Beef cattle selected for increased muscularity have a reduced muscle response and increased adipose tissue response to adrenaline

Abstract

The aim of this experiment was to evaluate the impact of selection for greater muscling on the adrenaline responsiveness of muscle, adipose and liver tissue, as reflected by changes in plasma levels of the intermediary metabolites lactate, non-esterified fatty acids (NEFA) and glucose. This study used 18-month-old steers from an Angus herd visually assessed and selected for divergence in muscling for over 15 years. Ten low muscled (Low), 11 high muscled (High) and 3 high muscled heterozygotes for myostatin mutation (High Het ) steers were challenged with adrenaline doses ranging between 0.2 to 3.0 mg/kg live weight. For each challenge, 16 blood samples were taken between 230 and 130 min relative to adrenaline administration. Plasma was analysed for NEFA, lactate and glucose concentration and area under curve (AUC) over time was calculated to reflect the tissue responses to adrenaline. Sixteen basal plasma samples from each animal were also assayed for growth hormone. Muscle glycogen and lactate concentration were analysed from four muscle biopsies taken from the semimembranosus, semitendinosus and longissimus thoracis et lumborum of each animal at 14, 90 and 150 days on an ad libitum grain-based diet and at slaughter on day 157. In response to the adrenaline challenges, the High steers had 30% lower lactate AUC than the Low steers at challenges greater than 2 mg/kg live weight, indicating lower muscle responsiveness at the highest adrenaline doses. Aligning with this decrease in muscle response in the High animals were the muscle glycogen concentrations which were 6.1% higher in the High steers. These results suggest that selection for muscling could reduce the incidence of dark, firm, dry meat that is caused by low levels of glycogen at slaughter. At all levels of adrenaline challenge, the High steers had at least 30% greater NEFA AUC, indicating that their adipose tissue was more responsive to adrenaline, resulting in greater lipolysis. In agreement with this response, the High steers had a higher plasma growth hormone concentration, which is likely to have contributed to the increased lipolysis evident in these animals in response to adrenaline. This difference in lipolysis may in part explain the reduced fatness of muscular cattle. There was no effect of selection for muscling on liver responsiveness to adrenaline.

Can potential for dark cutting be predicted by phenotype? Relationship between sex, carcass characteristics, and the incidence of dark cutting beef

“…McGilchrist et al (2012) reflected that increased muscularity and fatness most likely indicated improved nutrition of the cattle and increased muscle glycogen reserves; however, increased musculature in the study of Hawrysh et al (1985) was linked to increased dark cutting and was most likely indicative of rapidly growing cattle with reduced muscle energy reserves. Additionally, muscular animals with an increased proportion of fast muscle fibers (Wegner et al 2000) may be most at risk of dark cutting with acute activity such as fighting or physical combat before slaughter, which greatly lowers muscle glycogen (Lacourt and Tarrant, 1985;McGilchrist et al 2011). The steers that produced dark cutting carcasses in the present study appeared most similar to the cattle of Hawrysh et al (1985) that cut dark, and DMI was compromised in dark cutting cattle in the present study when live weight at the start of finishing was taken into consideration, suggesting that these steers may not have been ingesting sufficient feed for glycogen storage relative to their size.…”

Beef carcasses with larger eye muscle areas, lower ossification scores and improved nutrition have a lower incidence of dark cutting

“…Shift in fibre type proportions is likely to lead to metabolic changes in muscle, as has been shown by McGilchrist et al (2011aMcGilchrist et al ( , 2011b and Martin et al (2011) who demonstrated that more muscular cattle and lambs are more responsive to insulin, less responsive to adrenaline and had a higher storage of muscle glycogen (McGilchrist, et al, 2011a). Therefore younger cattle and more muscular cattle are both likely to have increased glycogen at slaughter and a reduced incidence of dark cutting syndrome.…”

“…More muscular animals have higher muscle to fat ratio (O'Rourke, et al, 2009;Perry, Yeates, & McKiernan, 1993) and have more stress responsive adipose tissue to adrenaline (McGilchrist, et al, 2011a), which partially explains the leanness of these animals. This indicates that under stressful conditions pre-slaughter, high muscled cattle utilise adipose tissue for energy production to a greater extent than low muscled cattle.…”

Whole body insulin responsiveness is higher in beef steers selected for increased muscling

“…Therefore, in our study, the effect of increased adiposity and decreased glucose utilisation per gram of adipose tissue may be additive in reducing the responsiveness to insulin of lower muscled, fatter cattle. However, the high-muscled cattle are leaner suggesting that there may also be a catabolic mechanism controlling accretion of adipose in these animals (McGilchrist et al, 2011).…”

“…The intravenous glucose tolerance test was performed in conjunction with seven adrenaline challenges (McGilchrist et al, 2011) in a randomized design over 4 days (two challenges assigned per animal per day at 0900 h and 1400 h) following the completion of the insulin infusion challenges. The glucose (50% Glucose (vv) Baxter Healthcare) challenge was administered at 200 mg/kg live weight through the infusion catheter.…”