Thermal control of respiratory evaporative heat loss in exercising dogs.

Experiments were carried out to determine whether respiratory evaporative heat loss (REHL) in exercising dogs is entirely under thermal control or whether a nonthermal input is additionally involved. To determine body core thermosensitivity, hypothalamic perfusion thermodes and intravascular heat exchanges were chronically implanted in the animals. This allowed the temperature of these two areas to be independently manipulated. At 30 degrees C air temperature, REHL was measured in three dogs during rest or while running on a treadmill (6 km . h-1, 0 degree gradient). During exercise, the threshold temperature was lowered by 9 degrees C, and the slope of the heat-loss response was reduced to one-third as compared with rest when hypothalamic temperature alone was clamped at various levels between 30 degrees and 42 degrees C. However, when extrahypothalamic body core temperature was additionally clamped, the decrease in threshold during exercise was reduced to 0.43 degrees C, while the slope of the response was identical to that during rest. The results suggest that by taking account of total body core thermosensitivity, instead of hypothalamic thermosensitivity, the alleged role of a nonthermal input is greatly reduced. In addition, the results showed that the major pat of central thermosensitivity must be attributed to the extrahypothalamic body core.