Age-related increase in muscle sympathetic nerve activity is associated with abdominal adiposity

Tonic sympathetic nerve activity (SNA) increases with age, but the mechanisms are unknown. There is evidence that SNA is positively related to total and abdominal body fat, which also increase with age. We tested the hypotheses that I) the elevation in SNA with age is partially accounted for by higher abdominal and/or total body fat and 2) skeletal muscle is a target of the adiposity-related sympathetic effects. Direct microneurographic recordings of skeletal muscle SNA (MSNA) were obtained during supine rest in 16 older(64 +/- 1 yr,means +/- SE) and 16 young(24 +/- 1 yr) adult males. Central body fat was estimated by waist circumference (WC) and fat mass (FM) by hydrostatic weight. MSNA, WC, and FM were higher in the older vs. young males (44 +/- 2 vs. 22 +/- 2 bursts/min, 91 +/- 2 vs. 79 +/- 1 cm, and 19 +/- 2 vs. 9 +/- 1 kg, respectively; all P < 0.0001). Although univariate correlations were high for MSNA and both WC (r = 0.77) and FM (r = 0.75), stepwise multiple regression analysis revealed WC to be the best predictor of MSNA (R-2 = 0.60, P < 0.0001), with FM explaining only an additional 2% of the variance (not significant). Statistically covarying for WC reduced but did not eliminate the difference in adjusted age-group means for MSNA (39 +/- 3 vs. 26 +/- 2 bursts/min, P = 0.003). We conclude that I)the elevated SNA in older adults is partially related to higher body fat, particularly in the abdominal region, and 2) skeletal muscle is a target of the adiposity-related sympathetic effects observed with aging.