Influence of spontaneously occurring bursts of muscle sympathetic nerve activity on conduit artery diameter

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Credeur DP, Holwerda SW, Restaino RM, King PM, Crutcher KL, Laughlin MH, Padilla J, Fadel PJ. Characterizing rapid-onset vasodilation to single muscle contractions in the human leg. J Appl Physiol 118: 455-464, 2015. First published December 24, 2014 doi:10.1152/japplphysiol.00785.2014 following single muscle contractions has been examined in the forearm of humans, but has not yet been characterized in the leg. Given known vascular differences between the arm and leg, we sought to characterize ROV following single muscle contractions in the leg. Sixteen healthy men performed random ordered single contractions at 5, 10, 20, 40, and 60% of their maximum voluntary contraction (MVC) using isometric knee extension made with the leg above and below heart level, and these were compared with single isometric contractions of the forearm (handgrip). Single thigh cuff compressions (300 mmHg) were utilized to estimate the mechanical contribution to leg ROV. Continuous blood flow was determined by duplex-Doppler ultrasound and blood pressure via finger photoplethysmography (Finometer). Single isometric knee extensor contractions produced intensity-dependent increases in peak leg vascular conductance that were significantly greater than the forearm in both the above-and belowheart level positions (e.g., above heart level: leg 20% MVC, ϩ138 Ϯ 28% vs. arm 20% MVC, ϩ89 Ϯ 17%; P Ͻ 0.05). Thigh cuff compressions also produced a significant hyperemic response, but these were brief and smaller in magnitude compared with single isometric contractions in the leg. Collectively, these data demonstrate the presence of a rapid and robust vasodilation to single muscle contractions in the leg that is largely independent of mechanical factors, thus establishing the leg as a viable model to study ROV in humans. vascular conductance; hyperemia; exercise onset; blood flow AT THE ONSET of dynamic exercise, skeletal muscle blood flow responses are characterized by an initial rapid increase, followed by a slower rise to steady state (20,39,44). In an effort to focus on the rapidity of the blood flow responses to exercise, studies have been performed to assess the vasodilation following single 1-s isometric muscle contractions. This initial response following a single muscle contraction, termed rapidonset vasodilation (ROV), is proportional to the intensity of contraction and peaks within five cardiac cycles postcontraction (8,11,30,48,51). ROV is considered to be an important initiating event to exercise hyperemia (8, 9). Previous investigations into ROV in humans have focused on the forearm as a model of study (2,6,12,48). However, to our knowledge, no attention has been given to the lower extremities. The lower limbs exhibit large fluctuations in blood flow throughout the day due to periods of sitting, standing, and walking (32,36,47), and importantly, skeletal muscle contractions performed in this region are our primary means of locomotion. There are also known differences in resistance vessel function between the upper and lower extremities (...

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Characterizing rapid-onset vasodilation to single muscle contractions in the human leg

Credeur

Holwerda

Restaino

et al. 2015

Self Cite

“…Importantly, the skin was marked for probe placement to ensure all measurements were made at the same location. The left brachial artery was imaged via duplex Doppler ultrasound (GE Logiq P5 or Logiq 7) with a linear transducer (10)(11)(12). For all Doppler measurements, the velocity cursor was set midvessel with a 60°angle of insonation, and sample volume was adjusted to encompass the entire vessel lumen without extending beyond it.…”

Section: Experimental Measurementsmentioning

confidence: 99%

Regulation of regional cerebral blood flow during graded reflex-mediated sympathetic activation via lower body negative pressure

Kaur

Vranish

Barbosa

et al. 2018

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The role of the sympathetic nervous system in cerebral blood flow (CBF) regulation remains unclear. Previous studies have primarily measured middle cerebral artery blood velocity to assess CBF. Recently, there has been a transition towards measuring internal carotid artery (ICA) and vertebral artery (VA) blood flow using duplex Doppler ultrasound. Given that the VA supplies autonomic control centers in the brainstem, we hypothesized that graded sympathetic activation via lower body negative pressure (LBNP) would reduce ICA but not VA blood flow. ICA and VA blood flow were measured during two protocols: Protocol-1, low-to-moderate LBNP (-10, -20, -30, -40 Torr) and Protocol-2, moderate-to-high LBNP (-30, -50, -70 Torr). ICA and VA blood flow, diameter, and blood velocity were unaffected up to -40 LBNP. However, -50 and -70 LBNP evoked reductions in ICA and VA blood flow (e.g., -70 LBNP: %∆VA-baseline= -27.6±3.0) that were mediated by decreases in both diameter and velocity (e.g., -70 LBNP: %∆VA-baseline diameter= -7.5±1.9 and %∆VA-baseline velocity= -13.6±1.7), which were comparable between vessels. Since hyperventilation during -70 LBNP reduced PCO, this decrease in PCO was matched via voluntary hyperventilation. Reductions in ICA and VA blood flow during hyperventilation alone were significantly smaller than during -70 LBNP and were primarily mediated by decreases in velocity (%∆VA-baseline velocity= -8.6±2.4; %∆VA-baseline diameter= -0.05±0.56). These data demonstrate that both ICA and VA were unaffected by low-to-moderate sympathetic activation, whereas robust reflex-mediated sympatho-excitation caused similar magnitudes of vasoconstriction in both arteries. Thus, contrary to our hypothesis, the ICA was not preferentially vasoconstricted by sympathetic activation.

“…The MSNA signal was calibrated by assigning the voltage of the three largest bursts during baseline the value of 100 arbitrary units (AU), and all other bursts within a trial were normalized with respect to this value. MSNA was analyzed using a custom-designed LabVIEW program (13,14), which generated synchronized beat-by-beat data of all recorded variables gated by the R wave of the electrocardiogram. Sympathetic activity was quantified using standard measures, including burst frequency (bursts/ min), burst incidence (bursts/100 heartbeats), total MSNA (AU/beat), and total activity (AU/min).…”

Section: Subjectsmentioning

confidence: 99%

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

Greaney

Stanhewicz

Kenney

et al. 2014

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...