Enzyme activities in single electrophysiologically identified crab muscle fibres.

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Abstract
The superficial muscle fibers in the proximal part of the closer muscle in the crab Eriphia can be separated into four fiber groups (I-IV) on the basis of electrophysiological and histochemical characteristics. The activity levels of glyceraldehydephosphate dehydrogenase (GAPDH), lactate dehydrogenase (LDH), citrate synthase (CS), NADP-isocitrate dehydrogenase (IDH) and 3-hydroxyacyl-CoA dehydrogenase (HAD), determined in single electrophysiologically identified fibers, differed significantly among the different fiber groups. In addition, fibers belonging to the same group, with similar electrophysiological characteristics, demonstrated variability with regard to metabolic enzyme activities. Nevertheless, comparison of absolute enzyme activities and enzyme activity ratios permitted the discrimination of at least three groups. These groups corresponded with those defined according to electrophysiological and histochemical characteristics. The group I fibers (tonic fibers) are intermediate in oxidative potential and show the lowest values of glycolytic enzymes. The group II and group III fibers can be regarded as fast oxidative fibers. The high ratio between activity levels of enzymes for glycolytic and oxidative metabolism found for group IV fibers (fast fibers) demonstrated that this group depends strongly on anaerobic metabolism.