Ethnic differences in insulinemia and sympathetic tone as links between obesity and blood pressure.

Hyperinsulinemia and increased sympathetic nervous system (SNS) activity are thought to be pathophysiological links between obesity and hypertension. In the present study, we examined the relation among heart rate (HR), blood pressure (BP), and percent body fat (hydrodensitometry or DEXA), fasting plasma insulin concentration, and muscle sympathetic nerve activity (MSNA, microneurography) in male, normotensive whites (n=42) and Pima Indians (n=77). Pima Indians have a high prevalence of obesity and hyperinsulinemia but a relatively low prevalence of hypertension. Compared with whites, Pima Indian men had a higher percent body fat (28% versus 21%) and higher fasting insulin concentrations (210 versus 132 pmol/L) but lower MSNA (27 versus 33 bursts/min) (all P<0.001). In both ethnic groups, HR and BP were positively related to percent body fat and MSNA, and both were significant independent determinants of HR and BP in multiple regression analyses. However, MSNA was positively related to percent body fat and the fasting insulin concentration in whites (r=0.60 and r=0.47, both P<0.01) but not in Pima Indians (r=0.15 and r=0.03, NS) (P<0.01 for ethnic differences in the slope of the regression lines). These results confirm the physiological importance of the SNS in normal BP regulation but indicate that the roles of hyperinsulinemia and increased SNS activity as mediators for the relation between obesity and hypertension can differ between different ethnic groups. The lack of an increase in SNS activity with increasing adiposity and insulinemia in Pima Indians may contribute to the low prevalence of hypertension in this population.