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This work investigated the metabolic basis for the variability of carcass and i.m. adiposity in cattle. Our hypothesis was that the comparison of extreme breeds for adiposity might allow for the identification of some metabolic pathways determinant for carcass and i.m. adiposity. Thus, 23- to 28-mo-old steers of 3 breeds, 2 with high [Angus or Japanese Black x Angus (J. Black cross)] and 1 with low (Limousin) i.m. and carcass adiposity, were used to measure activities or mRNA levels, or both, of enzymes involved in de novo lipogenesis [acetyl-coA carboxylase, fatty acid synthase (FAS), glucose-6-phosphate dehydrogenase (G6PDH), malic enzyme], circulating triacylglycerol (TAG) uptake (lipoprotein lipase), and fatty acid esterification (glycerol-3-phosphate dehydrogenase), as well as the mRNA level of leptin, an adiposity-related factor. In a first study, enzyme activities were assayed in the s.c. adipose tissue (AT), the oxidative rectus abdominis, and the glycolytic semitendinosus muscles from steers finished for 6 mo. Compared with Angus or J. Black cross, Limousin steers had a 27% less (P = 0.003) rib fat thickhess, and 23 and 29% less (P [less than or equal to] 0.02) FAS and G6PDH activities in s.c. AT. In rectus abdominis and semitendinosus, the 75% less (P < 0.001) TAG content was concomitant with 50% less (P < 0.001) G6PDH activity. In a second study, enzyme activities plus mRNA levels were assayed in an oxido-glycolytic muscle, the longissimus thoracis (LT), in the i.m. AT dissected from LT, and in s.c. AT from the same Limousin steers and from Angus steers finished for 10 mo. Compared with Angus, the 50% less (P < 0.001) rib fat thickness in Limousin contrasted with the 1.1- to 5.8-fold greater (P [less than or equal to] 0.02) mRNA levels or activities, or both, of acetyl-coA carboxylase, G6PDH, lipoprotein lipase, and glycerol-3-phosphate dehydrogenase in s.c. AT. Conversely, the 90% less (P < 0.001) TAG content in Limousin LT was concomitant to the 79 and 83% less (P [less than or equal to] 0.002) G6PDH activity and leptin mRNA level. Such differences could arise from a greater number of adipocytes in LT from Angus steers because no difference was found between Limousin and Angus for G6PDH activity and leptin mRNA in i.m. AT. We conclude that FAS and G6PDH in s.c. AT could be involved in differences in carcass adiposity, but this relationship disappeared when the fatness increased strongly. Leptin and G6PDH are related to the expression of marbling whatever the body condition and thus could be relevant indicators of marbling in beef cattle. Key words: adipose tissue, bovine, leptin, lipogenic activity, muscle

Marbling is an important criterion for beef quality grading in many countries. The purpose of the current study was to utilize the natural genetic variation to identify major metabolic indicators of marbling in cattle differing in genotypes. Rectus abdominis (RA, oxidative), semitendinosus (glycolytic), and longissimus thoracis (LT, oxido-glycolytic) muscles were taken from steers of different genotypes that expressed high [Angus, n = 16; and crossbred (Angus x Japanese Black), n = 10] or low (Limousin, n = 12) levels of marbling in their meat. Muscles from Angus and crossbred steers were characterized, as expected, by a greater triacylglycerol (TAG) content (P < 0.001) and also by greater protein contents of fatty acid-binding protein specific for heart and muscles (H-FABP; P < 0.001 for RA and P < 0.05 for LT muscle) or for adipocytes (A-FABP; P < 0.001 for RA and LT muscles). Moreover, oxidative enzyme activities ([beta]-hydroxyacyl-CoA dehydrogenase, citrate synthase, isocitrate dehydrogenase, cytochrome-c oxidase) were greater (P < 0.01 to 0.001) in the 3 muscles studied, whereas glycolytic enzyme activities (phosphofructokinase and lactate dehydrogenase) were lower (P < 0.001) in RA muscle in Angus and crossbred steers compared with Limousin steers. Significant correlations were observed between TAG content and H- and A-FABP protein contents, and oxidative (r [less than or eqaul to] +0.55, P < 0.001) or glycolytic enzyme activities (r [less than or equal] -0.47, P < 0.001), when the 3 genotypes and muscles studied were considered as a whole. In addition, A-FABP protein content and some oxidative enzyme activities were significantly correlated with TAG content independently of the genotype and muscle effects. In conclusion, A-FABP protein content, as well as oxidative enzyme activities, may be used as indicators of the ability of steers from extreme genotypes to deposit intramuscular fat. Key words: triacylglycerol, marbling, fatty acid-binding protein, muscle metabolism, genotype, steer

An experiment was conducted with 42 growing Montbeliard steers to study the effect of feed restriction, followed by refeeding, on circulating concentrations of thyroxine ([T.sub.4]) and triiodothyronine ([T.sub.3]) and on hepatic and muscle activities of 5'-deiodinase (5'D). At 9 mo of age, 21 steers were diet-restricted for 3 mo (ADG, 641 g/d), prior to a 4-mo compensatory growth period with ad libitum access to the same diet (ADG, 1,240 g/d). They were compared to 21 control steers continuously gaining 1,100 g/d between 9 and 16 mo of age. Blood samples were collected every 14 d and samples of liver and semitendinosus and triceps brachii (triceps) muscles were obtained at slaughter at the end of the restriction and refeeding periods (12 and 16 mo of age, respectively). Compared to control steers, feed restriction decreased plasma concentrations of ([T.sub.4]) after 56 to 83 d of feed restriction (P < 0.05), whereas ([T.sub.3]) concentration decreased only after 83 d of feed restriction (P < 0.05). No differences in hepatic and muscle 5'D activities were observed after 87 d of feed restriction and decreased growth rate (12 mo of age). During the refeeding period (compensatory growth), circulating concentrations of ([T.sub.4]) and ([T.sub.3]) were restored to control levels within 14 d. Moreover, ([T.sub.3]) concentration rose above that of control steers after 56 d of refeeding and remained higher for the duration of the experiment (P < 0.05). Hepatic 5'D activity was higher (P = 0.07) in compensated than in control steers at the end of refeeding period (16 mo of age) and higher (P < 0.01) after compensation at 16 mo than during restriction at 12 mo. Activities of 5'D in semitendinosus and triceps muscles were higher (P < 0.001) in 16-mo-old than in 12-mo-old steers, but no differences were observed due to feed restriction or compensatory growth. These results indicate that nutritional status regulates both thyroidal secretion and extrathyroidal ([T.sub.3]) production in cattle. The data also suggest that extrathyroidal ([T.sub.3]) production may be involved in the mechanism of compensatory growth in cattle. Key Words: Cattle, Compensatory Growth, Deiodination, Restricted Feeding, Thyroxine, Triiodothyronine