Muscle sympathetic nerve activity during cold stress and isometric exercise in healthy older adults

Abstract

Cardiovascular mortality increases in cold weather in older adults, and physical activity may impart even greater cardiovascular risk than cold exposure alone. Human aging is associated with exaggerated pressor responses to whole body cooling; however, the sympathetic response to cold stress alone and in combination with isometric exercise is unknown. We hypothesized that cold stress would 1) increase muscle sympathetic nerve activity (MSNA) and 2) augment the MSNA response to isometric handgrip in older adults. Whole body cooling (water-perfused suit) was conducted in 11 young (23 ± 1 yr) and 12 healthy older adults (60 ± 2 yr). Blood pressure (BP; Finometer) and MSNA (microneurography) were measured throughout cooling and during isometric handgrip at 30% maximal voluntary contraction performed at a mean skin temperature (Tsk) of 34 and 30.5°C. MSNA was greater in older adults at Tsk = 34.0°C and throughout cooling (P < 0.05). MSNA increased during cooling in older, but not young, adults (young: Δ0 ± 1 vs. older: Δ8 ± 1 bursts/min; P < 0.05). The cooling-induced increase in BP was greater in older adults (P < 0.05). During handgrip, the increases in MSNA and BP were not different between conditions in either young (Δ14 ± 2 Tsk 34°C vs. Δ12 ± 3 Tsk 30.5°C bursts/min; Δ20 ± 3 Tsk 34°C vs. Δ19 ± 3 Tsk 30.5°C mmHg; both P > 0.05) or older adults (Δ12 ± 1 Tsk 34°C vs. Δ8 ± 1 Tsk 30.5°C bursts/min; Δ18 ± 3 Tsk 34°C vs. Δ17 ± 2 Tsk 30.5°C mmHg; both P > 0.05). In summary, MSNA increased during cold stress in older, but not young, adults. Furthermore, concomitant cold stress did not alter the sympathetic responses to isometric exercise in either age group, suggesting preserved sympathetic responsiveness during exercise in the cold in healthy aging.

Keywords: aging; blood pressure; handgrip; whole body cooling.