Exercise thermoregulation in men after 1 and 24-hours of 6 degrees head-down tilt.

BACKGROUND: Exercise thermoregulation is dependent on heat loss by increased skin blood flow (convective and conductive heat loss) and through enhanced sweating (evaporative heat loss). Reduction of plasma volume (PV), increased plasma osmolality, physical deconditioning, and duration of exposure to simulated and actual microgravity reduces the ability to thermoregulate during exercise. HYPOTHESIS: We hypothesized that 24 h of head down tilt (HDT24) would alter thermoregulatory responses to a submaximal exercise test and result in a higher exercise rectal temperature (Tre) when compared with exercise Tre after 1 h of head down tilt (HDT1). METHODS: Seven men (31+/-SD 6 yr, peak oxygen uptake (VpO2peak) of 44+/-6 ml x kg(-1) x min(-1)) were studied during 70 min of supine cycling at 58+/-SE 1.5% VO2peak at 22.0 degrees C Tdb and 47% rh. RESULTS: Relative to pre-tilt sitting chair rest data, HDT1 resulted in a 6.1+/-0.9% increase and HDT24 in a 4.3+/-2.3% decrease in PV (delta = 10.4% between experiments, p<0.05) while plasma osmolality remained unchanged (NS). Pre-exercise Tre was elevated after HDT24 (36.71 degrees C +/-0.06 HDT1 vs. 36.93 degrees C+/-0.11 HDT24, p<0.05). The 70 min of exercise did not alter this relationship (p<0.05) with respective end exercise increases in Tre to 38.01 degrees C and 38.26 degrees C (degrees = 1.30 degrees C (HDT1) and 1.33 degrees C (HDT24)). While there were no pre-exercise differences in mean skin temperature (Tsk), a significant (p<0.05) time x treatment interaction occurred during exercise: after min 30 in HDT24 the Tsk leveled off at 31.1 degrees C, while it continued to increase reaching 31.5 degrees C at min 70 in HDT1. A similar response (NS) occurred in skin blood velocity. Neither local sweating rates nor changes in body weight during exercise of -1.63+/-0.24 kg (HDT1) or - 1.33+/-0.09 kg (HDT24) were different (NS) between experiments. CONCLUSION: While HDT24 resulted in elevated pre-exercise Tre, reduced PV, attenuation of Tsk and skin blood velocity during exercise, the absolute increase in exercise Tre was not altered. But if sweat rate and cutaneous vascular responses were similar at different core temperatures (unchanged thermoregulation), the Tre offset could have been caused by the HDT-induced hypovolemia.

Effect of glucose-water ingestion on exercise thermoregulation in men dehydrated after water immersion.

BACKGROUND: The influence of non-ionic osmols on thermoregulation is unclear. HYPOTHESIS: Hyperglycemia will attenuate the rise in exercise core temperature. METHODS: Dehydrated by 4-h of water immersion (34.5 degrees C) to the neck, 6 men, (35+/-SD 7 yr) participated in each of three trials where 2.0 g x kg(-1) body wt of oral glucose (33.8% weight per volume) was consumed followed by 80 min supine rest (Glu/Rest), or 70 min supine cycle exercise at 62.8%+/-SE 0.5% (1.97+/-0.02 L x min(-1)) peak O2 uptake, followed by 10 min supine recovery with prior (Glu/Ex) or without glucose (No Glu/Ex) ingestion. Blood samples were taken periodically for measurement of Hb, Hct, Na+, K+, Osm, and glucose; mean skin (Tsk) and rectal (Tre) temperatures, and sweating rate (resistance hygrometry) and skin blood velocity (laser Doppler) were measured intermittently. RESULTS: Mean percent changes in plasma volume (p<0.05) for the exercise trials were not different: -12.3+/-2.2% (No Glu/Ex) and -12.1+/-2.1% (Glu/Ex). Mean (+/-SE) pre-exercise plasma [glucose] for Glu/Ex was higher than that of No Glu/Ex (108.4+/-3.9 vs. 85.6+/-1.6 mg x dL(-1), respectively, p<0.05). Glu/Ex vs. No Glu/Ex data, respectively, at the end of exercise indicated that: Tre was lower by 0.4 degrees C (38.2+/-0.2 vs. 38.6+/-0.1 degrees C, p<0.05), Tsk was lower (32.0+/-0.3 vs. 32.4+/-0.2 degrees C, p<0.05), forearm sweating rate was lower (0.94+/-0.09 vs. 1.05+/-0.07 mg x cm(-2) x min(-1), p<0.05); and head (temporal) skin blood velocity was not different (1.67+/-0.21 vs. 1.51+/-0.24 Hz x 10(3), NS). CONCLUSIONS: Elevation of plasma [glucose] prior to supine submaximal exercise in dehydrated men attenuates the increase of Tre without alteration of heat production, total body sweating, serum electrolytes and osmolality, or exercise-induced hypoglycemia: the mechanism may be enhanced peripheral blood flow that could enhance body heat loss.