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-ad renergic agonist cimaterol (CIM) on fiber characteristics, capillary s upply, and metabolic enzyme activities in muscles of young Friesian bu lls. Four pairs of monozygotic twins in each of three live weight grou ps (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.k g LW(-1).d(-1) for 90 d The other twin served as control (C). Needle b iopsies were obtained from the longissimus (LM) and semitendinosus (ST ) muscles at d 82 to 84 of treatment, and muscle fibers were identifie d 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) d ecreased, 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 mu m(2), CIM: 3,934 mu m(2); P < .001). The number of capillaries per fib er 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 (lact ate dehydrogenase: +22%, hydroxyacyl-CoA dehydrogenase: -33%, and citr ate synthetase: -34%; all P < .001) and reduced the glycogen content b y 25% (P < .01). Although less pronounced, the same trend was seen for fiber types, capillary supply, and enzyme activities in ST. There was no WG x CIM interaction (P > .05) in these muscle measurements, indic ating similar response to CIM in different weight groups. The results indicate an extensive conversion of Type IIA to Type IIB fibers and a marked hypertrophy of Type IIB fibers in particular. Combined with the changes in enzyme activities these changes suggest a major shift in m uscle metabolism with CIM treatment.