Dissociation of thermogenic and trophic actions of norepinephrine in brown adipocytes of Richardson ground squirrels.

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Richardson ground squirrels are hibernators with seasonal changes in the thermogenic capacity of brown adipose tissue (BAT). The objective of this work was to evaluate whether norepinephrine (NE) acts both as a thermogenic and a growth factor in BAT. Brown adipocytes were isolated by collagenase digestion of axillary BAT and kept in a tissue culture incubator in methionine-free Dulbecco's modified Eagle's medium supplemented with albumin, antibiotics, and calf serum with or without NE for up to 24 h. For short-term incubations (<6 h), calf serum was omitted. Freshly isolated brown adipocytes responded to NE with 10- to 20-fold increases in rates of oxygen uptake. This ability to respond calorigenically to NE was maintained over a 24-h period. However, no significant increase in [35S]methionine incorporation into cellular proteins was observed when brown adipocytes were incubated for 1-6 or 24 h with NE. When labeled cell proteins were separated by SDS-PAGE and visualized by autoradiography, no selective change in protein labeling, particularly in the uncoupling protein (32 kDa) region was observed. Likewise, [35S]methionine incorporation into immunoprecipitable uncoupling protein was not affected by the presence of NE. These results suggest a dissociation between the thermogenic and growth effects of NE in BAT of Richardson ground squirrels.