Metabolism and morphology of brown adipose tissue from Brahman and Angus newborn calves

The objective of this study was to compare adipocyte morphology and lipogen esis between breed types (Angus vs Brahman) in brown adipose tissue (BAT) a nd white adipose tissue (WAT) from newborn calves, The Brahman calves (n = 7) were born during the fall season, whereas the Angus calves were born in fall (n = 6) or the following spring (n = 4). At parturition, Brahman cows were lighter than fall Angus cows, but were heavier than spring Angus cows (P < .05). Birth weights and perirenal BAT weights were greater in spring-b orn, but not in fall-born Angus calves, than in Brahman calves (P < .05). P all-born Angus BAT contained 63% more (P < .05) adipocytes/100 mg tissue an d contained a greater proportion(P < .05) of adipocytes with mean diameters of 40 to 50 mu m, and fewer adipocytes with diameters of 60 mu m or greate r, than Brahman BAT. Brahman BAT contained two-to-three times as many beta- receptors as Angus BAT (P < .05), although the dissociation constant (K-d) was not different between breed types. Mitochondria in Brahman BAT were pri marily spherical, whereas Angus BAT mitochondria were elongated, and mitoch ondrial cross-sectional area tended (P = .08) to be greater in Brahman BAT than in Angus BAT. The mitochondrial uncoupling protein (UCP) mRNA concentr ation (per 10(6) cells) was greater in Brahman BAT than in BAT from fall-bo rn Angus calves. Lipogenesis from acetate was greater in Angus BAT than in Brahman BAT (P < .05), and glucose and palmitate contributed a greater prop ortion of carbon to lipogenesis in Brahman BAT than in Angus BAT. These dif ferences in lipogenesis between breed types were not observed in s.c. WAT. The WAT from both breed types contained adipocytes with distinct brown adip ocyte morphology, suggesting an involution of BAT to WAT in utero. We concl ude that differences in UCP gene expression cannot cause the greater peak t hermogenesis of Angus calves; however, differences between breed types in l ipid metabolism and(or) mitochondrial morphology may contribute to this phe nomenon.