Effects of growth-promoting implants on morphology of Longissimus and Semitendinosus muscles in finishing steers

Growth-promoting implants lead to increased muscle accretion in ruminants. To elucidate the effects at a cellular level. muscle fiber distribution and cross-sectional area (CSA) of longissimus (LM) and semitendinosus (ST) mus cles were compared in implanted and control steers. Sixty-four Charolais st eers were assigned to one of four treatments (16 steers/treatment): (1) no implant, (2) Synovex-S(R) (estradiol benzoate + progesterone), (3) Ralgro(R ) (zeranol) or (4) Revalor-S(R) (trenbolone acetate + estradiol-17 beta). T he experiment was carried out using four slaughter groups (SGRP). Sixteen s teers each were slaughtered after 48, 104, 160 and 175 days (four steers/tr eatment) on trial. Steers on an implant treatment were first implanted at 1 5 months of age (day 0) and reimplanted at 56 and 112 days. Muscle fibers i n the LM and ST (for both live biopsy and post-mortem samples) were charact erized as either slow-twitch oxidative (SO), fast-twitch oxidative-glycolyt ic (FOG) and fast-twitch glycolytic (FG) fibers. Fiber distribution was min imally affected by SGRP in these physiologically mature steers. Implantatio n with Synovex did not alter fiber distribution in either muscle compared w ith control steers. Both Synovex-implanted and control steers showed a decr ease of FG and an increase of FOG fibers in the LM from day 0 to SGRP 2 fol lowed by an increase of FG and a decrease of FOG fibers. Ralgro- and Revalo r-implanted steers had an almost constant fiber distribution in the LM thro ughout the experiment resulting in higher precentages of FG fibers in SGRP 2 (P < 0.05) than SYN or CON steers. Biopsy samples of the LM muscle which were excised 51 days'(SGRP 1-3) or 65 days (SGRP 4) before slaughter proved to be suitable for the determination of fiber distribution in live animals . Fiber area increased in post-mortem samples of both muscles from SGRP 1-3 in all treatment groups followed by a plateau. Implantation with Revalor l ed to an additional increase in fiber area from SGRP 3 and 4 (P < 0.05). Sy novex did not affect fiber area compared with control steers whereas Ralgro and Revalor implants led to larger fibers in SGRP 3 and 4, respectively. I t can be concluded that some growth-promoting implants result in noticeable differences in muscle hypertrophic responses which coincide with their dif ferent effectiveness to enhance lean mass accretion. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.