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Oxidative Stress Does Not Modulate Metabolic Rate or Skeletal Muscle Sympathetic Activity with Primary Aging in Adult Humans

Department of Integrative Physiology (C.B., P.P.J., D.R.S.), University of Colorado, Boulder, Colorado 80309-0354; and Department of Medicine (D.R.S.), University of Colorado Health Sciences Center, Denver, Colorado 80262

Address all correspondence and requests for reprints to: Christopher Bell, Ph.D., Department of Integrative Physiology, 354UCB, University of Colorado, Boulder, Colorado 80309-0354. E-mail: Christopher.Bell{at}Colorado.edu.

Support of resting metabolic rate (RMR) by the ß-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 ß-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 µmol/liter) and older (70 ± 6 to 1022 ± 63 µmol/liter) adults, and reduced (P = 0.001) plasma concentrations of isoprostanes (young, -82.8 ± 47; older, -107 ± 29 pg/ml). Baseline RMR_FFM 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 RMR_FFM (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.

This research was supported by NIH Grants AG06537, AG00828, 1 P30 DK48520, and AHA 0225438Z.

Abbreviations: ß-AR, ß-Adrenergic receptor; ECG, electrocardiogram; FFM, fat free mass; MSNA, muscle sympathetic nerve activity; RMR, resting metabolic rate.

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