Thermogenesis and BAT activity in hypophysectomized rats with and without corticotropin replacement

Surgical hypophysectomy (Hypox) suppressed body weight, body energy gain, and net energetic efficiency when compared with pair-fed, sham-operated controls but caused marked increases in the thermic response to food, brown adipose tissue (BAT) mass, protein content, and BAT thermogenic activity (estimated from mitochondrial purine nucleotide binding). Chronic replacement of Hypox rats with ACTH restored adrenal weight, plasma corticosterone levels, body energy gain, energetic efficiency, and the thermic response to food to normal. However, the Hypox-replaced animals were apparently fatter (greater carcass energy density) and retained a greater mass of brown fat than controls. Purine nucleotide binding to BAT mitochondria was lower in the ACTH-replaced group than in Hypox rats but remained above the level in controls. These data indicate that hypophysectomy stimulates thermogenesis, probably as a result of decreased adrenal steroid release. However, ACTH does not totally prevent the effects of Hypox, and an involvement of other pituitary hormones and/or hypothalamic-releasing factors cannot be excluded.