Received July 30, 2004
Revised September 17, 2004
Accepted after revision January 24, 2005
Decreased MSNA does not explain increased vascular
conductance during contralateral isometric exercise in
humans
James P Fisher 1*,
Mikael Sander 2,
Ian Macdonald 3,
Michael J White 1
1 University of Birmingham
2 Copenhagen Muscle Research Centre
3 University of Nottingham Medical School
* To whom correspondence should be addressed. E-mail: j.p.fisher{at}bham.ac.uk.
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Abstract |
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At the onset of both electrically evoked (STIM) and
voluntary (VOL) isometric calf exercise there is an
increase in vascular conductance of the contralateral
lower limb suggesting withdrawal of muscle sympathetic
nerve activity (MSNA). Seven subjects performed STIM or
VOL ischaemic calf exercise at 30 % maximum voluntary
contraction in a seated position. Blood pressure, heart
rate and peroneal MSNA in the resting contralateral
lower limb were recorded. During both STIM and VOL
exercise blood pressure increased (P<0.05). Blood flow
increased by 40 ± 3 and 35 ± 3 % and conductance
increased by 37 ± 3 % and 31 ± 4 % (p<0.05) after 10 s
of STIM and VOL respectively and thereafter declined.
The time course and direction of these changes persisted
with subjects in a semi-supine position, confirming that
the transient conductance changes were not an artefact
of the dependent leg position. Thigh cuff inflation for
1 min without exercise caused a 47 ± 7.5 % (p<0.05)
reduction in MSNA, which recovered when the circulation
was restored. However, when cuff inflation was followed
by STIM or VOL exercise, MSNA did not fall further.
These data suggests that the transient increase in
vascular conductance at the onset of contralateral
electrically evoked or voluntary lower limb exercise is
unrelated to MSNA.
Key Words:
Muscle, Sympathetic nervous system, Vasodilatation
