Superoxide Excess in Hypertension and Aging: A Common Cause of Endothelial Dysfunction?
Objective: Excess superoxide (O2-) contributes to the endothelial dysfunction in 4 month (mo) old stroke prone spontaneously hypertensive rats (SHRSP). We sought to determine the relationship between endothelial function and age in normotensive Wistar-Kyoto (WKY) and SHRSP. Methods: Aortic rings were removed from female WKY and SHRSP at 4mo and 12mo of age. Nitric oxide (NO) bioavailability was measured by the size of contraction produced in response to the nitric oxide synthase (NOS) inhibitor L-NAME (0.1mmol/L). O2- generation by aortic rings was measured by lucigenin chemiluminescence before and after removal of the endothelium, or incubation with L-NAME (0.1mmol/L), or the NAD(P)H oxidase inhibitor diphenyleneiodinium (DPI, 0.1mmol/L). Results: Contraction to L-NAME (% of PE ± SEM) was significantly lower in 12mo WKY (296 ± 30, n=11) compared to 4mo WKY (523 ± 51, n=15; P=0.0009) and in also in 12mo SHRSP (220 ± 28, n=8) compared to 4mo SHRSP (341 ± 26, n=16; P=0.005). O2- generation (nmol/min/mg ± SEM) was significantly greater in 12mo WKY (2.83 ± 0.30, n=21) compared to 4mo WKY (1.06 ± 0.20, n=7; P=0.0001), but the difference between 12mo SHRSP (3.44 ± 0.31, n=13) and 4mo SHRSP (2.98 ± 0.49, n=9) did not achieve significance. Removal of the endothelium resulted in a significant reduction in O2- generation in 12mo SHRSP (2.49 ± 0.18 v 3.35 ± 0.29, n=12; P=0.009). In 12mo WKY the reduction was not statistically significant. L-NAME significantly reduced O2- generation in 12mo SHRSP (2.04 ± 0.54 v 1.5 ± 0.41, n=5; P=0.03), but made no difference to 12mo WKY. DPI significantly reduced O2- generation in 12mo WKY (2.83 ± 0.28 v 1.42 ± 0.27, n=4; P=0.009) and also 12mo SHRSP (2.25 ± 0.0.43 v 0.64 ± 0.07, n=7; P=0.01). Similar increases in O2- generation were observed with age and hypertension in carotid arteries (WKY: 4mo 0.88 ± 0.18, 12mo 3.88 ± 0.50; SHRSP: 4mo 3.35 ± 0.46, 12mo 4.89 ± 0.55). Conclusion: NO bioavailability decreases with age in WKY and SHRSP females. O2- generation increases with age in WKY and may contribute to the reduced NO by scavenging. Both NOS and NAD(P)H oxidases appear to be important contributors to the age related increase in O2-.