Greater resting muscle sympathetic nerve activity reduces cold pressor autonomic reactivity in older women, but not older men

Abstract

Previous work demonstrates augmented muscle sympathetic nerve activity (MSNA) responses to the cold pressor test (CPT) in older women. Given its interindividual variability, however, the influence of baseline MSNA on CPT reactivity in older adults remains unknown. Sixty volunteers (60-83y; 30 women) completed testing where MSNA (microneurography), blood pressure (BP), and heart rate (HR) were recorded during baseline and a 2-min CPT (~4°C). Participant data were terciled by baseline MSNA (n=10/group); comparisons were made between the high baseline men (HM) and women (HW), and low baseline men (LM) and women (LW). By design, HM and HW, vs. LM and LW, had greater baseline MSNA burst frequency (37±5 and 38±3 vs. 9±4 and 15±5 bursts/min) and burst incidence (59±14 and 60±8 vs. 16±10 and 23±7 bursts/100hbs; both P<0.001). However, baseline BP and HR were not different between the groups (all P>0.05). During the CPT, there were no differences in the increase in BP and HR (all P>0.05). Conversely, ΔMSNA burst frequency was lower in HW vs. LW (8±9 vs. 22±12 bursts/min; P=0.012) yet was similar in HM vs. LM (17±12 vs. 19±10 bursts/min, P=0.994). Further, ΔMSNA burst incidence was lower in HW vs. LW (9±13 vs. 28±16 bursts/100hbs; P=0.020), with no differences between HM vs. LM (21±17 vs. 31±17 bursts/100hbs; P=0.455). Our findings suggest that heightened baseline activity in older women attenuates the typical CPT-mediated increase in MSNA without changing cardiovascular reactivity. While the underlying mechanisms remain unknown, altered sympathetic recruitment or neurovascular transduction may contribute to these disparate responses.

Keywords: blood pressure; sex differences; stress responses; sympathetic activity.

Figure 1.

Representative tracings for finger blood pressure (BP), heart rate, and integrated muscle sympathetic nerve activity (iMSNA) during baseline and the cold pressor test (CPT). Tracings are from a high baseline man (A), high baseline woman (B), low baseline man (C), and low baseline woman (D).

Figure 2.

Cold pressor test (CPT) responses for finger systolic blood pressure (SBP; A), finger diastolic blood pressure (DBP; B), heart rate (C), stroke volume (D), cardiac output (E), and systemic vascular resistance (SVR; F) in older men and older women with high and low baseline (BL) muscle sympathetic nerve activity. Data are expressed as the change (Δ) in each value from baseline. Each variable includes a box and whiskers plot of the average 2-min CPT response for visualization. Data were analyzed using linear mixed effects models for the CPT reactivity time response (shaded gray region). Sample size for each group and variable was n = 10. Data are presented as means ± SD for the time response and as the median with 25th and 75th percentiles for the average response.

Figure 3.

Cold pressor test (CPT) responses for muscle sympathetic nerve activity (MSNA) burst frequency (A), burst incidence (B), average amplitude per burst (C), and total activity (D) in older men and older women with high and low baseline (BL) MSNA. Data are expressed as the change (Δ) in each value from baseline. Each variable includes a box and whiskers plot of the average 2-min CPT response for visualization. Data were analyzed using linear mixed effects models for the CPT reactivity time response (shaded gray region). Sample size for each group and variable was n = 10, except for amplitude and total activity, whereby only 9 older women with high baseline activity (inadequate signal quality for one participant) and 9 older men with low baseline activity (one participant determined as outlier using Grubbs test) were included. Data are presented as means ± SD for the time response and as the median with 25th and 75th percentiles for the average response. †P < 0.05 for group post hoc multiple comparison between high baseline women and low baseline women; §P < 0.05 for group post hoc multiple comparison between high baseline women and low baseline men.