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I. The cortisol status (total plasma cortisol concentration, free cortisol concentration, transcortin capacity) and the characteristics of skeletal muscle binding for cortisol and dexamethasone were examined in female lambs either implanted with Zeranol or trenbolone acetate or whose dietary intake was restricted.2. The skeletal muscle glucocorticoid receptor had a high affinity for the glucocorticoid triamcinolone (relative binding affinity 0.85) and cortisol (relative binding affinity 0.51) with virtually no affinity for trenbolone.3. Trenbolone acetate treatment reduced the binding capacity of sheep skeletal muscle for cortisol within 2 d of implantation. The other treatments had little effect except a small reduction in the animals where food intake was restricted. Similarly, binding capacity for dexamethasone was reduced by trenbolone acetate treatment but was not affected by the other treatments. This reduction in trenbolone acetate-treated animals is, at least in part, due to a reduction in glucocorticoid receptors. Transcortin capacity was elevated by Zeranol treatment but reduced with diet restriction or trenbolone treatment.5. No support for the suggestion of free cortisol concentration being important in the growth-promoting mechanism of trenbolone or Zeranol was obtained. 6.Although insulin concentrations were not significantly altered by treatment (P > 0.05), when combining all the animals there was evidence of a negative correlation between total cortiso1:insulin vaue (P < 0.05) or free cortisol :insulin value and growth rate (P i 0.001). Free cortisol was negatively correlated to growth rate (P < 0.05) and transcortin capacity positively correlated (P < 0.01).

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Concentrations of plasma total and plasma free cortisol and the extent of bonding of cortisol in the cytosol of skeletal muscle in sheep implanted with trenbolone, zeranol or receiving a restricted dietary intake were studied. No correlation between glucocorticoid status and growth was observed, although cytosolic binding was reduced with trenbolone treatment.