The effect of cimaterol on muscle fiber characteristics, capillary supply, and metabolic potentials of longissimus and semitendinosus muscles from young Friesian bulls.

The objective of this study was to determine the effect of the beta-adrenergic agonist cimaterol (CIM) on fiber characteristics, capillary supply, and metabolic enzyme activities in muscles of young Friesian bulls. Four pairs of monozygotic twins in each of three live weight groups (WG) were used (initial average live weight [LW]: 162, 299, and 407 kg, respectively). Within each pair, one twin was fed .06 mg of CIM.kg LW-1.d-1 for 90 d. The other twin served as control (C). Needle biopsies were obtained from the longissimus (LM) and semitendinosus (ST) muscles at d 82 to 84 of treatment, and muscle fibers were identified as slow-twitch (Type I) or fast-twitch (Type IIA or Type IIB) by the myosin ATPase stain. In LM, the proportion of Type I (C: 24.0%, CIM: 20.4%; P < .07) and Type IIA fibers (C: 24.2%, CIM: 8.6%; P < .001) decreased, whereas the proportion of Type IIB fibers increased (C: 51.7%, CIM: 71.1%; P < .001). Cimaterol increased the cross-sectional area of Type I (P < .02) and Type IIB fibers (P < .001), with no change in Type IIA fibers. Overall, the mean fiber area increased (C: 2,363 microns 2, CIM: 3,934 microns 2; P < .001). The number of capillaries per fiber did not change, but the number of capillaries per square millimeter decreased (P < .001) after CIM treatment. Cimaterol changed metabolic enzyme activities toward lower oxidative capacity of the muscle (lactate dehydrogenase: +22%, hydroxyacyl-CoA dehydrogenase: -33%, and citrate synthetase: -34%; all P < .001) and reduced the glycogen content by 25% (P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)