In-vivo non-invasive study of the thermoregulatory function of the blood vessels in the rat tail using magnetic resonance angiography.

In rats, a significant portion of total body heat loss occurs through sympathetically mediated changes in tail blood flow, making the rat tail a convenient model to study vasomotor activity during thermoregulation. Our aim was to perform a non-invasive study of the mechanisms of blood vessel control in the rat tail upon increasing body temperature. In anaesthetized rats, blood vessel temperature was monitored using non-invasive thermistors positioned on the skin surface, covering the ventral artery (Ta) and lateral vein (Tv), and changes in blood vessel size were measured using in-vivo magnetic resonance angiography (MRA). Two important regions of the tail (base and middle) were studied during a gradual rise of rectal temperature (Tr) from 37 to 40 degrees C. MRA data show that increasing Tr causes increased diameter of both arteries and veins of the tail, that venous diameter changes are greater than arterial diameter changes, and that diameter changes of both types of vessel are greater at the base of the tail than in the middle. Temperature data allowed calculation of (Ta - Tv), which we used as an index of flow through arteriovenous anastomoses (AVAs). The data suggest that AVAs near the base of the tail are important in heat exchange, and that they remain open only for Tr values between 38 and 39 degrees C. Copyright 2002 John Wiley & Sons, Ltd.