IMMUNOHISTOCHEMICAL, ULTRASTRUCTURAL AND MORPHOMETRIC EVIDENCE FOR BROWN ADIPOSE-TISSUE RECRUITMENT DUE TO COLD-ACCLIMATION IN OLD RATS

OBJECTIVE: The aim of this work was to study the reactivity to chronic cold stress of interscapular brown adipose tissue (IBAT) in old rats by stereological, immunohistochemical and ultrastructural methods. DES IGN: Five 2-year-old rats were cold-acclimated at 4 degrees C for 4 we eks and six rats were used as controls (20-23 degrees C). MEASUREMENTS : The following were measured: IBAT volume and weight; unilocular and multilocular brown adipocyte content; preadipocyte number; multilocula r cell mitochondrial area; cristae length and density per mitochondrio n, and immunoreactivity for the brown adipose tissue specific uncoupli ng protein (UCP). RESULTS: Following cold acclimation, IBAT increased significantly in weight, volume, relative mass and number of multilocu lar adipocytes (170%). The number of unilocular adipocytes did not var y significantly. Multilocular adipocytes of both cold-acclimated rats and controls expressed the uncoupling protein, but in the experimental group cristae length and density per mitochondrion were significantly higher. Multilocular adipocyte precursors were observed in only one c old-acclimated rat but not in controls. CONCLUSION: The response of br own adipose tissue of old rats to chronic cold stimulus is similar to that observed in young and adult rats, Cold acclimation induces brown adipocyte recruitment: their number increases significantly, they test UCP-positive and their mitochondria are significantly more active tha n in controls. On the other hand, the number of unilocular adipocytes is not significantly affected, which may serve to improve the utilizat ion of the heat produced by thermogenesis.