Norepinephrine turnover and energy expenditure in Pima Indian and white men

There is growing evidence of the involvement of sympathetic nervous system (SNS) activity in determining metabolic rate. Whole-body plasma norepinephrine turnover and its relationship to resting metabolic rate (RMR) and 24-hour energy expenditure (24EE) were compared in 14 Pima Indian men (25 +/- 4 years, 96 +/- 33 kg, 25% +/- 9% fat) and nine white men (25 +/- 3 years, 88 +/- 43 kg, 17% +/- 13% fat). Plasma norepinephrine turnover rate correlated strongly with body surface area (r = .76 and .54 for clearance and appearance, respectively) and fat-free mass (r = .74 and .52, respectively). However, independent of body size, there was no difference in either norepinephrine clearance or appearance rates between Pima Indian and white men. Norepinephrine appearance rate correlated positively with absolute values of 24EE and RMR, but not when adjusted for differences in body surface area or fat-free mass. However, norepinephrine appearance rate adjusted for differences in body size correlated with spontaneous physical activity. The results indicate that Pima Indian and white men have similar plasma norepinephrine appearance rates, but Pima Indians tend to be more resistant to beta-adrenergic stimulation. Although energy expenditure and SNS activity were not directly related, a higher SNS tone may either promote or reflect elevated levels of spontaneous physical activity and therefore influence both energy balance and body composition.