Age-related differences in the sympathetic-hemodynamic balance in men.

As humans age, the tonic level of activity in sympathetic vasoconstrictor nerves increases and may contribute to age-related increases in blood pressure. In previous studies in normotensive young men with varying levels of resting sympathetic nerve activity, we observed a balance among factors contributing to blood pressure regulation, such that higher sympathetic activity was associated with lower cardiac output and lesser vascular responsiveness to alpha-adrenergic agonists, which limited the impact of high sympathetic activity on blood pressure. In the present study, we tested the hypothesis that older normotensive men would exhibit a similar balance among these variables (sympathetic nerve activity, cardiac output, and alpha-adrenergic responsiveness) but that this balance would be shifted toward higher sympathetic nerve activity values. We measured muscle sympathetic nerve activity, cardiac output, arterial pressure, and forearm vasoconstrictor responses in 17 older men and compared these with previous data collected in 14 younger men. Muscle sympathetic activity (burst incidence) was positively related to diastolic blood pressure in the older men (r=0.49; P=0.05); this relationship was not observed in young men. In addition, there was no relationship between cardiac output and muscle sympathetic activity (r=0.29; P>0.05) or between muscle sympathetic activity and vasoconstrictor responses in the older men (eg, norepinephrine: r=-0.21; P>0.05). Although our older subjects were normotensive, the relationship between muscle sympathetic nerve activity and diastolic blood pressure and the lack of "balance" among the other variables suggest that these changes with aging may contribute to the risk of sympathetically mediated hypertension in older humans.