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Metabolic and endocrine effects of anabolic compounds in growing beef steers

University of British Columbia

The metabolic and endocrine effects of anabolic compounds, viz., Zeranol, diethylstilbestrol (DES) and Synovex-S were investigated in growing steers in order to understand their biochemical mechanisms of action. The growth promoting properties of certain anabolic compounds were established. A marked reduction in the plasma concentration of urea nitrogen, alpha amino nitrogen and amino acids was found in steers implanted with anabolic compounds. The results suggested that anabolic compounds significantly alter the nitrogen metabolism of steers by increasing the efficiency of utilization of absorbed nitrogenous compounds. Implantations of DES and Synovex-S were more effective than Zeranol in enhancing the growth rate and altering the nitrogen metabolism of the animals. The usefulness of measuring NT-methylhistidine in urine as a non-destructive, in vivo index of myofibrillar protein degradation and the developmental aspects of muscle protein metabolism in cattle were demonstrated. The implantation of anabolic compounds, in general, increased the efficiency of muscle protein synthesized and protein deposited by the steers. Implantations of DES and Synovex-S were more effective than Zeranol due to a reduction in the muscle protein degradation per unit synthesized. Hydroxyproline excretion in the urine was used as an in vivo indicator of collagen turnover in steers. As the animals gained weight and advanced in maturity, its excretion in the urine decreased indicating a reduction in collagen turnover. Implantation of DES increased the hydroxyproline excreted by steers, while, Zeranol and Synovex-S had very little effect. These results indicate that DES implantation increases the amount of collagen and its turnover in steers. DES and Synovex-S exerted a significant influence on the activity of the thyroid gland and caused an elevation in the circulating concentrations of free and total thyroxine. On the other hand, the implantation of Zeranol resulted in plasma thyroxine concentrations similar to or lower than in the controls. Plasma triiodothyronine concentrations were not influenced by any of the compounds studied. Increased plasma thyroxine concentration observed in the DES or Synovex-S implanted steers was shown to be due to an increase in the secretion and a decrease in the metabolic clearance rates of thyroxine. The implantation of Zeranol appeared to increase the secretion rate of thyroxine, and resulted in slightly depressed plasma thyroxine concentration due to a higher metabolic clearance rate. Implantations of anabolic compounds resulted in an increase in the circulating concentration of growth hormone. Implantations of DES and Synovex-S were more effective than Zeranol in increasing the plasma growth hormone concentration. The kinetic parameters of growth hormone metabolism in growing steers were determined. The anabolic compounds increased the secretion rate of growth hormone from the pituitary gland suggesting that these compounds evoke growth promotion in steers through changes in the endogenous growth hormone status. These changes involved an increased secretion rate of growth hormone with very little alteration in the metabolic clearance rate. The metabolism of insulin was influenced to a significant extent in steers implanted with anabolic compounds. Zeranol increased the insulin secretion rate to a greater extent than Synovex-S or DES. The data indicated that the implantation of anabolic compounds altered the metabolism of steers significantly and enhanced the secretion rate of thyroxine, growth hormone and insulin. They altered the metabolism of the steers in such a way that there was an efficient utilization of absorbed nutrients. The mechanisms of action of anabolic compounds have been discussed in detail and the directions for future research suggested.

Text

2010-05-02

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http://hdl.handle.net/2429/24292

Animal Science

University of British Columbia

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