Reflex and Local Thermal Control of Rabbit Ear Blood Flow

We measured thermal conductance (K) of the ears of 12 New Zealand White rabbits during periods of core heating in which mean skin temperature and ear temperatures (Te) were controlled. Right Te was always held at 28 degrees C; left Te was 15, 20, 24, 28, 31, 33 degrees C in separate core heating periods. Core heating was obtained by perfusing a previously implanted intravascular heat exchanger with hot water. Blood temperature (Tbl) was measured with a thermocouple that had been previously placed in contact with the common carotid artery. During heating, K rose with rising Tbl, but the relation between K and Tbl depended on Te. At high Te, K rose rapidly in the initial stages of core heating and soon reached its maximal value. At cool Te, K rose slowly with rising Tbl until high Tbl was reached; then K rose rapidly to maximal values. The maximal value of K reached at high Tbl was independent of Te. The results suggest that local temperature affects ear blood flow by affecting the way that ear blood vessels react to sympathetic reflex signals.