J. Hansen et al., DIFFERENTIAL SYMPATHETIC NEURAL CONTROL OF OXYGENATION IN RESTING ANDEXERCISING HUMAN SKELETAL-MUSCLE, The Journal of clinical investigation, 98(2), 1996, pp. 584-596
Metabolic products of skeletal muscle contraction activate metaborecep
tor muscle afferents that reflexively increase sympathetic nerve activ
ity (SNA) targeted to both resting and exercising skeletal muscle. To
determine effects of the increased sympathetic vasoconstrictor drive o
n muscle oxygenation, we measured changes in tissue oxygen stores and
mitochondrial cytochrome a,a(3) redox state in rhythmically contractin
g human forearm muscles with near infrared spectroscopy while simultan
eously measuring muscle SNA with microelectrodes. The major new findin
g is that the ability of reflex-sympathetic activation to decrease mus
cle oxygenation is abolished when the muscle is exercised at an intens
ity > 10% of maximal voluntary contraction (MVC). During high intensit
y handgrip (45% MVC), contraction-induced decreases in muscle oxygenat
ion remained stable despite progressive metaboreceptor-mediated reflex
increases in SNA, During mild to moderate handgrips (20-33% MVC) that
do not evoke reflex-sympathetic activation, experimentally induced in
creases in muscle SNA had no effect on oxygenation in exercising muscl
es but produced robust decreases in oxygenation in resting muscles. Th
e latter decreases were evident even during maximal metabolic vasodila
tion accompanying reactive hyperemia. We conclude that in humans sympa
thetic neural control of skeletal muscle oxygenation is sensitive to m
odulation by metabolic events in the contracting muscles. These events
are different from those involved in either metaboreceptor muscle aff
erent activation or reactive hyperemia.