example, in coronary

arterioles Smoothened antagonist from old and young female rats, treatment with either the SOD mimetic, Tempol (Sigma, St. Louis, MO, USA), or with catalase reduced flow-induced vasodilation and eliminated age-related differences in the maximal response to flow [39]. Treatment with the Cu/Zn SOD inhibitor, diethyldithiocarbamate, enhanced flow-induced vasodilation in arterioles from both young and old rats but did not eliminate age-related differences in flow-induced vasodilation. These findings suggest that with age, the dependence on H2O2-mediated vasodilation increases in coronary arterioles, although an ONOO•− component of the dilation persists. In contrast, in skeletal muscle arterioles from rats, H2O2-mediated vasodilation to flow decreases with age [40,85]. The source of ROS that act as signaling molecules in the aged microvascular endothelium has not been definitively determined;

however, recent reports indicate that an imbalance of ROS is a critical contributor to age-induced endothelial dysfunction in rodents [40,78,92]. Trott et al. [92] reported that either inhibition of NAD(P)H oxidase or scavenging of O2•− improved endothelial AT9283 molecular weight function in skeletal muscle

feed arteries of aged rats. These results imply Protein kinase N1 that either overproduction of O2•− or inadequate scavenging of O2•− contributes to endothelial dysfunction with age. In contrast, scavenging of endogenous O2•− by addition of exogenous SOD reduced endothelium-dependent vasodilation in arteries from young rats [92]. Similarly, scavenging of O2•− with Tempol impaired flow-mediated vasodilation in coronary arterioles from young but not old rats, indicating that the contribution of this ROS to endothelium-dependent vasodilation changes with age [40]. In coronary arterioles from old rats, endogenous SOD protein increased significantly but this increase in SOD was not paralleled by a rise in catalase protein, resulting in an imbalance of these antioxidant enzymes and overproduction of H2O2 [40]. These results suggest that balanced activity of antioxidant enzymes is necessary for maintenance of endothelial function with advancing age. Recent work also indicates that successful maintenance of endothelial function is critically dependent upon the ability to maintain antioxidant defense mechanisms [45,93,94]. Relocation of SOD-1 to the endothelial mitochondria has been reported to function as a compensatory mechanism that counters increased ROS production in the aged aorta [45].