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

Abstract

Greaney JL, Stanhewicz AE, Kenney WL, Alexander LM. Muscle sympathetic nerve activity during cold stress and isometric exercise in healthy older adults. J Appl Physiol 117: 648 -657, 2014. First published August 7, 2014 doi:10.1152/japplphysiol.00516.2014.-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 Ϯ 1 yr) and 12 healthy older adults (60 Ϯ 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 (T sk) of 34 and 30.5°C. MSNA was greater in older adults at T sk ϭ 34.0°C and throughout cooling (P Ͻ 0.05). MSNA increased during cooling in older, but not young, adults (young: ⌬0 Ϯ 1 vs. older: ⌬8 Ϯ 1 bursts/min; P Ͻ 0.05). The cooling-induced increase in BP was greater in older adults (P Ͻ 0.05). During handgrip, the increases in MSNA and BP were not different between conditions in either young (⌬14 Ϯ 2 T sk 34°C vs. ⌬12 Ϯ 3 Tsk 30.5°C bursts/min; ⌬20 Ϯ 3 Tsk 34°C vs. ⌬19 Ϯ 3 Tsk 30.5°C mmHg; both P Ͼ 0.05) or older adults (⌬12 Ϯ 1 Tsk 34°C vs. ⌬8 Ϯ 1 Tsk 30.5°C bursts/min; ⌬18 Ϯ 3 Tsk 34°C vs. ⌬17 Ϯ 2 Tsk 30.5°C mmHg; both P Ͼ 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. blood pressure; aging; handgrip; whole body cooling AGING IS A PRIMARY RISK FACTOR for cardiovascular disease, the leading cause of death in the United States (18). Furthermore, cardiovascular-related morbidity and mortality increase at low ambient temperatures, especially in older adults (5,38,44,50,63). The acute systemic physiological responses to cold exposure include peripheral and visceral vasoconstriction, elevated plasma norepinephrine, and increased blood pressure (BP) (9,52,62). Therefore, alterations in the acute responses to cold may contribute to the clinical relation between cold temperatures and increased cardiovascular-related mortality in primary aging. Perturbations that further challenge systemic physiological function during cold exposure, such as static or dynamic exercise, may result in increased sympathetic activation and excessive increases in BP, imparting even greater risk for an acute cardiovascular event in older adults. As such, understanding age-related changes in the acute sympathetic and...

Sympathetic regulation during thermal stress in human aging and disease

“…Whole-body cooling-induced decreases in mean skin temperature elicit a pronounced systemic pressor response in healthy young adults (Cui et al, 2007; Cui et al, 2005b; Durand et al, 2004; Greaney et al, 2014; Hess et al, 2009; Wilson et al, 2007); a similar pressor response also occurs during more severe cold stress during which reductions in core temperature are observed (Collins et al, 1985; Inoue et al, 1992; Wagner et al, 1985). The pressor response to cooling is likely mediated by increases in sympathetic activation, because skin surface cooling increases total peripheral resistance without altering cardiac output (Cui et al, 2005b; Durand et al, 2004; Raven et al, 1980).…”

“…Further, cold stress increases indirect indices of sympathetic nervous system activation (e.g., plasma NE) in young adults (Durand et al, 2004; Frank et al, 2000). Interestingly, findings from studies employing direct measures of MSNA during whole-body cooling in young adults are equivocal (Cui et al, 2007; Fagius et al, 1991; Greaney et al, 2014). For example, studies in which skin surface cooling was induced via a water-perfused suit (to decrease mean skin temperature from a thermoneutral of ~34 to ~30°C and without any reduction in core temperature) reported no change in MSNA during cold exposure (Cui et al, 2007; Greaney et al, 2014).…”

“…Interestingly, findings from studies employing direct measures of MSNA during whole-body cooling in young adults are equivocal (Cui et al, 2007; Fagius et al, 1991; Greaney et al, 2014). For example, studies in which skin surface cooling was induced via a water-perfused suit (to decrease mean skin temperature from a thermoneutral of ~34 to ~30°C and without any reduction in core temperature) reported no change in MSNA during cold exposure (Cui et al, 2007; Greaney et al, 2014). However, an increase in MSNA throughout cooling has been reported in studies in which participants were positioned in a temperature controlled box and exposed to low ambient air temperatures of ~10°C (Fagius et al, 1991).…”

Clinical experimental stress studies: methods and assessment

“…Recordings of efferent discharges in the postganglionic sympathetic C efferent fibers innervating muscle and skin provide direct information about muscle sympathetic nerve activity (MSNA) and SSNA. A number of studies have employed microneurography to measure the total muscle and/or skin sympathetic nerve activity in response to stress (Mano, 1997;Middlekauff et al, 2002;Greaney et al, 2014). MSNA is recorded by penetrating the electrode tip in the muscle nerve fascicles, while SSNA is recorded from the skin nerve fascicles.…”

Sympathetic control of reflex cutaneous vasoconstriction in human aging

“…Furthermore, studies have reported a blunted rise in blood pressure after a spontaneous burst of MSNA in older adults (140,153), suggesting potential age-related impairments in sympathetic vascular transduction to resistance arterioles. Thus age-related changes in the ability of MSNA to modulate vascular tone may contribute to altered blood pressure regulation during cold stress in older adults (61,68,157); however, this requires further study to extrapolate these findings to the skin circulation.…”