Evidence of viscerally-mediated cold-defence thermoeffector responses in man.

Abstract

Visceral thermoreceptors that modify thermoregulatory responses are widely accepted in animal but not human thermoregulation models. Recently, we have provided evidence of viscerally-mediated sweating alterations in humans during exercise brought about by warm and cool fluid ingestion. In the present study, we characterize the modification of shivering and whole-body thermal sensation during cold stress following the administration of a graded thermal stimuli delivered to the stomach via fluid ingestion at 52, 37, 22 and 7°C. Despite no differences in core and skin temperature, fluid ingestion at 52°C rapidly decreased shivering and sensations of cold compared to 37°C, whereas fluid ingestion at 22 and 7°C led to equivalent increases in these responses. Warm and cold fluid ingestion independently modifies cold defence thermoeffector responses, supporting the presence of visceral thermoreceptors in humans. However, the cold-defence thermoeffector response patterns differed from previously identified hot-defence thermoeffectors. Sudomotor activity is modified by both warm and cold fluid ingestion during heat stress, independently of differences in core and skin temperatures, suggesting independent viscerally-mediated modification of thermoeffectors. The present study aimed to determine whether visceral thermoreceptors modify shivering responses to cold stress. Ten males (mean±SD: age 27±5years; height 1.73±0.06m, weight 78.4±10.7kg) underwent whole-body cooling via a water perfusion suit at 5°C, on four occasions, to induce a steady-state shivering response, at which point two aliquots of 1.5mlkg-1 (SML) and 3.0mlkg-1 (LRG), separated by 20min, of water at 7, 22, 37 or 52°C were ingested. Rectal, mean skin and mean body temperature (Tb ), electromyographic activity (EMG), metabolic rate (M) and whole-body thermal sensation on a visual analogue scale (WBTS) ranging from 0mm (very cold) to 200mm (very hot) were all measured throughout. Tb was not different between all fluid temperatures following SML fluid ingestion (7°C: 35.7±0.5°C; 22°C: 35.6±0.5°C; 37°C: 35.5±0.4°C; 52°C: 35.5±0.4°C; P=0.27) or LRG fluid ingestion (7°C: 35.3±0.6°C; 22°C: 35.3±0.5°C; 37°C: 35.2±0.5°C; 52°C: 35.3±0.5°C; P=0.99). With SML fluid ingestion, greater metabolic rates and cooler thermal sensations were observed with ingestion at 7°C (M: 179±55W, WBTS: 29±21mm) compared to 52°C (M: 164±34W, WBTS: 51±28mm; all P<0.05). With LRG ingestion, compared to shivering and thermal sensations with ingestion at 37°C (M: 215±47W, EMG: 3.9±2.5% MVC, WBTS: 33±2mm), values were different (all P<0.05) following ingestion at 7°C (M: 269±77W, EMG: 5.5±0.9% MVC, WBTS: 14±12mm), 22°C (M: 270±86W, EMG: 5.6±1.0% MVC, WBTS: 18±19mm) and 52°C (M: 179±34W, EMG: 3.3±2.1% MVC, WBTS: 53±28mm). In conclusion, fluid ingestion at 52°C decreased shivering and the sensation of coolness, whereas fluid ingestion at 22 and 7°C increased shivering and sensations of coolness to similar levels, independently of core and skin temperature.