Department of Animal Biology, Section of Physiology, Faculty of Veterinary Medicine, University of CoÂrdoba,Spain Skeletal Muscle Fibre Characteristics in Young and Old Bulls and Metabolic Response after a Bull®ght E. I. AGU È ERA 1,4 , A. MUN Ä OZ 2 , F. M. CASTEJO Â N 1 and B. ESSE Â N-GUSTAVSSON 3 Addresses of authors: 1 Department of Animal Biology (Physiology), Faculty of Veterinary Medicine, University of Co Ârdoba; 2 University of Cardenal-Herrera, Valencia, Spain; 3 Department of Large Animal Clinical Sciences, Swedish University of Agricultural Sciences, Uppsala, Sweden; 4 Corresponding author With 3 tables (Received for publication August 3, 2000) Summary Fibre type composition, activities of enzymes such as citrate synthase (CS), 3-hydroxyacyl coenzyme A dehydrogenase (HAD), lactate dehydrogenase (LDH), as well as glycogen, lactate and pH levels were analysed in muscle biopsies (m. gluteus medius) obtained after bull®ghting from 10 young and 10 old bulls. No changes were seen in ®bre type composition between groups, but the older bulls had higher HAD and LDH activities. Low glycogen concentrations and low pH values were found in both groups, but the lactate concentration after bull®ghting was higher in the older group of bulls. The histochemical stain for glycogen revealed that type IIB ®bres in both young and old bulls contained more glycogen than seen in type IIA and type I ®bres. These results show that young and old bulls have similar muscle ®bre type composition, but the metabolic capacity differs, with a higher glycolytic capacity and lactate production in older bulls. Furthermore, it seems that the physical and emotional stress in connection with a bull®ght causes a marked depletion of glycogen, especially of type I and IIA ®bres. Introduction Skeletal muscle ®bres can be histochemically classi®ed into different ®bre types based on differences in the pH dependence of the myosin adenosine triphosphatase (m-ATPase) activity (Brooke and Kaiser, 1970). At alkaline pH, slow-contracting, type I ®bres, have a low ATPase activity, whereas fast-contracting, type II ®bres, have a high ATPase activity (Burke and Edgerton, 1975). Type II ®bres can be further subdivided into type IIA and type IIB ®bres on the basis of their differences in m-ATPase under different acidic conditions (Brooke and Kaiser, 1970). With another histochemical staining technique [Periodic Acid Schiff (PAS)] it is possible to evaluate the glycogen content within the muscle ®bres (Kugelberg and Edstro Èm, 1968). Evaluation of PAS stains of repeated muscle biopsies from horses and humans have been used to indicate how muscle ®bres are recruited during work (Lindholm et al., 1974; Saltin and Gollnick, 1983; Valberg, 1986). A high degree of depletion after exercise indicates that glycogen has been one of the main substrates for energy release during ®bre recruitment. This technique has also been used in pigs and cattle to study if glycogenolysis differs among J. Vet. Med. A 48, 313±319 (2001) Ó 2001 Blackwell Wissenschafts-Verlag, Berlin ISSN 0931±184X U. S. Copyright Clearance Center Code Statement: 0931±184X/2001/4805±0313 $15.00/0 www.blackwell.de/synergy