Thermoregulation in winter swimmers and physiological significance of human catecholamine thermogenesis

Thermoregulation in control subjects and cold-adapted winter swimmers was e xamined during 1 h of cold water immersion (13 degrees C). It was found tha t the thermoregulatory functions of winter swimmers differ from those of na n-cold-adapted subjects. As evident from the relationship between rectal te mperature and the magnitude of cold thermogenesis, in controls a significan t part of cold thermogenesis during the early phase of cooling was induced by changes in peripheral temperature input, while in the late phase of cool ing it was the central temperature input which was mainly engaged in induct ion of cold thermogenesis. In winter swimmers the magnitude of cold thermog enesis was solely related to changes in rectal temperature, indicating the predominance of the central temperature input in activation of heat product ion mechanisms. The thermoregulatory threshold for induction of cold thermo genesis was lowered (by 0.34 degrees C), but the apparent hypothalamic ther mosensitivity was the same as in non-cold-adapted subjects. These differenc es are indicative of adaptation of thermoregulatory control centres. Additi onally, the activity of thermoregulatory effecters was also changed. Shiver ing was induced later during cooling (after 40 min) in winter swimmers than in controls, which suggests an important participation of non-shivering th ermogenesis in the early thermogenic response. Winter swimmers also showed bradycardia and a greater reduction in plasma volume during cooling. The da ta indirectly indicate restriction of heat loss from the body. Only a non-s ignificant increase in quantity of subcutaneous fat was observed in winter swimmers. Thus, winter swimmers mere able to survive a significantly greate r temperature gradient between body and environment than non-cold-adapted s ubjects by modifying the sensory functions of hypothalamic thermoregulatory centres to lower heat loss and produce less heat during cold exposure. Add itionally, the capacity of the total cold thermogenesis due to potentiation of non-shivering heat production was also increased. Heat produced due to thermogenic action of adrenaline may represent more than a quarter of the t otal cold thermogenesis. In conclusion, the data suggest that winter swimme rs exhibit metabolic, hypothermic and insulative types of cold adaptation.