Oxidative stress does not modulate metabolic rate or skeletal muscle sympathetic activity with primary aging in adult humans

Support of resting metabolic rate (RMR) by the beta-adrenergic receptors of the sympathetic nervous system is attenuated with age and contributes to declines in RMR. This may be mediated by an age-associated increase in oxidative stress that can suppress beta-adrenergic responsiveness and/or modulate sympathetic activity. To address these issues, RMR was determined in 12 young (23 +/- 1 yr, mean +/- SE) and 21 older (68 +/- 3 yr) adults before and during systemic infusion of ascorbic acid [bolus, 0.06 g/kg fat free mass (FFM); drip, 0.02]. Ascorbic acid increased plasma concentrations similarly in young (72 +/- 5 to 1107 +/- 114 mumol/liter) and older (70 +/- 6 to 1022 +/- 63 mumol/liter) adults, and reduced (P = 0.001) plasma concentrations of isoprostanes (young, -82.8 +/- 47; older, -107 +/- 29 pg/ml). Baseline RMRFFM was lower (5719 +/- 215 vs. 6703 +/- 328 kJ/d; P = 0.001) and muscle sympathetic nerve activity (MSNA) was greater (MSNA, 28 +/- 2 vs. 23 +/- 3 bursts/min; P < 0.05) in older compared with young. However, neither RMRFFM (young, +117 +/- 63; older, +163 +/- 48 kJ/d; P = 0.14) or MSNA (young, 0 +/- 2; older, -1 +/- 1 bursts/min; P = 0.71) changed in either age group during ascorbic acid infusion compared with saline control. These results indicate that increased oxidative stress: 1) is not a mechanism contributing to decreases in RMR with primary aging; and 2) does not modulate MSNA in healthy adult humans.