NONSHIVERING THERMOGENESIS IN MARSUPIALS - ABSENCE OF THERMOGENIC RESPONSE TO BETA-3-ADRENERGIC AGONISTS

The status of nonshivering thermogenesis (NST) in marsupials remains c ontroversial. Although morphological studies have failed to find evide nce for the presence of brown adipose tissue (BAT) in adults or juveni les of species from all extant familes of marsupial, a number of studi es have investigated the metabolic response of marsupials to noradrena line (NA) and yielded conflicting results. In eutherian mammals, NA st imulates NST in BAT by acting on beta 3-receptors, and in the experime nts reported here we investigated the response of adult and juvenile b rush tail possums (Trichosurus vulpecula), a Brazilian opossum (Monode lphis domestica), adult and juvenile red-necked (Bennett's) wallabies (Macropus rufogriseus) and the laboratory rat to selective beta 3-agon ists (ICI D7114 and BRL 35135) and to NA. Wallabies were tested with t he beta 3-agonists only. Although NA and both beta 3-agonists caused a n 85% increase in oxygen consumption in rats, there was no significant effect on any of the marsupials. These results clearly indicate no be ta 3-stimulated NST in these marsupials. All reports of metabolic resp onses to NA are from macropods, and a recent study demonstrates that N A and other alpha-adrenergic agonists stimulate thermogenesis in a sma ll macropod, the bettong (Bettongia gaimardi), by acting on alpha 1-re ceptors. Thermogenic responses to NA seems to be restricted to macropo ds, showing the danger of characterising the response of any one marsu pial species as being representative of marsupials as a group. (C) 199 7 Elsevier Science Inc.