Aging enhances autonomic support of blood pressure in women

Abstract

The autonomic nervous system plays a central role in both acute and chronic blood pressure regulation in humans. The activity of the sympathetic branch of the autonomic nervous system is positively associated with peripheral resistance, an important determinant of mean arterial pressure in men. In contrast, there is no association between sympathetic nerve activity and peripheral resistance in women before menopause, yet a positive association after menopause. We hypothesized that autonomic support of blood pressure is higher after menopause in women. We examined the effect of ganglionic blockade on arterial blood pressure and how this relates to baseline muscle sympathetic nerve activity in 12 young (25±1 years) and 12 older postmenopausal (61±2 years) women. The women were studied before and during autonomic blockade using trimethaphan camsylate. At baseline, muscle sympathetic nerve activity burst frequency and burst incidence were higher in the older women (33±3 versus 15±1 bursts/min; 57±5 versus 25±2 bursts/100 heartbeats, respectively; P<0.05). Muscle sympathetic nerve activity bursts were abolished by trimethaphan within minutes. Older women had a greater decrease in mean arterial pressure (-29±2 versus -9±2 mm Hg; P<0.01) and total peripheral resistance (-10±1 versus -5±1 mm Hg/L per minute; P<0.01) during trimethaphan. Baseline muscle sympathetic nerve activity was associated with the decrease in mean arterial pressure during trimethaphan (r=-0.74; P<0.05). In summary, our results suggest that autonomic support of blood pressure is greater in older women compared with young women and that elevated sympathetic nerve activity in older women contributes importantly to the increased incidence of hypertension after menopause.

Keywords: blood pressure; menopause; sympathetic nerve activity.

Figure 1

Compressed experimental recording from a representative subject before and during ganglionic blockade. A close-up of the MSNA and blood pressure recording is shown in the box. Note the disappearance of sympathetic bursts during ganglionic blockade with trimethaphan (TMP) infusion.

Figure 2

Age-related differences in the magnitude of change in hemodynamic variables between control and trimethaphan (TMP). CO=cardiac output; HR=heart rate; MAP=mean arterial pressure; and TPR=total peripheral resistance. Mean ± SEM, *p<0.05 vs. Young.

Figure 3

Pearson product correlations between autonomic nervous system measurements and the change in mean arterial pressure (MAP) during trimethaphan (TMP). Closed symbols indicate young women and open symbols indicate older women. MSNA=muscle sympathetic nerve activity, shown as burst incidence; NE=norepinephrine. The association between MSNA burst frequency and the change in MAP was also significant.