SELECTIVE BRAIN COOLING IN THE HORSE DURING EXERCISE AND ENVIRONMENTAL HEAT-STRESS

Five horses were exercised on a treadmill [to central blood temperatur e (T-core) approximate to 42.5 degrees C]. Three of those horses were heated at rest in a climate room (53 degrees C, 90% relative humidity) (to T-core approximate to 41.5 degrees C). Temperatures were measured in the rectum, hypothalamus (T-hyp), cerebrum, and cavernous sinus (T -sinus), on the skin of the head and midside, and T-core. When T-core increased above 38.5 degrees C, T-hyp remained 0.6 +/- 0.1 degrees C ( SE) lower during heat exposure and 1 +/- 0.2 degrees C lower during ex ercise. During heat exposure, T-sinus was 2.2 +/- 0.4 degrees C below T-core, and during exercise, T-sinus was 5 +/- 0.9 degrees C below T-c ore. Upper respiratory tract bypass during exercise in one horse resul ted in substantial reductions in T-core - T-hyp to 0.4 +/- 0.3 degrees C and T-core - T-sinus to 0.9 +/- 0.2 degrees C. Thus the horse, a sp ecies without a carotid rete, can selectively cool the brain during ex ercise or heat exposure; this occurs, at least in part, via cool blood within the cavernous sinus, presumably resulting principally from coo ling of venous blood within the upper respiratory tract.