Nonshivering thermogenesis in marsupials: absence of thermogenic response to beta 3-adrenergic agonists

The status of nonshivering thermogenesis (NST) in marsupials remains controversial. Although morphological studies have failed to find evidence for the presence of brown adipose tissue (BAT) in adults or juveniles of species from all extant families of marsupial, a number of studies have investigated the metabolic response of marsupials to noradrenaline (NA) and yielded conflicting results. In eutherian mammals, NA stimulates NST in BAT by acting on beta 3-receptors, and in the experiments reported here we investigated the response of adult and juvenile brush tail possums (Trichosurus vulpecula), a Brazilian opossum (Monodelphis domestica), adult and juvenile red-necked (Bennett's) wallabies (Macropus rufogriseus) and the laboratory rat to selective beta 3-agonists (ICI D7114 and BRL 35135) and to NA. Wallabies were tested with the beta 3-agonists only. Although NA and both beta 3-agonists caused an 85% increase in oxygen consumption in rats, there was no significant effect on any of the marsupials. These results clearly indicate no beta 3-stimulated NST in these marsupials. All reports of metabolic responses to NA are from macropods, and a recent study demonstrates that NA and other alpha-adrenergic agonists stimulate thermogenesis in a small macropod, the bettong (Bettongia gaimardi), by acting on alpha 1-receptors. Thermogenic responses to NA seems to be restricted to macropods, showing the danger of characterising the response of any one marsupial species as being representative of marsupials as a group.