Brown adipose tissue metabolism in vivo and serum insulin concentrations in rabbits soon after birth.

1. In rabbits kept unfed for 4 or 24 or 48 hr after delivery by Caesarean section at term, noradrenaline infusion (I.V. for 30 min) caused a similar increase in oxygen consumption but the increase in serum free fatty acid concentration was greatest in rabbits kept unfed for 48 hr.2. The brown adipose tissue of anaesthetized rabbits under 3 hr old took glucose from the circulation but did not release fatty acids. In similar rabbits noradrenaline infusion stimulated the tissue to generate heat, but there was no release of fatty acids even though the rate of triglyceride hydrolysis was high (as judged by the rate of glycerol release).3. In rabbits kept unfed for 48 hr from birth in a warm environment, brown adipose tissue released small amounts of fatty acids but continued to take glucose from the circulation. Heat production in response to noradrenaline infusion was accompanied by an increased release of fatty acids. The fat content of the brown adipose tissue did not fall with starvation.4. The mean serum insulin concentration of rabbits at birth was 54 muu./ml. compared to 23 muu./ml. in the mother. In new-born rabbits kept unfed for 48 hr the insulin concentration had fallen to 14 muu./ml.5. It is concluded (i) that at birth brown adipose tissue has the capacity to generate heat but the tissue is slow to release its stores of fat in response to starvation, (ii) that brown adipose tissue has a high rate of glucose uptake even during starvation and (iii) that the high circulating concentration of insulin may be responsible for the tissue's slow adaptation to the demands of starvation.