3.5 Arousal (continued)

Experiments using rabbits show that a marked lowering of T _a initially lowers the temperature in many parts of its body, including areas that contain BAT. However, the temperature of the area with BAT rapidly returns to normal (37° C) because of thermogenesis within the BAT. In contrast, muscle temperature continues to fall because it has no capacity for temperature-related thermogenesis. The temperature of organs surrounded by, or near to BAT, show a smaller decrease in temperature. These results explain how core temperature, and hence the functioning of essential organs, is protected.

Mammals contain white adipose tissue (WAT) and, usually, BAT. The proportion of the total mass that is BAT varies from species to species, some having none but some having up to about 5% of body mass. The extent to which BAT produces heat in response to cold stress depends on the amount of BAT. Broadly speaking, mammals with small neonates, small mammals that are cold-acclimatized, and mammals arousing from hibernation, all contain a significant amount of BAT and are substantially dependent on it for thermogenesis. BAT is very much more thermogenic than any other tissue, by about a factor of ten (per unit mass), and is located in discrete depots with a good vascular supply. These depots contain brown adipocytes, which have a characteristic size and structure and are well endowed with numerous, large mitochondria with many cristae and a high concentration of cytochromes. It is the degree of vascularization and the concentration of cytochromes that give BAT its brown colour. Brown adipocytes also contain more glycogen than white adipocytes. White adipocytes are mainly unilocular (one large vacuole containing fat), whereas brown adipocytes that are thermogenically active are multilocular (a large number of small vacuoles containing fat). Brown adipocytes uniquely contain uncoupling protein (UCP). BAT is well innervated by neurons of the sympathetic branch of the autonomic nervous system. Neurons innervating brown adipocytes contain the neurotransmitter, noradrenalin; those innervating blood vessels contain another neurotransmitter, neuropeptide Y.

WAT is primarily a store of fat that is mobilized to provide lipid fuel for tissues remote from itself. In contrast, the function of BAT during cold stress is that of thermogenesis. When a BAT depot is cold-stressed, there is rapid cell division that increases the mass of BAT. The diameter of blood capillaries increases, and there are also changes in the histological make-up. Cold stress also leads to an increase in BAT sympathetic nervous activity and in the tissue concentration of UCP.

Non-BAT mitochondria (whether in WAT or any other tissue) are capable of transforming about 40% of the chemical energy of the respired substrates to the chemical energy of ATP; the other 60% appears as heat. However, because the rate of respiration is coupled to ATP production, the rate of respiration, and thus oxygen consumption and heat production, is comparatively low. ATP is produced by a mechanism that depends on the inner mitochondrial membrane being impermeable to hydrogen ions except via channels that are part of the ATP synthase complex. In BAT mitochondria however, fuel oxidation is uncoupled from ATP production, thus ensuring that all the energy released from oxidation is released as heat. As a consequence of uncoupling, which is brought about by the uncoupling protein UCP, the rate of heat production is greatly increased (Figure 20).

UCP is situated in the inner mitochondrial membrane and acts as a proton translocator providing a route by which hydrogen ions (built up in the intermembrane space as a consequence of electron transport) are able to re-enter the matrix without passing through the ATP synthase complex.