Influence of ambient temperature on calorigenic action of thyroid hormones in young mice.

In 5- and 7-day-old HAICR mice the calorigenic action of thyroxine (T4) and triiodothyronine (T3) was investigated at different ambient temperatures. After treating animals with 5 microgram T4 or 1 microgram T3 per g body weight and day, their oxygen consumption was compared with that of litter mate controls at 23 degree, 27 degree, 31 degree and 35 degree C. The hormone action was highly dependent on ambient temperature, 1 microgram T3 being most effective. A thyroid hormone effect could never be observed at 31 degree C, the animals' nest temperature. By application of propranolol, which almost selectively blocks nonshivering thermogenesis (NST), we could show that T3 increases minimal oxygen consumption (MOC) as well as the capacity for NST. In addition, it could be shown that also oxygen consumption of liver tissue of 5-day-old mice is elevated by preceding treatment with thyroid hormones. The ambient temperature effects are discussed in relation to the thermoregulatory situation of the mice during the early postnatal period.