Differential sympathetic activation in muscle and skin neural districts in the metabolic syndrome

The present study was designed to determine whether and to what extent the activation of the sympathetic nervous system reported in the metabolic syndrome is generalized to the whole cardiovascular system or if it is rather confined to selected vascular districts. In 16 untreated patients with metabolic syndrome, 12 essential hypertensive subjects, 12 obese subjects, and 14 lean healthy normotensive controls, we measured blood pressure (Finapres, Englewood, CO), heart rate (electrocardiogram), venous plasma norepinephrine (high-performance liquid chromatography), and postganglionic sympathetic nerve traffic in the skeletal muscle and in the skin districts (microneurography). The muscle and skin nerve traffic measurements were obtained in a randomized sequence. Measurements also included skin sympathetic nerve responses to an arousal (acoustic stimulus). The 4 groups of subjects had superimposable ages. Muscle sympathetic nerve traffic values were significantly higher in subjects with hypertension and in those with obesity than in controls (51.2 ± 2.8 and 52.0 ± 3.0 vs 37.2 ± 3.3 bursts per 100 heart beats, respectively; P <.01 for all). A further significant increase in muscle sympathetic nerve traffic was detected in subjects with the metabolic syndrome (61.0 ± 3.2 bursts per 100 heart beats, P <.05). In contrast, skin sympathetic nerve traffic was not significantly different in the 4 groups of individuals (13.0 ± 0.7, 14.3 ± 1.3, 12.5 ± 0.8 vs 15.4 ± 1.0 bursts per minute, respectively; P = not significant). The skin sympathetic responses to an acoustic stimulus were also similar in the different groups. The present data provide the first direct evidence that in the metabolic syndrome the sympathetic activation is not uniformly distributed over the cardiovascular system. This may depend on the fact that muscle and skin sympathetic nerve activities are regulated by mechanisms that are affected in a different fashion by the various components of the disease.