Enzyme activities in single electrophysiologically identified crab muscle fibres.

The superficial muscle fibres in the proximal part of the closer muscle in the crab Eriphia can be separated into four fibre 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 fibres, differed significantly among the different fibre groups. In addition, fibres 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 fibres (tonic fibres) are intermediate in oxidative potential and show the lowest values of glycolytic enzymes. The group II and group III fibres can be regarded as fast oxidative fibres. The high ratio between activity levels of enzymes for glycolytic and oxidative metabolism found for group IV fibres (fast fibres) demonstrated that this group depends strongly on anaerobic metabolism.