Distinct subtypes of type I fibers of human skeletal muscle

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Abstract
Two distinct and consistent type I fiber subtypes were identified in histochemically normal biopsies (vastus lateralis or biceps brachialis) from 12 healthy persons, both sexes, ages 15-42. The type I fibers were uniformly light with the regular ATPase (9.4) and uniformly dark with the acid-preincubated reverse ATPase reactions. The subtype IA fibers were stained darker than subtype IB fibers with the NAD-dehydrogenase-tetrazolium reductase, Oil red O, NADH phosphate-tetrazolium reductase, nonspecific esterase, succinic-tetrazolium reductase, NAD-linked malic-tetrazolium reductase, and NAD-linked lactic-tetrazolium reductase, and stained lighter with the phosphorylase, periodic acid-Schiff for glycogen and menadione-mediated .alpha.-glycerophosphate-tetrazolium reductase reactions. The IB fibers were not as light (or as dark, depending on the stain) as any of the type II fibers. In one pathologic case, all the hypotrophic type I muscle fibers with central nuclei were subtype IB. In case of chronic infantile spinal atrophy the muscle showed some type-grouping and marked type I fiber predominance; the great majority of type I fibers were subtype IB, among which were islands (subtype-groups) of IA fibers. Subtyping of type I fibers may be informative in evaluating the pathologic process of certain human neuromuscular diseases.