“Non-shivering” thermogenesis in adults is due to:
A. Thyroid hormoneB. Brown fat between the shoulders
C. Noradrenaline from adrenal medulla
D. Muscle metabolism
E. Diet induced thermogenesis
Answer. C. Noradrenaline from
adrenal medulla ; (E) Diet induced thermogenesis.
A source of considerable heat,
particularly in infants, is brown fat. This fat has a high rate of metabolism
and its thermogenic function has been likened to that of an electric blanket.
Brown adipose tissue is involved in
metabolism, particularly at times when heat generation is necessary. Thus, the
tissue is extremely active in some species in arousal from hibernation, in
animals exposed to cold (nonshivering
thermogenesis), and in heat production in the newborn animal. Though not a
prominent tissue in humans, it is present in normal individuals, where it could
be responsible for “diet-induced thermogenesis.” It is noteworthy that brown
adipose tissue is reduced or absent in obese persons. The tissue is
characterized by a well-developed blood supply and a high content of
mitochondria and cytochromes but low activity of ATP synthase. Metabolic
emphasis is placed on oxidation of both glucose and fatty acids. Norepinephrine liberated from sympathetic
nerve endings is important in increasing lipolysis in the tissue and increasing
synthesis of lipoprotein lipase to enhance utilization of triacylglycerol-rich
lipoproteins from the circulation. Oxidation and phosphorylation are not
coupled in mitochondria of this tissue, and the phosphorylation that does occur
is at the substrate level, eg, at the succinate thiokinase step and in
glycolysis. Thus, oxidation produces much heat, and little free energy is
trapped in ATP. A thermogenic uncoupling protein, thermogenin, acts as a proton
conductance pathway dissipating the electrochemical potential across the
mitochondrial membrane.
