Enhanced skin sympathetic nervous activity by hyperthermia was
suppressed hyperosmolality but not by hypovolemia in young men
Yoshi-ichiro Kamijo, Hiroyuki
Mitono, Shigeki Ikegawa, and Hiroshi Nose
Department of Sports Medical
Sciences,
Human beings have a higher ability to dissipate
heat generated by active muscles during exercise from the skin surface by a
larger amount of skin blood flow (SkBF) than other animal species. However, such
a high SkBF may compete with muscles blood flow during exercise when an
increase in cardiac out put is limited.
Mitono et al. (2005) found that the suppression
of cutaneous vasodilation at the onset of exercise, in proportion to exercise
intensity, was attenuated when the concomitant increase in plasma osmolality (Posmol)
was prevented by intravenous hypotonic saline infusion, suggesting that the
increase in Posmol is an afferent signal to redistribute blood flow
from skin to active muscles at the onset of exercise. Recently, Ikegawa et al.
(2009) investigated the role of plasma volume (PV) expansion in an enhanced
thermoregulation after aerobic training, and found that the increased
sensitivity of cutaneous vasodilation at a given increases in esophageal
temperature (Tes) after training was returned to the baseline when
the increase in PV was reduced to the baseline, suggesting that PV was a key
factor to increase the sensitivity of cutaneous vasodilation after aerobic
training. These results suggest that osmoreceptors and/or baroreflexes play an
important role in regulating SkBF during exercise. Despite this, the efferent
path for these responses remains unknown.
To determine the efferent path for the
suppression of cutaneous vasodilation in hyperosmolality and hypovolemia, we continuously
measured skin sympathetic nerve activity (SSNA) during passive warming for 50
min in control (C; normovolemia and isosmolality), 10% hypovolemia attained by
diuretics (LPV), and 5msOm/kgH2O hyperosmolality
attained by diuretics and hypertonic infusion (HOS). As Tes
increased, cutaneous vascular conductance increased in all groups, but the
increase was attenuated in LPV and HOS
compared with C. Although burst
frequency of SSNA (BF) increased as Tes increased in all groups, the
increase was attenuated only in HOS while that in LPV was not, similar to that in C.
These results suggest that the suppression
of cutaneous vasodilation in hyperosmolality was caused by the decrease in BF of
SSNA but was not in hypovolemia.
Key words: Skin Blood flow, SSNA,
Hypovolemia, Hypersomolality