Influence of age and sex on the pressor response following a spontaneous burst of muscle sympathetic nerve activity

Abstract

The sympathetic nervous system is critical for the beat-to-beat regulation of arterial blood pressure (BP). Although studies have examined age- and sex-related effects on BP control, findings are inconsistent and limited data are available in postmenopausal women. In addition, the majority of studies have focused on time-averaged responses without consideration for potential beat-to-beat alterations. Thus we examined whether the ability of muscle sympathetic nerve activity (MSNA) to modulate BP on a beat-to-beat basis is affected by age or sex. BP and MSNA were measured during supine rest in 40 young (20 men) and 40 older (20 men) healthy subjects. Beat-to-beat fluctuations in mean arterial pressure (MAP) were characterized for 15 cardiac cycles after each MSNA burst using signal averaging. The rise in MAP following an MSNA burst was similar between young men and women (+2.64 ± 0.3 vs. +2.57 ± 0.3 mmHg, respectively). However, the magnitude of the increase in MAP after an MSNA burst was reduced in older compared with young subjects (P < 0.05). Moreover, the attenuation of the pressor response was greater in older women (+1.20 ± 0.1 mmHg) compared with older men (+1.72 ± 0.2 mmHg; P < 0.05). Interestingly, in all groups, MAP consistently decreased after cardiac cycles without MSNA bursts (nonbursts) with the magnitude of fall greatest in older men. In summary, healthy aging is associated with an attenuated beat-to-beat increase in BP after a spontaneous MSNA burst, and this attenuation is more pronounced in postmenopausal women. Furthermore, our nonburst findings highlight the importance of sympathetic vasoconstrictor activity to maintain beat-to-beat BP, particularly in older men.

Fig. 1.

Examples of the signal averaging for a young and older man (A) and a young and older woman (B) illustrating the changes in mean arterial pressure (MAP) following spontaneous bursts of muscle sympathetic nerve activity (MSNA). The dashed lines represent the average MAP response for 10 MSNA bursts, and the thick line represents the mean MAP response for all MSNA bursts in the baseline segment.

Fig. 2.

Summary data for young and older men and women showing beat-to-beat (A) and peak (B) changes in MAP following a MSNA burst. Values are means ± SE. *Significantly different from young men and women (P < 0.05); †significantly different from older men (P < 0.05).

Fig. 3.

Summary data for young and older men and women showing beat-to-beat (A) and peak (B) changes in MAP following cardiac cycles that were not associated with MSNA burst (i.e., nonburst). Values are means ± SE. ‡Significantly different from older women and young men and women (P < 0.05).

Fig. 4.

Summary data for young and older men and women showing beat-to-beat (left) and peak (right) changes in total vascular conductance (TVC) for cardiac cycles with (A) and without (B) a MSNA burst. Values are means ± SE. *Significantly different from young men and women (P < 0.05).

Fig. 5.

Summary data for young and older men and women showing beat-to-beat changes in cardiac output (CO) for cardiac cycles with (A) and without (B) a MSNA burst. Values are means ± SE. *Significantly different from young men and women (P < 0.05).

Fig. 6.

Summary data showing beat-to-beat changes in MAP following a MSNA burst when taking burst size into consideration for young and older men and women. Each subject had their sympathetic bursts divided into quartiles according to the height of the burst. Values are means ± SE.