Histochemical, Enzymatic, and Contractile Properties of Skeletal Muscles of Three Anuran Amphibians

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The histochemical, enzymatic and contractile properties of several hindlimb muscles from Bufo boreas, Rana pipiens and Xenopus laevis were compared. On the basis of histochemical staining, 3 fiber types are described: 1) fast-twitch, glycolytic (FG)-large (140 μm diam) fibers that stain darkly for myosin adenosine triphosphatase and α-glycerophosphate dehydrogenase and lightly for succinate dehydrogenase and lightly for succinate dehydrogenase; 2) fast-twitch, oxidative, glycolytic (FOG)-smaller (70 μm) fibers that stain darkly for all 3 strains; and 3) tonic-small (75 μm) fibers that stain lightly for all 3 stains. Tonic fibers are restricted to peripheral fiber layers or pockets. The bulk of the muscle is composed of 80% FG and 20% FOG fibers by area. This fiber-type profile is similar among locomotory muscles within a given species and among species. Despite this histochemical similarity in fiber profile, biochemical enzymatic activity (citrate synthase and lactate dehydrogenase) varies in correlation with organismal metabolic activity. The isometric twitch contraction time and half-relaxation time were measured to assess contractile kinetics. In addition, the maximum rate of tension rise during a tetanus was measured. These contractile properties varied among species, with Bufo muscles contracting more slowly than those of Rana and Xenopus. A given muscle from the 3 species had a similar response to repetitive fatiguing stimulation, although Bufo muscles accumulated half as much lactate as those of Rana and Xenopus. Apparently, the muscles of Bufo have a lower tolerance to lactate accumulation and require less energy to generate isometric tension than the muscles of the other 2 species. The differences in organismal locomotory and metabolic patterns in anuran amphibians are correlated with quantitative differences in enzymatic and contractile properties and not with variations in muscle fiber types or proportion.

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