Sympathetic activity during passive heat stress in healthy aged humans

Abstract

Key points: Cardiovascular adjustments to heat stress are attenuated in healthy aged individuals, which could contribute to their greater prevalence of heat-related illnesses and deaths during heat waves. The attenuated cardiovascular adjustments in the aged could be due to lower increases in sympathetic nerve activity during heat stress. We examined muscle sympathetic nerve activity (MSNA) and plasma catecholamine concentrations in healthy young and aged individuals during whole-body passive heat stress. The main finding of this study is that increases in MSNA and plasma catecholamine concentrations did not differ between young and aged healthy individuals during passive heating. Furthermore, the increase in these variables did not differ when a cold pressor test and lower body negative pressure were superimposed upon heating. These findings suggest that attenuated cardiovascular adjustments to heat stress in healthy aged individuals are unlikely to be related to attenuated increases in sympathetic activity.

Abstract: Cardiovascular adjustments during heat stress are generally attenuated in healthy aged humans, which could be due to lower increases in sympathetic activity compared to the young. We compared muscle sympathetic nerve activity (MSNA) between 11 young (Y: 28 ± 4 years) and 10 aged (A: 70 ± 5 years) subjects prior to and during passive heating. Furthermore, MSNA responses were compared when a cold pressor test (CPT) and lower body negative pressure (LBNP) were superimposed upon heating. Baseline MSNA burst frequency (Y: 15 ± 4 vs. A: 31 ± 3 bursts min(-1) , P ≤ 0.01) and burst incidence (Y: 26 ± 8 vs. A: 50 ± 7 bursts (100 cardiac cycles (CC))(-1) , P ≤ 0.01) were greater in the aged. Heat stress increased core temperature to a similar extent in both groups (Y: +1.2 ± 0.1 vs. A: +1.2 ± 0.0°C, P = 0.99). Absolute levels of MSNA remained greater in the aged during heat stress (burst frequency: Y: 47 ± 6 vs. A: 63 ± 11 bursts min(-1) , P ≤ 0.01; burst incidence: Y: 48 ± 8 vs. A: 67 ± 9 bursts (100 CC)(-1) , P ≤ 0.01); however, the increase in both variables was similar between groups (both P ≥ 0.1). The CPT and LBNP further increased MSNA burst frequency and burst incidence, although the magnitude of increase was similar between groups (both P ≥ 0.07). These results suggest that increases in sympathetic activity during heat stress are not attenuated in healthy aged humans.

Figure 1

Effect of whole-body passive heat stress on skin and core temperatures of healthy young and aged subjects Mean skin (left panel) and core (right panel) temperatures at normothermic baseline and during whole-body passive heat stress sufficient to elevate core temperature by 0.6°C (+0.6°C) and 1.2°C (+1.2°C) in healthy young (n = 11) and aged (n = 10) subjects. Values are mean ± 95% confidence intervals. ^*Significantly different from baseline (P ≤ 0.05).

Figure 2

Effect of whole-body passive heat stress on heart rate and mean arterial pressure in healthy young and aged subjects Heart rate (left panel) and mean arterial pressure (right panel) at normothermic baseline and during whole-body passive heat stress sufficient to elevate core temperature by 0.6°C (+0.6°C) and 1.2°C (+1.2°C) in healthy young (n = 11) and aged (n = 10) subjects. Values are mean ± 95% confidence intervals. ^*Significantly different from baseline (P ≤ 0.05). ^†Significantly different between young and aged (P ≤ 0.05). ^‡Significantly different change from baseline between young and aged (P ≤ 0.05).

Figure 3

Effect of whole-body passive heat stress on MSNA burst frequency and burst incidence in healthy young and aged subjects Muscle sympathetic nerve activity burst frequency (left panel) and burst incidence (right panel) at normothermic baseline and during whole-body passive heat stress sufficient to elevate core temperature by 0.6°C (+0.6°C) and 1.2°C (+1.2°C) in healthy young (n = 11) and aged (n = 10) subjects. Values are mean ± 95% confidence intervals. ^*Significantly different from baseline (P ≤ 0.05). ^†Significantly different between young and aged (P ≤ 0.05).

Figure 4

Effect of whole-body passive heat stress on total MSNA in healthy young and aged subjects Changes from normothermic baseline in total muscle sympathetic nerve activity during whole-body passive heat stress sufficient to elevate core temperature by 0.6°C (+0.6°C) and 1.2°C (+1.2°C) in healthy young (n = 9) and aged (n = 7) subjects. Values are mean ± 95% confidence intervals. ^*Significantly different from baseline (P ≤ 0.05).

Figure 5

Effect of whole-body passive heat stress on plasma adrenaline and noradrenaline concentrations in healthy young and aged subjects Plasma adrenaline (left panel) and noradrenaline (right panel) concentrations at normothermic baseline and during whole-body passive heat stress sufficient to elevate core temperature by 0.6°C (+0.6°C) and 1.2°C (+1.2°C) in healthy young (n = 11) and aged (n = 10) subjects. Values are mean ± 95% confidence intervals. ^*Significantly different from baseline (P ≤ 0.05). ^†Significantly different between young and aged (P ≤ 0.05).