Thermoregulatory strategies during short-term exercise at different intensities

Previously we have observed a decline in sweating rate (SR) during high intensity exercise in an incremental test protocol. In man, the sweating sensitivity (SR/degree C) has been shown to vary with exercise intensity. The aim of the present study was to determine if thermoregulatory responses in the horse, a prolific sweater with a high rate of metabolic heat production, were modified by exercise intensity. The effect of 3 separate exercise intensities (50 (S), 70 (M) and 90% (F) VO2peak on neck (SRN) and gluteal sweating rate (SRG), pulmonary artery (TPA), rectal (TREC) and skin temperatures on the neck (TSK-N) and gluteal region (TSK-G) were investigated. Horses were exercised for approximately 2 min at each exercise intensity in a randomised order. The rate of increase in TPA increased with exercise intensity (P < 0.05) (0.27 +/- 0.05, 0.43 +/- 0.04 and 0.77 +/- 0.05 degree C/min during S, M and F, respectively. The TPA and TSK temperatures at the onset of sweating were not different between exercise intensities (P > 0.05). Despite the increased rate of heat production with increasing exercise intensity, sweating sensitivity decreased on both the neck (P < 0.05) (18.2 +/- 3.4 (S), 12.0 +/- 3.5 (M) and 2.9 +/- 0.7 (g/m2/min)/degree C (F)[ and gluteal region (P < 0.05) (8.9 +/- 2.0 (S), 5.1 +/- 1.7 (M) and 0.8 +/- 0.3 (g/m2/min)/degree C (F)[. During exercise at 90% VO2peak sweating sensitivity and the rate of increase in skin temperature were markedly reduced compared to at 70% VO2peak. The present investigation demonstrates that the drive to thermoregulate is overridden during short-term, high-intensity exercise, resulting in greater heat storage (TPA) as a result of a lower SR sensitivity.