SELECTIVE BRAIN COOLING IN GOATS - EFFECTS OF EXERCISE AND DEHYDRATION

1. Measurements of brain and central blood temperature (T(br) and T(bl)), metabolic rate (MR) and respiratory evaporative heat loss (REHL) were made in trained goats walking on a treadmill at 4.8 km h-1 at treadmill inclines of 0, 5, 10, 15 and 20 % when they were fully hydrated and at 0 % when they had been deprived of water for 72 h. 2. In hydrated goats, exercise MR increased progressively with increasing treadmill incline. Both T(bl)), and T(br) rose during exercise, but T(bl) always rose more than T(br) and selective brain cooling (SBC = T(bl)) - T(br)) increased linearly with T(bl). Significant linear relationships were also present between REHL and T(bl) and between SBC and REHL. Neither the slope of the regression relating SBC to T(bl) nor the threshold T(bl) for onset of SBC was affected by exercise intensity. Manual occlusion of the angularis oculi veins decreased SBC in a walking goat, while occlusion of the facial veins increased SBC. 3. Dehydrated goats had higher levels of T(bl), T(br) and SBC during exercise, but the relationship between SBC and T(bl) was the same in hydrated and dehydrated animals. In dehydrated animals, REHL at a given T(bl) was lower and SBC was thus maintained at reduced rates of REHL. 4. It is concluded that SBC is a linear function of body core temperature in exercising goats and REHL appears to be a major factor underlying SBC in exercise. The maintenance of SBC in spite of reduced REHL in dehydrated animals could be a consequence of increased vascular resistance in the facial vein and increased flow of cool nasal venous blood into the cranial cavity.