Temperature dependence of O2 consumption; opposite effects of leptin and etomoxir on respiratory quotient in mice.

OBJECTIVES: The aims were to compare the temperature dependence of the metabolic rate in young ob/ob mice with that in mature ob/ob and db/db mice and to examine the effect on the metabolic substrate preference of leptin and etomoxir in ob/ob, C57BL/6J (wild-type), and db/db mice. RESEARCH METHODS AND PROCEDURES: In vivo oxygen consumption and carbon dioxide production were continuously measured by indirect calorimetry, and body temperature and total locomotor activity were measured by an implanted transponder. Leptin, etomoxir, or vehicle was administered intraperitoneally.
RESULTS: The temperature dependence of the metabolic rate of mature ob/ob and db/db mice were similar to that in wild-type mice. In young 6-week-old ob/ob mice, the metabolic rate was almost doubled at 15 degrees C. Leptin (2 x 3 mg/kg) decreased the respiratory quotient (RQ) and carbon dioxide production but did not alter oxygen consumption, body temperature, or locomotor activity in ob/ob and C57BL/6J mice and had no effect in the db/db mice. Etomoxir (2 x 30 mg/kg) enhanced RQ and decreased oxygen consumption, carbon dioxide production, and body temperature in ob/ob, C57BL/6J, and db/db mice. Total locomotor activity was reduced in ob/ob and C57BL/6J mice. DISCUSSION: In young ob/ob mice, the temperature sensitivity was enhanced compared with mature mice. Leptin and etomoxir had opposite effects on metabolic substrate preference. Leptin and lowered environmental temperature increased the relative fat oxidation as indicated by decreased RQ, possibly through activation of the sympathetic nervous system.