Metabolism and celluarity of adipose tissues from calf- and yearling-fed nuclear transfer clone steers.
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We hypothesized that age was a more important determinant than diet in the development of intramuscular adipose tissue. Two groups of Brangus nuclear transfer cloned steers were used in these experiments. The first group (n = 8) was calf- (CF) or yearling-fed (YF) to a constant age endpoint (16 mo) (Exp. 1). The second group (n = 10) was CF or YF to a constant live weight endpoint (530 kg) (Exp. 2). The CF steers were started on a high energy finishing diet immediately after weaning, and the YF steers were allowed to graze bermuda grass pasture for 120 d before starting the feeding period. The CF steers in Exp. 2 were approximately 80 d younger the steers from the other groups at slaughter. At slaughter, subcutaneous (s.c.) and intramuscular (i.m.) adipose tissues were dissected from the longissimus muscle and used for the measurement of lipogenesis, DNA synthesis, and cellularity. When fed to a constant age, s.c. and i.m. adipocyte cell diameter and number of adipocytes/100 mg adipose tissue (i.e., cell density) were the same in CF and YF steers. However, when fed to a constant weight, the younger, CF steers had smaller (P < 0.05) s.c. and i.m. adipocytes than the YF steers. Rates of lipogenesis and DNA synthesis were highest in the adipose tissues of the constant age, YF steers, which also exhibited 50% less s.c. fat and 25% less marbling adipose tissue than the other treatment groups. We conclude that age was a primary determinant of adipocyte size, but not adipose tissue mass. Also, preadipocytes in the leaner, constant-age YF steers were more actively proliferating and differentiating as these animals accumulated more adipose tissue.