The relationship between the thermogenic response of brown adipose tissue and age during noradrenaline infusion in the young rabbit.

This work was undertaken to determine if the thermogenic activity of brown fat decreased with age in young rabbits despite the morphological evidence indicating persistence of the brown adipocytes at 4 weeks of age. Data obtained by infusing five doses of noradrenaline and measuring oxygen consumption were used to construct cumulative dose-response curves for five age groups between 3 and 32 days of age. Blood flow to brown fat and other tissues was measured by the microsphere method at 1 and 3 weeks of age. The noradrenaline-induced increase in oxygen consumption when expressed as a percentage of resting oxygen consumption in millilitres per 100 g of body weight decreased (p less than 0.05) with age. However, the absolute noradrenaline-induced increase in metabolic rate (millilitres per minute) increased with age. Total blood flow to brown fat (millilitres per minute) during noradrenaline infusion was unchanged between 1 and 3 weeks of age, but when the blood flow was expressed in millilitres per minute per gram of tissue flow decreased significantly (p less than 0.05) probably because of infiltration of brown fat with white fat. These data suggest that the amount of brown fat and its thermogenic capacity remain relatively constant between 1 and 3 weeks of age, but as a thermogenic organ, brown fat becomes proportionally less effective with age because of the large increase in body mass.