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Abstract: We evaluated the effect of plasma hyperosmolality on behavioral thermoregulation in mice, using a new experimental system. The system consisted of Plexiglas box (dimensions: 50×12×19cm) with five computer-controlled Peltier boards (dimensions: 10×10cm) at the bottom. Experiments were conducted in two different settings of the system. An operant behavior setting: each board was first set to 39°C, and the right-end board was changed to 20°C for 1min when a mouse moved to a specific position. A temperature mosaic setting: each board was randomly set to 15°C, 22°C, 28°C, 35°C, or 39°C with a 6-min interval, but each board temperature was different from the others at a given time point. Mice were injected subcutaneous (s.c.) isotonic or hypertonic saline (154mM (IS group) or 2,500mM (HS group), 10ml/kg body wt), and exposed to either setting for 90min. In the operant setting, the HS group showed fewer operant behavior counts than the IS group (11±5 and 25±4 counts, respectively; P<0.05) with greater increase in body temperature (1.6±0.4°C vs. 0.0±0.2°C, respectively; P<0.05). In the mosaic setting, the HS group selected the board temperature of 35°C more frequently than the other temperatures (P<0.05) with the same increase in body temperature. These results may suggest that plasma hyperosmolality modulates behavioral thermoregulatory response to heat and induce regulated hyperthermia. [Copyright &y& Elsevier]

Hyperosmolality in extracellular fluid in humans attenuates autonomic thermoregulation in heat, such as sweating and blood flow in the skin. However, exercise training minimizes the attenuation. The aim of the present study was to clarify the influence of hyperosmolality on thermal perception and to assess the training effect of exercise. Ten sedentary (SED) and 10 endurance-trained (TR) healthy young men were infused with 0.9% (normal saline [NS]) or 3% NaCl (hypertonic saline [HS]) for 120 min on two separate days. After infusion for 20 min, heat stimulus to the skin of the whole body was produced by a gradual increase in hot water-perfused suit temperature (33 °C, 36 °C, and 39 °C), which was first used in the normothermic condition and then in the mild hyperthermic condition (0.5–0.6 °C increase in esophageal temperature) and controlled by immersion of the lower legs in a water bath at 34.5 °C and 42 °C, respectively. Thermal sensation and comfort were rated at the time of each thermal condition. Plasma osmolality increased by ~ 10 mosmL/kg·H 2 O in the HS trial. In the mild hyperthermic condition, increases in sweat rate and cutaneous vascular conductance were lower in the HS than in the NS trial in both the SED and TR groups ( p < 0.05). In the SED group, thermal sensation in the mild hyperthermic condition was lower in the HS than in the NS trial ( p < 0.05); there was no significant difference between the trials in the TR group. These results might indicate that hyperosmolality attenuates thermal sensation with heat and that exercise training eliminates the attenuation. [ABSTRACT FROM AUTHOR]