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1 Shinshu University Graduate School of Medicine, Department of Sports Medical Sciences, Matsumoto, Japan
2 Shinshu University Graduate School of Medicine, Department of Anesthesiology and Resuscitation, Matsumoto, Japan
* To whom correspondence should be addressed. E-mail: nosehir{at}sch.md.shinshu-u.ac.jp.
It is well known that hyperosmolality suppresses thermoregulatory responses and that
plasma osmolality (Posmol) increases with exercise intensity. We examined whether the
decreased esophageal temperature thresholds for cutaneous vasodilation (THFVC) and sweating
(THSR) after ten-day endurance training (ET) are caused by either attenuated increase in Posmol at a given exercise intensity or blunted sensitivity of hyperosmotic suppression. Nine young
male volunteers exercised on a cycle ergometer at 60% peak oxygen consumption rate (Vo
2peak),
1 hr.day-1, for 10 days, at 30°C. Before and after ET, thermoregulatory responses were
measured during 20-min exercise at pre-training 70% Vo
2peak in the same environment as during
ET under isoosmotic- or hyperosmotic conditions. Hyperosmolality by ~10 mosmol.kgH2O-1
was attained by acute hypertonic saline infusion. After ET, Vo
2peak and blood volume (BV) both
increased by ~4% (P < 0.05), followed by a decrease in THFVC (P < 0.05), but not by that in
THSR. Although there was no significant decrease in Posmol at the thresholds after ET, the
sensitivity of increase in THFVC at a given increase in Posmol
THFVC/Posmol, °C.(mosmol. kgH2O-1)-1], determined by hypertonic infusion, was reduced to
0.021 ± 0.005 from 0.039 ± 0.004 before ET (P < 0.05). The individual reductions in
THFVC/Posmol after ET were highly correlated with their increases in BV around THFVC (r =
-0.89, P < 0.005). In contrast, there was no alteration in the sensitivity of the hyperosmotic
suppression of sweating after ET. Thus, the downward shift of THFVC after ET was partially
explained by the blunted sensitivity to hyperosmolality, which occurred in proportion to the
increase in BV.
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