Abstract

Marbling, or intramuscular fat, is an important factor in meat quality. As a key regulator of lipid metabolism, AMP activated protein kinase (AMPK) may be associated with intramuscular fat accumulation and glycogen content in bovine muscle, and muscle fiber type composition in beef cattle. Steers with high intramuscular fat content (High IMF, 5.71 ± 0.36 %, n=5) and low intramuscular fat content (Low IMF, 2.09 ± 0.19 %, n=5) in the longissimus muscle (LM) were selected for analysis. High IMF steers had a reduced AMPK activity, reduced acetyl-CoA carboxylase phosphorylation, decreased glycogen content, and decreased phosphorylation of GSK3 in the longissimus muscle. No noticeable difference in myosin isoform composition was observed between Low and High IMF muscle. This data shows LM lipid and glycogen metabolisms are affected by AMPK activity. High IMF steers also had decreased Warner-Bratzler shear force values leading us to investigate tenderness in these animals. Postmortem proteolysis is an important determinant in beef tenderness. Caspase 3 is a protease that functions in apoptosis and has been shown recently to degrade myofibrillar proteins suggesting it could be involved in postmortem proteolysis. Caspase 3 activity in the LM immediately postmortem was higher for the cattle with increased Warner-Bratzler shear force values. In 5 additional steers we sampled LM over a 10 d period. Western blot analysis LM samples showed only the pro-caspase 3 form and failed to detect the activated enzyme. No significant activation of caspase 3 was detected, only a decreased activity at 72 h and 240 h postmortem was observed. Hence, caspase 3 is not anticipated to be involved in postmortem tenderization of beef.

Nutrient restriction (NR) of the dam during gestation can affect muscle development and adipose tissue deposition of offspring. Early to mid-gestation is important for muscle and adipose tissue development in beef cattle. Experiments showed that NR and a low plane of nutrition decreased subcutaneous fat thickness of steers at slaughter and kidney, pelvic and heart fat when nutrient restriction was applied during early to mid-gestation. Additionally, NR and NR plus protein supplementation during early to mid-gestation increased muscle growth when compared to control offspring.