Myofibrillar-protein isoforms and sarcoplasmic-reticulum Ca2+-transport activity of single human muscle fibres

Abstract

The polymorphism of myofibrillar proteins and the Ca2+ uptake activity of sarcoplasmic reticulum were analyzed in single fibers from human skeletal muscles. Two populations of histochemically identified type-I fibers were found differing in the number of light chain isoforms of the constituent myosin; the pattern of light chains of fast myosin of type-IIA and type-IIB fibers was indistinguishable. Regulatory proteins, troponin and tropomyosin, and other myofibrillar proteins, such as M- and C-proteins, showed specific isoforms in type-I and type-II fibers. Tropomyosin presented different stoichiometries of the .alpha.- and .beta.-subunits between the 2 types of fibers. Sarcoplasmic reticulum volume, as indicated by the maximum capacity for calcium oxalate accumulation, was almost identical in type-I and type-II fibers; the rate of Ca2+ transport as twice as high in type-II as compared with type-I fibers. In normal human muscle fibers, there is a tight segregation of fast and slow isoforms of myofibrillar proteins that is very well coordinated with the relaxing activity of the sarcoplasmic reticulum. These findings may represent a molecular correlation with the differences of the twitch-contraction time between fast and slow human motor units. This tight segregation is partially lost in the muscle fibers of elderly individuals.