Thermoregulatory reflex control of cutaneous vasodilation in healthy aging

ABSTRACT

Reflex cutaneous vasodilation during heating is attenuated in healthy human aging secondary to blunted increases in efferent skin sympathetic nervous system activity (SSNA) and reductions in end-organ sensitivity. Whether age-related alterations in the mean body temperature ($\bar{T}$_b) threshold for increasing SSNA and/or the sensitivity of responses are evident with aging have not been examined. We tested the hypotheses that the T_b threshold for SSNA and cutaneous vascular conductance (CVC) would be increased, but the sensitivity would be reduced, with aging. Reflex vasodilation was induced in 13 young (23 ± 3 y) and 13 older (67 ± 7 y) adults using a water-perfused suit to systematically increase mean skin and esophageal temperatures. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry) in the innervated dermatome were continuously measured. SSNA was normalized to baseline; CVC was normalized as a percentage of maximal CVC. Baseline $\bar{T}$_b was lower in older adults (36.0 ± 0.4°C vs 36.4 ± 0.3°C; p = 0.005). During passive heating, the ∆$\bar{T}$_b thresholds for increasing SSNA and CVC were greater (1.3 ± 0.4°C vs 0.9 ± 0.3°C; p = 0.007 and 1.3 ± 0.4°C vs 0.8 ± 0.3°C; p = 0.002, respectively) in older adults. The slope of the relation between both SSNA (0.31 ± 0.23 vs 0.13 ± 0.10 V⋅s⋅°C ⁻¹; p = 0.01) and CVC (87.5 ± 50.1 vs 32.4 ± 18.1%max⋅°C⁻¹; p = 0.002) vs $\bar{T}$_b was lower in older adults. The relative $\bar{T}$_b threshold for activation of SSNA and the initiation of reflex cutaneous vasodilation is higher in older adults, and once activated, the sensitivity of both responses is diminished, supporting the concept that the efferent component of the thermoregulatory reflex arc is impaired in healthy aging.

Abbreviations: CI: confidence interval; CVC: cutaneous vascular conductance; SSNA: skin sympathetic nervous system activity; $\bar{T}$_b: mean body temperature; T_es: esophageal temperature; $\bar{\mathrm{T}}$_sk: mean skin temperature.

KEYWORDS:

• Passive heat stress
• skin blood flow
• skin sympathetic nervous system activity (SSNA)
• microneurography
• onset threshold

Key points summary

• Reflex cutaneous vasodilation during heating is attenuated in healthy human aging secondary to blunted increases in efferent skin sympathetic nervous system activity (SSNA) and reductions in end-organ sensitivity.

• The present findings demonstrate that the relative $\bar{T}$_b threshold for activation of SSNA and the initiation of reflex vasodilation is higher in older adults, and once activated, the sensitivity of both responses is diminished, supporting the concept that the efferent component of the thermoregulatory reflex arc is impaired in healthy aging.

• The delayed activation and reduced sensitivity of peripheral thermoeffector mechanisms likely have significant functional consequences for heat dissipation and the modulation of body temperature during heat exposure in older adults.

Author contributions

JLG: conceived and designed research, data collection, analysis, interpretation, and manuscript preparation; AES: conceived and designed research, data collection, analysis, interpretation, and manuscript preparation; STW: data analysis, interpretation, and manuscript preparation; WLK: conceived and designed research, data analysis, interpretation, and manuscript preparation. All authors approved the final version of the manuscript. All laboratory work was conducted at PSU.

Acknowledgments

We greatly appreciate the effort expended by the volunteer participants. We thank Susan Slimak, RN and Jane Pierzga, MS for their assistance.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Noll Chair Endowment and the National Institutes of Health (NIH) awards [HL133414 (JLG), HL138133 (AES), and. AG07004 (WLK)]. SWT was support by AG049676.