Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 18 TRENBOLONE ACETATE AND TESTOSTERONE STIMULATE GROWTH IN RATS THROUGH DIFFERENT MECHANISMS M.N. SILLENCE* and M.L. MATTHEWS* Trenbolone acetate (TBA) is a synthetic anabolic androgen currently undergoing registration for use in Australia. In addition to stimulating growth in cattle and sheep, it is effective in female, but not in male rats. Corticosteroid hormones are well-known to retard growth rate at high concentrations, and Thomas and Rodway (1982) have found reduced plasma concentrations of catabolic corticosteroid hormones in TBA-treated female rats and sheep. The aim of the present study was to examine the adrenal response of male rats to TBA and to compare in females the effects of TBA and testosterone. Wistar rats (7-8 weeks old) were housed individually, allowed free access to a standard laboratory diet, and maintained on a $2 h light 12 h darkness cycle Animals were weighed and injected at 1000 h daily with lights on at 0700 h. Steroids were dissolved in peanut oil, and injected for five days. subcutaneously. Male rats (n=8) received either peanut oil (0.2 ml), or TBA (O-8 mg/kg) t females (n=16) received either peanut oil, TBA (0.8 mg/kg), or testosterone propionate (0.8 mg/kg). Rats were killed at 1700 h, the time of day when plasma concentrations of corticosterone peak. Blood was collected and plasma was analysed for corticosterone and ACTH concentration. Table 1 Growth rate (g/day) and plasma concentrations of corticosterone (nmol/l) and ACTH (ng/l) in rats treated daily for 5 days with androgenic anabolic steroids In male rats, TBA caused no significant increase in growth rate, and no significant change in plasma concentrations of corticosterone. In contrast, female rats treated with TBA showed a significant (PxO.05) improvement in growth rate (97%), and a significant (P<O.OS) reduction in plasma concentrations of corticosterone (45%). This result suggests that TBA promotes growth in female rats through the suppression of adrenal activity. Testosterone treatment caused an increase in the growth of female rats also. However, the growth response of females to testosterone was significantly (P<O.OS) less than to TBA, and was accompanied by an increase in plasma concentrations of corticosterone (52%). This result suggests that TBA and testosterone promote growth through different mechanisms, and that the growth response to testosterone may be constrained by increased plasma concentrations of corticosterone. Plasma concentrations of ACTH were not altered significantly by either treatment. Thus, the adrenal gland is a more likely site of action than the pituitary, for the inhibitory effect of TBA and for the stimulatory action of testosterone on plasma corticosterone. It is concluded that the suppression of adrenal activity by anticatabolic agents can have a profound influence on growth in normal female rats. THOMAS, K-M. and RODWAY, R.G. (1982). Proc. Nut. Soc. 41: 138A. * CSIRO, Div Tropical Animal Production, P-0. Box 5545, Rockhampton, Qld. 4702. 549