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

Background: Exercise thermoregulation is dependent on heat loss by increase d skin blood flow (convective and conductive heat loss) and through enhance d sweating (evaporative heat loss). Reduction of plasma volume (PV), increa sed plasma osmolality, physical deconditioning, and duration of exposure to simulated and actual microgravity reduces the ability to thermoregulate du ring exercise. Hypothesis: We hypothesized that 24 h of head down tilt (HDT 24) would alter thermoregulatory responses to a submaximal exercise test an d result in a higher exercise rectal temperature (Tre) when compared with e xercise Tre after 1 h of head down tilt (HDT1). Methods: Seven men [31 +/- SD 6 yr, peak oxygen uptake (VO(2)peak) Of 44 +/- 6 ml.kg(-1).min(-1)] were studied during 70 min of supine cycling at 58 +/- SE 1.5% VO(2)peak at 22. 0 degrees C Tdb and 47% rh. Results: Relative to pre-tilt sitting chair res t 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 HD T24 (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) wi th respective end exercise increases in Tre to 38.01 degrees C and 38.26 de grees C [Delta = 1.30 degrees C (HDT1) and 1.33 degrees C (HDT24)]. While t here were no pre-exercise differences in mean skin temperature ((T) over ba r sk), a significant (p < 0.05) time X treatment interaction occurred durin g exercise: after min 30 in HDT24 the (T) over bar sk leveled off at 31.1 d egrees C, while it continued to increase reaching 31.5 degrees C at min 70 in HDT1. A similar response (NS) occurred in skin blood veracity. Neither l ocal 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 ex periments. Conclusion: While HDT24 resulted in elevated pre-exercise Tre, r educed PV, attenuation of Tsk and skin blood velocity during exercise, the absolute increase in exercise Tre was not altered. But if sweat rate and cu taneous vascular responses were similar at different core temperatures (unc hanged thermoregulation), the Tre offset could have been caused by the HDT- induced hypovolemia.