Thermogenesis, nonshivering

Brown adipose tissue is the site of nonshivering thermogenesis. It is found in hibernating and newborn animals and is present in small quantity in humans. Thermogenesis results from the presence of an uncouphng protein, thermogenin, in the inner mitochondrial membrane. 

Newborn babies contain brown fat in their necks and upper backs that serves the function of nonshivering thermogenesis. Muscle contractions that take place in the process of shivering use ATP and produce heat, but nonshivering thermogenesis is a hormonal stimulus for heat generation without the associated muscle contractions of shivering. 

In the neonate, neural NE promotes nonshivering thermogenesis, i.e., heat production via stimulation of lipolysis and fatty acid oxidation in brown adipose tissue ( 63) with the support of thyroid hormone. Brown adipose tissue contains a high density of mitochondria with an uncoupling protein that allows the cells to oxidize fatty acids and generate heat as a major product NE released at nerve endings stimulates cAMP-mediated lipolysis in these cells and promotes thermogenesis by this y83-mechanism (Chapter 14). 

The entire process of heat generation from brown fat, called nonshivering thermogenesis, is regulated by norepinephrine. (In shivering thermogenesis, heat is produced by nonvoluntary muscle contraction.) Norepinephrine, a neurotransmitter released from specialized neurons that terminate in brown adipose tissue, initiates a cascade mechanism that ultimately hydrolyzes fat molecules. The fatty acid products of fat hydrolysis activate the uncoupler protein. Fatty acid oxidation continues until the norepinephrine signal is terminated or the cell s fat reserves are depleted. 

F. 21.12. Brown fat is a tissue spiecialized for nonshivering thermogenesis. Cold or excessive food intake stimulates the release of norepinephrine from the sympathetic nerve endings. As a result, a lipase is activated that releases fatty acids for oxidation. The proton conductance protein, thermogenin, is activated, and protons are brought into the matrix. This stimulates the electron transport chain, which increases its rate of NADH and FAD(2H) oxidation and produces more heat. 

Connolly, E., Nedergaard, J. Cannon, B. (1988). Shivering and nonshivering thermogenesis in birds a mammalian view. In Physiology of Cold Adaptation in Birds, ed. C Bech R.E. Reinertsen, pp. 37-48. New York Plenum. 

Duchamp, C, Barre, H., Rouanet, J.-L. el al. (1991). Nonshivering thermogenesis in king penguin chicks. I. Role of skeletal muscle. Am. ]. Physiol, 261, R1438-45. 

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