In vitro muscle cell protein synthesis and degradation, nitrogen balance and the feedlot response to trenbolone acetate, estradiol, and somatotropin in finishing beef steers
Steroidal implants have been used for 40 years to increase gain and gain efficiency of cattle by increasing protein deposition. However, the mechanism in which these hormones increase protein deposition is not fully understood.
This dissertation is an in-depth study that utilized cell culture techniques to examine the indirect effects of serum from steers treated with steroid hormones and somatotropin on muscle cell proliferation and protein synthesis/degradation. Also, this dissertation studied the effect of steroid hormones on nitrogen balance, feedlot response and cellular composition of anterior pituitary cells in finishing steers. Treating steers with steroidal hormones alone and in combination with somatotropin increased the percentage of nitrogen retained relative to controls and steers given somatotropin alone. Both steroids and growth hormone depressed plasma urea nitrogen concentrations on days 1 and 4 of the collection period additively. Serum from steers given steroidal implants and growth hormone increased in vitro muscle cell proliferation and muscle cell protein synthesis, while having minimal effect on protein degradation. Directly administering steroids to cultured muscle cells had no effects on protein synthesis or degradation. Implanting steers with steroidal hormones increased average daily gain and gain efficiency and depressed plasma urea nitrogen relative to control cattle within the first 28 days following implantation. There was no consistent response to serum from these steers in muscle cell protein synthesis or degradation in vitro. Implanting steers 3 and 5 weeks prior to slaughter increased average daily gain while decreasing the marbling score in the carcasses. Implants did not cause dark cutting carcasses. Steroidal implants increased the percentage of somatotropes and decreased the percentage of mammosomatotropes in the anterior pituitaries of finishing beef steers. This dissertation concludes that steroid hormones do not have a direct effect on muscle cell protein deposition. Indirectly via factors in the serum, these hormones increase protein deposition by increasing protein synthesis. These steroid hormones given in combination with somatotropin increase protein synthesis and nitrogen retention. The mechanism for the increased protein deposition due to steroids may be the change in the percentage of acidophils in the anterior pituitary.