In-vivo study of the thermoregulation of the rat tail using magnetic resonance angiography (MRA)

In the rat, almost 20% of the total body heat-loss occurs by sympathetically mediated increases in blood flow through a system of arteriovenous anastomoses (AVAs) in the skin of the tail which are absent at the base and abundant at the tip. To study the mechanisms of thermoregulation in the rat tail we monitored online the blood vessel temperature and the arterial and venous vessel size and their mutual vascular volume interactions using in vivo MRA. During a gradual rise in rectal temperature from 36 degrees Celsius to 40 degrees Celsius, tail surface temperatures were measured at ventral (T a ) and lateral (T v ) sits overlying the respective vascular bundles. At the base, middle and tip, diameter of the ventral artery and the lateral veins of the heat-loaded animal increased clearly upon rising body temperature. Calculation of (T a - T v ) in function of the rectal temperature during heating showed that at the tail base (T a - T v ) was maximum at rectal temperature of 38 degrees Celsius and minimum at 39 degrees Celsius. At the middle and the tip of the tail, a steady rise of (T a - T v ) was observed. If we assume that vasodilatation is a synchronical process along the length of the tail, then the difference in (T a - T v ) is due to the presence of AVAs.