Selective brain cooling in resting and exercising Norwegian reindeer (Rangifer tarandus tarandus).

The threshold body core temperature for selective brain cooling (SBC) as well as the slope of brain cooling were determined in three Norwegian reindeer (Rangifer tarandus tarandus) during rest and during exercise. Brain temperature was measured in the hypothalamus (Thypo) and blood temperature (Tblood) was measured either in the right carotid artery or in a few cases in the right atrium of the heart. During rest the animals were subjected to ramp-like increases of Tblood by means of a thermostatically controlled water circulated heat exchanger (HE) introduced into the rumen via a chronically implanted rumen cannula. During exercise the animals ran on a treadmill at a speed of between 5.5-8.0 km hr-1 and a slope of 13.5 degrees for periods of 30-60 min. The elevation of Tblood during both rest and exercise resulted in significant amounts of SBC. The mean threshold for SBC (Thypo = Tblood) during rest was 38.7 degrees C. The threshold for SBC was elevated significantly to 39.5 degrees C during exercise. The mean slope of SBC (increase of SBC per degree increase of Tblood) was 0.82 both during rest and exercise.