Relative Contributions of Acetate, Lactate and Glucose to Lipogenesis in Bovine Intramuscular and Subcutaneous Adipose Tissue

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Abstract
Intramuscular adipose tissue from the fifth–eighth thoracic vertebrae region of the longissimus dorsi muscle, and portions of the overlying subcutaneous adipose tissue, were obtained at 16 and 18 months of age from Angus steers fed ad libitum either a corn silage (low energy) or ground corn (high energy) diet. Carcass weight, backfat thickness, and kidney, pelvic and heart fat were significantly greater in the steers fed the high energy diet; however, there were no significant differences in marbling scores between diet groups. Additionally, feeding steers the high energy diet did not result in differences in adipocyte diameter or number of adipose cells per gram tissue in either adipose tissue depot. Intramuscular adipocytes had a peak diameter (the diameter represented by the greatest number of cells) of 104 ± 2 µm; peak diameter in subcutaneous adipose tissue was 141 ± 5 µm. The activities of ATP-citrate lyase and NADP-malate dehydrogenase increased with age in the subcutaneous adipose tissue; feeding steers the high energy diet increased enzyme activities further. Age and diet had no effect on enzyme activities in intramuscular adipose tissue. A similar pattern was observed for the incorporation of lipogenic precursors into fatty acids. Acetate provided 70–80% of the acetyl units to in vitro lipogenesis in subcutaneous adipose tissue, but only 10–25% in intramuscular adipose tissue. Conversely, glucose provided 1–10% of the acetyl units in subcutaneous adipose tissue, but 50–75% in the intramuscular depot. The contribution of lactate to lipogenesis was similar in both tissues (15–30%). The pentose cycle was estimated to contribute all of the necessary NADPH required for lipogenesis in the younger steers, and 40–85% (depending on diet and tissue site) in the 18-month-old steers. There was no effect of diet on, or difference between adipose tissue depots for, the relative contribution of the pentose cycle to lipogenesis. The data suggest that different regulatory processes control de novo fatty acid synthesis in intramuscular and subcutaneous adipose tissue.