Proceedings of The Physiological Society

Trinity College Dublin (2003) J Physiol 551P, PC37

Communications

Increase in contralateral lower limb vascular conductance at the onset of voluntary and electrically evoked isometric exercise in man is not explained by a decreased muscle sympathetic nerve activity

James P. Fisher *, Mikael Sander †, Ian MacDonald ‡ and Michael J. White *

* School of Sport and Exercise Sciences, University of Birmingham, Birmingham B15 2TT, UK, †Copenhagen Muscle Research Centre, Rigshospitalet, Copenhagen, Denmark and ‡University of Nottingham Medical School, Nottingham NG7 2UH, UK


We have recently reported a transient increase in conductance of the contralateral lower limb occurring 10-15 s after the onset of either voluntary or electrically evoked isometric exercise of the human calf muscles (Fisher & White, 2003). One possible explanation of this observation is that muscle sympathetic nerve activity (MSNA) is decreased soon after exercise onset.

With local ethical committee approval and after all subjects gave informed written consent seven healthy subjects, mean age 24 ± 1.4 years (6 males) performed electrically evoked (STIM) and voluntary (VOL) isometric contractions at 30 % maximum voluntary contraction force, using the protocols described in Fisher & White, (2003). In the last 5 s of the pre-exercise rest period and throughout exercise, circulation through the exercising limb was occluded by thigh cuff inflation to 200 mmHg. HR (ECG) and blood pressure (MAP) (Finapres) and MSNA, recorded from the peroneal nerve of the contralateral lower limb, were continuously monitored throughout the protocol (Hansen & Sander, 2003). On a separate occasion, using the same protocol, contralateral lower limb blood flow was measured every 15 s using venous occlusion plethysmography. Vascular conductance was calculated from blood flow/MAP. In three subjects the effects of thigh cuff inflation for 1 min without exercise on contralateral limb MSNA were investigated. All values are presented as 15 s ensemble average changes from rest ± S.E.M. Statistical analysis was performed using repeated measures ANOVA (P < 0.05) and Student's paired t tests with Bonferroni correction for multiple comparisons.

In both STIM and VOL, conductance had increased significantly (P < 0.05) after 10 s of exercise, declining thereafter (Fisher & White, 2003). At this time HR had increased by 3.08 ± 1.61 and 6.29 ± 1.79 beats min-1 for STIM and VOL respectively (P < 0.05) falling back to resting levels by 30 s of exercise. During both protocols exercise MAP increased (P < 0.05). There were no differences between STIM and VOL for any of the above variables, which changed as has previously been reported (Fisher & White, 2003). Thigh cuff inflation for 1 min without exercise caused a 47 ± 7.5 % (P < 0.05) reduction in contralateral limb MSNA, which recovered when circulation was restored. However, when cuff inflation and the associated MSNA decrease was followed by STIM and VOL exercise, MSNA did not fall further (Fig. 1).

These data do not support the hypothesis that the increase in conductance seen at the onset of STIM or VOL exercise is due to reduction in total MSNA at this time.

Where applicable, experiments conform with Society ethical requirements