HISTOCHEMICAL REACTIONS OF FIBERS IN A FAST TWITCH MUSCLE OF THE CAT

Serial sections of flexor digitorum longus muscle (fdl) of the cat were examined histochemically for 4 enzyme systems: ATPase with alkaline and acid pre-incubation, phosphorylase and succinic dehydrogenase (SDHase). The number of types into which fibers should be divided was assessed by estimating enzyme reaction intensity from measurements of light transmission through photomicrographys. The enzyme reaction intensities of fibers generally distributed continuously. The distribution histograms showed 2 (phosphorylase and SDHase) or 3 (acid and alkaline ATPase) clear peaks. Combinations (18) of reaction intensities (profiles) were seen of which 8 were very rare. The distribution of profiles differed between individuals but were similar in right and left muscles. Areas of fibers were measured from muscles which were fixed at the length at which twitch tension was maximal. The variance in fiber area with any 1 profile was significantly less than the variance in fiber area of all fibers within a muscle. There were significant differences between the mean areas of fibers with different profiles. If only 3 enzyme reactions are considered (acid and alkaline ATPase and phosphorylase) the majority of fibers fall into 1 of the 3 classes commonly accepted for other muscles. The remainder would fit into this classification with the minimal assumption of only 1 error of fiber typing resulting from the continuous distributions of enzyme reaction intensities. The SDHase reaction was not strongly correlated with the 3 classes and could be used to divide the fibers further into 6 groups. Differences between means of fiber areas were significant for all pairs out of these 6 groups except 1. The grouping reflects a dual system of enzymes, the 2 systems being ATPases and phosphorylase and SDHase. A possible role of nervous activity in determining this dual system is discussed. The hypothesis involves 2 partly independent characteristics of motoneuronal activity: the frequency of impulses and the total number of impulses. The measurements are correlated with other physiological variables in the individual animals. The mean areas of fibers in all groups increased with body wt. There were changes in the proportions of light and dark SDHase fibers related to wt. The total area contributed by dark alkaline ATPase fibers decreased and that by intermediate alkaline ATPase fibers increased with increasing twitch time to peak. Specific tension of the group of slower muscle fibers in fdl was estimated to be 0.29 N mm-2 compared with 0.39 N mm-2 for the faster fibers.