Interscapular brown adipose tissue recruitment is hindered by a temperature environment of 33°C: uncoupling protein-1 underexpression is not associated with obesity development in rats

Brown adipose tissue (BAT) generates heat due to unique thermogenic UC-mitochondria, an event known as nonshivering thermogenesis. Cold, adrenergic agents, hormones, etc., activate nonshivering thermogenesis, resulting in lipid mobilization, an increase in the mitochondria and mitochondrial cristae, and increased uncoupling protein-1 (UCP I) expression and its incorporation into mitochondrial cristae. BAT precursor cells mature and contribute to BAT growth in a process known as BAT recruitment. For the first time, we herein report the effect of a thermoneutral environment of 33 degrees C on interscapular BAT (IBAT) in rats delivered and raised at 33 degrees C. The control animals were housed at 20 degrees C. Thermoneutral IBAT was atrophic (73 mg vs. 191 mg) but with more adipocyte precursor cells; euthermia (37.6 degrees C) was maintained without nonshivering thermogenesis. Although IBAT was inactive, the thermoneutral animals did not develop obesity, and on the contrary, the thermoneutral environment of 33 degrees C hindered the rats' growth, weight (65 gm vs. 139 gm), volume (67 gm vs. 136 gm) and length (12 cm vs. 16 cm). The thermoneutral brown adipocytes were smaller (7234 mu m(3) vs. 9198 mu m(3)) with more lipids (4919 mu m(3) vs. 4507 mu m(3)) and a smaller mitochondrial cristae area (52504 mu m(2) vs. 61288 mu m(2)/adipocyte). Lipoprotein lipase mRNA expression was 11% (vs. 58% in control) and UCP1 mRNA expression was 34% (vs. 93% control). UCPI immunoelectron microscopic study detected 160 UCP I-gold particles (vs. 700 in control) per UC-mitochondrion; thermoneutral brown adipocytes had 9-fold fewer UCPI-gold particles (0.34x10(6) vs. 2.99x10(6) UCP I-gold particles), and thermoneutral UC-mitochondria developed specific intramitochondrial tubular inclusions.