FIBER-TYPE COMPOSITION OF THE MIDDLE GLUTEAL MUSCLE OF MULES

This study compared the muscle fiber type distribution in the middle g luteal muscles of mules and horses, Nine mules and ten Quarter Horse-t ype horses from a commercial pack outfit were sampled with a Bergstrom biopsy needle, The middle gluteal muscle was sampled at a location 10 cm dorsocaudal to the tuber coxae at an angle of 45 degrees. Muscle s amples were histochemically analyzed for myosin ATPase (pH 9.5 preincu bation) and succinic dehydrogenase activity. Comparing myosin ATPase a nd succinic dehydrogenase activities, muscle fibers were identified as either type I, type IIA, or type IIB, For each sample, 150 fibers wer e counted, Mules had more type I fibers (39.7+/-2.3 vs 20.8+/-2.2%, P less than or equal to.0001) but a fewer % of type IIA fibers (39.3+/-1 .35 vs 47.4+/-1.37%, P=.0007) and fewer type IIB fibers (21.1+/-2.3 vs 31.5+/-2.4%, P=.0062), Mules did have a greater percentage of oxidati ve fibers, type I plus type IIA, (79.00+/-2.3 vs 68.2+/-2.5%, P=.0055) . The remaining muscle samples were assayed for activity of citrate sy nthase and beta-hydroxyacyl-CoA dehydrogenase (HOAD). There were no si gnificant differences between mules and horses for either citrate synt hase (P=.49) or HOAD (P=.76).