Control of norepinephrine turnover in brown adipose tissue of Syrian hamsters.

Sympathetic nervous system activity in brown adipose tissue (BAT) of normal and myopathic Syrian hamsters was assessed by measuring norepinephrine turnover rate (NETR) using [3H]norepinephrine. Acute exposure of normal hamsters to cold (4 degrees C) for 4 or 24 h increased norepinephrine secretion but not resynthesis. By 3 days and at 2 wk in the cold, NETR increased but returned to a normal level by 6 wk. Hamsters were initially hypothermic (to 3 days) then normothermic (2 and 6 wk). Adaptation of normal or myopathic hamsters to high-fat diet or short photoperiod (4 h light, 20 h dark) for up to 12 wk did not alter NETR in BAT. Serum triiodothyronine (T3) concentration increased rapidly in the cold to reach a maximum level by 24 h at which it remained for 6 wk and was not correlated with changes in NETR in BAT. The high-fat diet did not alter T3 level in normal hamsters; it increased the low T3 level in myopathic hamsters. Short photoperiod induced a transient increase in T3 level in normal hamsters but not in myopathic hamsters. We conclude that the hypertrophied state of BAT in Syrian hamsters adapted to cold, to high-fat diet, or to short photoperiod is not maintained by elevated norepinephrine secretion and that some other factor(s) must be involved. A selective increase in concentration of BAT mitochondrial uncoupling protein in hamsters may be dependent on raised T3 level in serum and/or production in BAT, since both occur in response to cold acclimation but not in response to high-fat diet or short photoperiod.