Greaney Jody L, Stanhewicz Anna E, Kenney W Larry, Alexander Lacy M
Department of Kinesiology, Noll Laboratory, The Pennsylvania State University, University Park, PA, USA.
J Physiol. 2015 May 1;593(9):2199-211. doi: 10.1113/JP270062. Epub 2015 Mar 24.
The reduction in skin blood flow during whole-body cooling is impaired in healthy older adults. However, the relative contributions of altered skin sympathetic nerve activity (SSNA), transduction of this efferent neural outflow to the cutaneous vasculature, and peripheral vascular responsiveness to adrenergic stimuli to the impaired reflex vasoconstrictor response to whole-body cooling in human ageing remain unclear. We report that the SSNA response to whole-body cooling is blunted in healthy older adults, and this attenuated sympathetic response is related to a marked impairment in reflex cutaneous vasoconstriction. Further, the reflex SSNA response to a non-thermoregulatory stimulus was preserved in older adults during cooling. We additionally show that cutaneous vascular responsiveness to adrenergic stimuli is not reduced in older adults. These results further our understanding of the physiological mechanisms underlying impaired thermal-cardiovascular integration in healthy ageing.
Reflex cutaneous vasoconstriction is impaired in older adults; however, the relative roles of altered skin sympathetic nerve activity (SSNA) and end-organ peripheral vascular responsiveness are unclear. We hypothesized that in older adults whole-body cooling would elicit a blunted SSNA response and cutaneous adrenergic responsiveness would be reduced. Twelve young adults (Y; 24 ± 1 years) and 12 older adults (O; 57 ± 2 years) participated in two protocols. In Protocol 1, SSNA (peroneal microneurography) and red cell flux in the affected dermatome (laser Doppler flowmetry; dorsum of foot) were measured during whole-body cooling (mean skin temperature (Tsk ) 30.5°C; water-perfused suit). Mental stress was performed at mean Tsk 34.0°C (thermoneutral) and at 30.5°C. In Protocol 2, an intradermal microdialysis fibre was placed in the skin of the lateral calf for graded infusions of noradrenaline (norepinephrine) (NA; 10(-12) to 10(-2) m). Cutaneous vascular conductance (CVC = flux/mean arterial pressure) was expressed as a change from baseline (ΔCVCbase ). Vasoconstriction was attenuated in O. SSNA increased significantly during cooling in Y (+184 ± 37%; P < 0.05) but not O (+51 ± 12%; P > 0.05). Mental stress at Tsk 30.5°C further increased SSNA in both groups. There was no age-related difference in adrenergic responsiveness to exogenous NA (logEC50 : -6.41 ± 0.24 in Y, -6.37 ± 0.25 in O; P > 0.05). While the SSNA response to whole-body cooling is impaired with ageing, SSNA can be further increased by a non-thermoregulatory stimulus. Cutaneous adrenergic sensitivity is not reduced in O. These findings suggest that alterations in afferent signalling or central processing likely contribute to blunted SSNA responses to cooling and subsequent impairments in reflex cutaneous vasoconstriction in ageing.
健康的老年人在全身冷却过程中皮肤血流量的减少受到损害。然而，皮肤交感神经活动（SSNA）改变、这种传出神经冲动向皮肤血管系统的传导以及外周血管对肾上腺素能刺激的反应性对人类衰老过程中全身冷却时反射性血管收缩反应受损的相对贡献仍不清楚。我们报告称，健康的老年人对全身冷却的SSNA反应减弱，这种交感反应减弱与反射性皮肤血管收缩的明显受损有关。此外，老年人在冷却过程中对非体温调节刺激的反射性SSNA反应得以保留。我们还表明，老年人对肾上腺素能刺激的皮肤血管反应性并未降低。这些结果加深了我们对健康衰老过程中热心血管整合受损的生理机制的理解。
老年人的反射性皮肤血管收缩受损；然而，皮肤交感神经活动（SSNA）改变和终末器官外周血管反应性的相对作用尚不清楚。我们假设，在老年人中，全身冷却会引发减弱的SSNA反应，并且皮肤肾上腺素能反应性会降低。12名年轻成年人（Y；24±1岁）和12名老年人（O；57±2岁）参与了两项实验方案。在实验方案1中，在全身冷却（平均皮肤温度（Tsk）30.5°C；水灌注服）过程中测量SSNA（腓骨微神经ography）和受影响皮节的红细胞通量（激光多普勒血流仪；足背）。在平均Tsk 34.0°C（热中性）和30.5°C时进行精神应激。在实验方案2中，将一根皮内微透析纤维置于小腿外侧皮肤，用于分级输注去甲肾上腺素（NA；10^(-12)至10^(-2) m）。皮肤血管传导率（CVC = 通量/平均动脉压）表示为相对于基线的变化（ΔCVCbase）。O组的血管收缩减弱。Y组在冷却过程中SSNA显著增加（+184±37%；P < 0.05），而O组未增加（+51±12%；P > 0.05）。在Tsk 30.5°C时的精神应激使两组的SSNA进一步增加。对外源性NA的肾上腺素能反应性在年龄上无差异（logEC50：Y组为-6.41±0.24，O组为-6.37±0.25；P > 0.05）。虽然随着年龄增长，对全身冷却的SSNA反应受损，但非体温调节刺激可进一步增加SSNA。O组的皮肤肾上腺素能敏感性未降低。这些发现表明，传入信号或中枢处理的改变可能导致衰老过程中对冷却的SSNA反应减弱以及随后反射性皮肤血管收缩受损。
