Hanson, Brady E.

AbstractHeightened muscle sympathetic nerve activity (MSNA) contributes to impaired vasodilatory capacity and vascular dysfunction associated with aging and cardiovascular disease. The contribution of elevated MSNA to the vasodilatory response during passive leg movement (PLM) is not fully understood. This study tested the hypothesis that elevated MSNA diminishes the vasodilatory response to PLM in healthy young males (n = 11, 25 ± 2 yr). Post exercise circulatory occlusion (PECO) following 2 min of isometric handgrip (HG) exercise performed at 25% (ExPECO 25%) and 40% (ExPECO 40%) maximum voluntary contraction was used to incrementally engage the metaboreceptors and augment MSNA. Control trials were performed without PECO (ExCON 25% and ExCON 40%) to account for changes due to HG exercise. PLM was performed 2 min after exercise and hemodynamics were assessed. MSNA was recorded by microneurography in the peroneal nerve (n = 8). Measures of MSNA (i.e., burst incidences) increased during ExPECO 25% (+15 ± 5 burst/100 bpm) and ExPECO 40% (+22 ± 4 burst/100 bpm) and returned to pre‐HG levels during ExCON trials. Leg vascular conductance (vasodilation) during PLM was reduced by 16% and 44% during ExPECO 25% and ExPECO 40%, respectively. These findings indicate elevated MSNA attenuates the vasodilatory response to PLM and the magnitude of reduction in vasodilation during PLM is graded with the degree of sympathoexcitation.

Abstract Heightened muscle sympathetic nerve activity (MSNA) contributes to impaired vasodilatory capacity and vascular dysfunction associated with aging and cardiovascular disease. The contribution of elevated MSNA to the vasodilatory response during passive leg movement (PLM) has not been adequately addressed. This study sought to test the hypothesis that elevated MSNA diminishes the vasodilatory response to PLM in healthy young males (n = 11, 25 ± 2 year). Post exercise circulatory occlusion (PECO) following 2 min of isometric handgrip (HG) exercise performed at 25% (ExPECO 25%) and 40% (ExPECO 40%) of maximum voluntary contraction was used to incrementally engage the metaboreceptors and augment MSNA. Control trials were performed without PECO (ExCON 25% and ExCON 40%) to account for changes due to HG exercise. PLM was performed 2 min after the cessation of exercise and central and peripheral hemodynamics were assessed. MSNA was directly recorded by microneurography in the peroneal nerve (n = 8). Measures of MSNA (i.e., burst incidences) increased during ExPECO 25% (+ 15 ± 5 burst/100 bpm) and ExPECO 40% (+ 22 ± 4 burst/100 bpm) and returned to pre-HG levels during ExCON trials. Vasodilation, assessed by the change in leg vascular conductance during PLM, was reduced by 16% and 44% during ExPECO 25% and ExPECO 40%, respectively. These findings indicate that elevated MSNA attenuates the vasodilatory response to PLM and that the magnitude of reduction in vasodilation during PLM is graded in relation to the degree of sympathoexcitation.

While rapid-onset vasodilation (ROV) has been shown to decrease in older individuals, it is unclear if sex contributes to the decline with aging. We sought to identify if sex-related differences exist in the ROV response to single forearm contractions in young and older adults. Our data suggest sex-related differences are present among older but not young individuals, with women having an attenuated response. These data indicate sex plays a role in decreased vasodilation with aging.

Rapid‐onset vasodilation (ROV) in response to a single muscle contraction is attenuated with aging. Moreover, sex‐related differences in muscle blood flow and vasodilation during dynamic exercise have been observed in young and older adults. The purpose of the present study was to explore if sex‐related differences in ROV exist in young (n=36; 21 male/15 female; 25 ± 1 yr) and older (n=32; 19 male/13 female; 66 ± 1 yr) adults. Subjects performed single forearm contractions at relative intensities of 10%, 20%, and 40% maximal voluntary contraction (MVC). Brachial artery blood velocity and diameter were measured with Doppler ultrasound, and forearm vascular conductance (FVC; ml·min −1 ·100 mmHg −1 ) was calculated from blood flow (ml·min −1 ) and blood pressure (mmHg) and used as a measure of peak ROV (peak change in FVC from baseline). Due to significant differences in MVC and consequent differences in weight used for each relative intensity between males and females, the data were analyzed two additional ways. First, responses were normalized for workload (i.e. peak FVC per kg of weight lifted). Second, the peak ROV responses in a subset of subjects (n=44; 12 young male, 10 young female, 11 older male, 11 older female) with similar absolute workloads (10–12kg) were compared. Lastly, the magnitude of change in peak ROV with increasing intensity (calculated as the slope of a linear regression line between 10, 20, and 40% MVC) was calculated and compared between groups. Peak ROV was attenuated in females across all relative intensities in the young and older groups (P < 0.05). When normalized for workload, peak ROV was attenuated with age in both males and females at all intensities (P<0.05) except at 10% for females (P=0.47). However, sex‐related differences were not observed within age when normalized by workload at any intensity (P>0.05). In the subset of subjects with a similar absolute workload (~11kg), age‐related differences in ROV remained in both the female and male groups (P<0.05). Older females demonstrated an attenuated peak ROV compared to their older male counterparts (91 ± 6 vs. 121 ± 11 ml·min −1 ·100 mmHg −1 , P<0.03), whereas no sex differences in peak ROV were observed between young females and males (134 ± 8 vs. 154 ± 11 ml·min −1 ·100 mmHg −1 , P=0.15). Examining the slope of the peak ROV response across contraction intensities revealed a main effect of age (P<0.05), with the slope being blunted in older compared to young subjects. When separated by sex, the slope was smaller in older compared to young females (5.5 ± 0.7 vs. 8.9 ± 0.9 FVC units/intensity, P<0.05) but not between older and young males (7.0 ± 0.50 vs. 8.2 ± 1.0 FVC units/intensity, P=0.38). Our data suggest that sex‐related differences in the rapid vasodilatory response to single muscle contractions exist in older but not young adults, such that older females have a blunted response compared to older males.