The effects of cortisol on skeletal muscle metabolism and meat tenderness in cattle

Consumers recognize tenderness to be one of the most important determinants of beef quality; however, the metabolic causes of tenderness or toughness are unknown. Therefore, it was the intent of this research to determine how Cortisol (C), an endocrine mediator of stress, affects skeletal muscle metabolism and meat tenderness in cattle. Specific objectives were to determine (1) factors affecting C clearance rate; (2) if the catabolic actions of C affect the serum enzymes creatine phosphokinase (CPK) and aspartate aminotransferase (AT); (3) if orally administered C affects serum CPK and C and the optimum dosage for maximal CPK elevation; (4) the effects of elevated C in vivo on skeletal muscle synthesis and degradation in vitro; and (5) the relationship between C and meat tenderness using the myofibril fragmentation index (MFI).

The clearance of C from the circulatory system was faster (P < .001) in Bos taurus than in Bos indicus steers; however, the distribution volume and total clearance was greater for the Bos indicus steers. The rate of C clearance was not affected (P > .1) by estradiol and testosterone implantation. Cortisol infusion for 5 h increased average serum CPK (315 vs. 190 IU/1; P < .01) and AT (35.6 vs. 25.1 IU/1; P < .1). A linear relationship (P < .0001) was observed between C infusion time and serum CPK and AT. Oral administration of C at the dosage of 25 mg C/kg body weight (BW) increased (P < .01) serum C .5 h after C dosing, but failed (P > .1) to elevate serum CPK. However, both serum CPK and C were elevated (P < .001) with 50 and 100 mg C/kg BW. In a following study, C was fed to steers at a dosage of 75 mg C • kg BW"1 • d~^ for 7 d. Longissimus muscle biopsies were taken on d 0 and d 7. Protein synthesis and degradation rates were determined in vitro by amino acid (i.e., tyrosine [TYR]) incorporation and release, respectively. The incorporation of radiolabeled TYR proceeded at a linear rate for both d 0 ^2 = .48, respectively; P < .0001; x = hours). The intercepts of the in vitro regression lines, an indication of in vivo synthesis, did not differ (P > .1) between d 0 and 7; however, the rate of protein synthesis (slopes) were different (P < .05). The release of TYR from the muscle was linear for d 0 and 7 biopsies (Y = .89 + .37x, r^ = .78; Y = 1.80 + .46x, r^ = .65, respectively; P < .0001; x = hours). The intercepts of the regression lines tended to be different (P = .07) between d 0 and 7; however, the rate of protein degradation was not different (P > .1). In a group of cattle with a diverse genetic background including Bos indicus breeding, serum C was not highly related to meat tenderness.

Thus, elevated serum C concentrations increases muscle protein turnover, primarily by increasing muscle protein degradation. However, either normal variation in endogenous levels of serum C are insufficient to result in differences in beef tenderness or serum C levels must be elevated significantly before effects on muscle tenderness will be observed. In conclusion, it may require long exposures to stress as in shipping and/or a long arrival-to-slaughter layover in order to elicit the detrimental effects implied with C feeding or infusion.