Investigation of the mechanisms that influence the accretion of bovine intramuscular and subcutaneous adipose tissue

Abstract

The understanding of the mechanisms that differ between breeds of cattle and their ability to deposit intramuscular adipose tissue is imperative to profitable beef production. Thus, the interactions among breeds, metabolic substrates and specific hormones in bovine intramuscular and subcutaneous adipose tissue were investigated. Subcutaneous and intramuscular adipose tissues were obtained from 10 Angus and 9 Santa Gertrudis steers immediately postmortem. The adipose tissues were incubated for 2 h and 48 h with and without 1 mU/ml insulin and 30 mg/ml bovine serum albumin (BSA) to measure the incorporation of ¹⁴C-labeled acetate and glucose (5 mM, plus 5 mM unlabeled lactate) into lipid fractions. At the same chronological age, Angus steers had a more youthful (P<.05) lean maturity score, higher USDA marbling scores and higher USDA quality grades than carcasses from Santa Gertrudis steers. The lower marbling scores and quality grades in the Santa Gertrudis steers were paralleled by an increase in the number of fat cells per gram, smaller mean fat cell volumes and smaller mean diameters for intramuscular adipose tissue relative to Angus steers. The Angus steers had higher rates of pentose cycle reductase activities in intramuscular adipose tissue relative to the Santa Gertrudis steers, which may allow for the production of more glycerol and reducing equivalents (NADPH) from glucose. The increased availability of glycerol and NADPH in the intramuscular adipose tissue in the Angus steers presumably resulted in increased marbling scores and higher USDA quality grades. No differences were observed between Angus and Santa Gertrudis steers for lipogenesis in 2 h subcutaneous adipose tissue incubations. Insulin and BSA did not affect lipogenesis in subcutaneous or intramuscular adipose tissues in 2 h or 48 h adipose tissue cultures. Angus intramuscular adipose tissue had a higher incorporation of acetate into lipids and produced more glyceride-glycerol and glyceride-fatty acids from glucose than the intramuscular adipose tissue from the Santa Gertrudis steers. These results clearly indicate that insulin does not stimulate lipogenesis in bovine adipose tissue. However, there may have been a breed difference for the effects of insulin in bovine adipose tissue. Angus steers produced carcasses with more intramuscular adipose tissue, possibly as a result of increased glyceride fatty acid production from glucose and increased pentose cycle reductase enzyme activities.