Adiposity-independent sympathetic activity in black men

Abstract

Obesity is thought to lead to sympathetic overactivity as a compensatory adjustment to weight gain. However, most of the experimental support for the hypothesis has been derived from white cohorts. Our previous study in blacks indicated that sympathetic nerve activity (SNA) is closely correlated with body mass index only in women, whereas, in black men, SNA is elevated and dissociated from adiposity (Abate et al., Hypertension 38: 379-383, 2001). To further determine whether total and regional adiposity are determinants of SNA in blacks, we performed a prospective weight loss study in 12 normotensive obese black men and 9 obese black women. SNA, body mass index, and abdominal fat mass were measured before and 16 wk after hypocaloric diet. The major new findings are that, in obese black men, the dietary-induced weight loss of 11.3+/-0.8 kg resulted in reduction in plasma leptin, insulin, and visceral abdominal fat but had no effect on SNA (from baseline of 26+/-4 to 28+/-3 bursts/min, P=not significant). In contrast, in black women, weight loss of 8.0+/-0.9 kg caused similar reductions in plasma leptin, insulin, and visceral abdominal fat and led to a reduction in SNA by 40% (from baseline of 22+/-2 to 13+/-3 bursts/min, P<0.05). In conclusion, these new data from this prospective study provide strong support for a major adiposity-independent sympathetic activity in black men and adiposity-related sympathetic activity in black women.

Fig. 1.

Recordings of muscle sympathetic nerve activity (SNA) in a 32-year-old black woman (top) and a 24-year-old black man (bottom) studied at baseline and 16 wk after dietary weight loss. On these mean-voltage displays of muscle sympathetic-nerve activity to the muscle, each peak represents a spontaneous burst of sympathetic nerve discharge. In the 32-year-old black woman, SNA reduced by 50% after 16 wk of hypocaloric diet, whereas SNA remained unchanged in the 24-year-old black man despite a similar amount of weight loss.

Fig. 2.

A: summary data of all subjects showing muscle SNA in bursts/min and bursts/100 heart beats (bursts/110RR). B: total and visceral abdominal fat measured by MRI in black women (n = 9) and black men (n = 12) at baseline and 16 wk after dietary weight loss. *P < 0.05 vs. baseline. Data are expressed as means ± SE.