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

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 (60-83 yr; 30 women) completed testing where MSNA (microneurography), blood pressure (BP), and heart rate (HR) were recorded during baseline and a 2-min CPT (-4 degrees 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, versus 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/100 hbs; 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, DMSNA burst frequency was lower in HW versus 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). Furthermore, DMSNA burst incidence was lower in HW versus LW (9 +/- 13 vs. 28 +/- 16 bursts/100 hbs; P = 0.020), with no differences between HM versus LM (21 +/- 17 vs. 31 +/- 17 bursts/100 hbs; 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. Although the underlying mechanisms remain unknown, altered sympathetic recruitment or neurovascular transduction may contribute to these disparate responses.