Fibre types inLimulus telson muscles: morphology and histochemistry

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Abstract
Using a variety of techniques, we have demonstrated the presence of at least two fibre types inLimulus median telson levator muscle. By light and electron microscopy, large (21 56 μm2 mean cross-sectional area) fibres have A-bands of 4.1 μm, one-half I bands of 2.15 μm and Z lines ⩽ 0.5 μm in width. Few mitochondria are found in these fibres, which comprise 54% of those present in a given microscope field and which occupy 82% of the total cross-sectional area. Small fibres (484 μm2 mean cross-sectional area) have A bands of 6.3 μm, one-half I bands of 3.1 μm and Z lines between 0.5 and 1.0 μm in width and are rich in mitochondria. Although small fibres comprise nearly one-half (46%) of the fibres in a field, they occupy only 18% of the total cross-sectional area. Histochemical staining for alkaline-stable myofibrillar ATPase activity and mitochondrial reduced β-nicotinamide adenine nucleotide (β-NADH) tetrazolium reductase activity confirms the presence of two fibre types. The large fibres react positively for the myofibrillar ATPase activity and negatively for the mitochondrial enzyme activity. The reverse is seen with the small fibres. Some fibres of intermediate size, having intermediate staining characteristics, were also observed. Native gel electrophoresis of both myofibrillar and purified myosin preparations supports the observed differences in myofibrillar ATPase activity in that two myosin isozymes are resolved on pyrophosphate gels. Although the thick filaments isolated from unstimulated small fibres are longer (>6.0 μm) than those isolated from unstimulated large fibres (4.26 μm), all have a similar appearance with respect to the arrangement of myosin heads on their surfaces, and similar diameters. The implications of the observed heterogeneity of fibre types is discussed with reference to previously reported phenomena inLimulus telson muscle, including changes in length of thick filaments on fibre stimulation and the shape of the length-tension curve obtained from fibre bundles.

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