Age and gender influence muscle sympathetic nerve activity at rest in healthy humans

Muscle sympathetic nerve activity at rest increases with age in humans. The respective influences of the aging process per se and gender on this increase and whether age and gender effects on muscle sympathetic nerve activity can be identified with plasma norepinephrine concentrations, however, have not been established. To examine these issues, nine young women (aged 24±1 years; mean±SEM), eight young men (aged 26±1 years), seven older women (aged 63±1 years), and eight older men (aged 66±1 years) were studied. All were healthy, normotensive (blood pressure <140/90 mm Hg), nonobese (<20% above ideal weight), unmedicated, nonsmokers engaged in minimal to recreational levels of chronic physical activity. Arterial blood pressure (manual sphygmomanometry, brachial artery), heart rate, muscle sympathetic nerve activity (peroneal microneurography), and antecubital venous plasma norepinephrine concentrations (radioenzymatic assay) were determined during quiet supine resting conditions. Body weight was higher in men, but there were no age-related differences, whereas estimated body fat (sum of skinfolds) was higher in women and in the older groups (p<0.05). Estimated daily energy expenditure, arterial blood pressure, and heart rate were not different among the groups. Both muscle sympathetic nerve activity burst frequency and burst incidence at rest were progressively higher in the young women, young men, older women, and older men (10±1 versus 18±2 versus 25±3 versus 39±5 bursts/min and 16±1 versus 30±4 versus 40±3 versus 61±6 bursts/100 heartbeats, respectively; all p<0.05 versus each other). In contrast, average levels of plasma norepinephrine concentrations were not different among the groups. There was, however, a positive relation between plasma norepinephrine concentrations and muscle sympathetic nerve activity (r=0.65; p<0.0003) when the individual data were pooled. There were no strong or consistent associations between muscle sympathetic nerve activity and any other variable. We conclude that the rise in muscle sympathetic nerve activity with aging in resting humans appears to be independent of age-related differences in ischemic heart disease, obesity, chronic physical activity, or arterial blood pressure, and thus is likely related to some factor associated with the aging process per se. Gender, however, appears to be an important determinant of muscle sympathetic nerve activity at rest in both young and older humans. Finally, the age- and gender-specific influences on muscle sympathetic nerve activity are not necessarily reflected by plasma norepinephrine concentrations.