Selective brain cooling in the horse during exercise and environmental heat stress

Five horses were exercised on a treadmill [to central blood temperature (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 exercise. 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-core. Upper respiratory tract bypass during exercise in one horse resulted 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 species without a carotid rete, can selectively cool the brain during exercise or heat exposure; this occurs, at least in part, via cool blood within the cavernous sinus, presumably resulting principally from cooling of venous blood within the upper respiratory tract.