In-vivo non-invasive study of the thermoregulatory function of the blood vessels in the rat tail using magnetic resonance angiography G. Vanhoutte, 1 * M. Verhoye, 1 E. Raman, 2 M. Roberts 3 and A. Van der Linden 1 1 Bio Imaging Lab, University of Antwerp, Antwerp, Belgium 2 Vision Lab, University of Antwerp, Antwerp, Belgium 3 Department of Biology, Lin®eld College, McMinnville, OR 97128, USA Received 12 June 2001; Revised 20 February 2002; Accepted 20 February 2002 ABSTRACT: 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 (T a ) and lateral vein (T v ), 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 (T r ) from 37 to 40°C. MRA data show that increasing T r 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 (T a  T v ), 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 T r values between 38 and 39°C. Copyright  2002 John Wiley & Sons, Ltd. KEYWORDS: angiography; thermoregulation; rat tail INTRODUCTION A major function of cutaneous blood flow is to control the loss of heat from the body surface. Moreover, body appendages, such as tails, are important heat exchange sites. Heat loss from the surface is achieved by skin blood flow, which can be modified by a sympathetically mediated mechanism activated by changing core tem- peratures, as well as by locally mediated responses of vascular smooth muscle activated by changing tail temperature. 1 The rat’s tail is well suited to function as a heat-loss organ. It lacks fur, has a large surface area-to-volume ratio, and is well vascularized with arteriovenous anastomoses (AVAs). 2 AVAs are non-nutrient connec- tions between arteries and veins that have a relatively large diameter. There is evidence that AVAs in body appendages play an important role in thermoregulation as in the rabbit ear increased body temperature increases AVA flow. 3 AVAs are also thought to open during exposure to extreme cold to prevent tissue freezing. 4,5 Although Gemmel and Hales 6 reported that there are many more AVAs in the tip of the rat’s tail than in the middle or the base, the significance of this observation is uncertain since vessels near the tail base would be expected to be more important for heat exchange than those near the tip, where blood temperature would be lower. The thermoregulatory function of the tail of the rat has been studied with thermometry, calorimetry and plethys- mography. Several mathematical models for the heat loss-blood flow relationship have been developed for the rat tail, and researchers have investigated how tail blood flow and heat-loss vary with core temperature. Thompson and Stevenson 7 and Little and Stoner 8 showed that tail heat loss increases with increasing core temperature. Rand et al. 9 found that, when ambient temperature increases, both heat-loss and blood flow increase. Raman et al. 10 showed that tail blood flow and heat loss increase with increasing body temperature, and that there is a redistribution of blood flow between the deep and superficial vessels of the tail, dependent on the tail temperature but independent of the rectal temperature. NMR IN BIOMEDICINE NMR Biomed. 2002;15:263–269 Published online in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/nbm.768 *Correspondence to: G. Vanhoutte, Groenenborgerlaan 171, Bio- Imaging Lab, 2020 Antwerpen, Belgium. Email: grevaho@ruca.ua.ac.be Contract/grant sponsor: University of Antwerp, RUCA. Contract/grant sponsor: Murdock Charitable Trust, Medical Research Foundation of Oregon. Abbreviations used: AVA, arterio venous anastomoses; Q, blood flow; S, cross-section area; T r , rectal temperature; T a , arterial temperature (measured on the skin); T v , venous temperature (measured on the skin); T t , tail skin temperature. Copyright  2002 John Wiley & Sons, Ltd. NMR Biomed. 2002;15:263–269