Recombinant bovine somatotropin (rbST) increases size and proportion of fast-glycolytic muscle fibers in semitendinosus muscle of creep-fed steers

The objective of this study was to determine the effect of recombinant bovi ne somatotropin (rbST) on muscle fiber histology and histochemistry in cree p-fed beef steers. Crossbred steer calves were assigned to one of two treat ment groups: control (sham-injected; n = 12) or rbST-injected (0.09 mg.kg(- 1).d(-1); n = 12). Calves were injected every 14 d starting at d 28 of age until weaning at 205 d of age. Biopsies of the semitendinosus muscle were p erformed on d 100, and slaughter samples of semitendinosus muscle were coll ected for muscle fiber analyses on d 206. The rbST-treated calves had large r (P = 0.045) fast-twitch-glycolytic (FG) fibers [2,564 +/- 10 vs 2,351 +/- 11 mum(2) cross-sectional area, respectively] than controls. No difference s (P = 0.36) between rbST-treated and control steers in cross-sectional are a were detected for slow-twitch-oxidative (SO) [1,192 +/- 20 vs 1,148 +/- 2 2 mum(2), respectively] or fast-twitch-oxidative-glycolytic (FOG) fibers [1 ,484 +/- 35 vs 1,403 +/- 38 mum(2), respectively]. The percentage distribut ion for FOG fibers was greater for control calves than for the rbST-treated calves (38.4 vs 34.9 +/- 0.1%, respectively; P = 0.014), whereas the perce ntage distribution for FG fibers was greater in the rbST-treated calves tha n for control calves (53.5 vs 48.4 +/- 0.2%, respectively; P = 0.03). The p ercentage distribution for SO fibers tended to be greater for the control c alves than for the rbST-treated calves (13.1 vs 11.7 +/- 0.1%, respectively ; P = 0.07). The percentage of FG fibers increased with age (45.4 vs 56.6 /- 0.8%, respectively; P = 0.001), whereas the percentage distribution of S O (14.3 vs 10.5 +/- 0.5%, respectively) and FOG fibers (40.3 vs 32.9 +/- 0. 70/0, respectively) decreased (P = 0.901) from d 100 to d 206. The increase d longissimus muscle area and dissectable lean tissue in rbST-treatcd calve s are associated with a greater percentage of FG fibers, which possess larg er cross-sectional areas than the other fibers.