CONTRACTILE PROPERTIES OF SINGLE SLOW- AND FAST-TYPE SKELETAL MUSCLE FIBERS FROM AGED RATS 992

Abstract

Significant alterations in mammalian skeletal muscle morphologic and biochemical properties are known to occur with aging; however, little information is available on the fiber-type specific contractile properties. The purpose of this study was to determine the fiber-type specific contractile properties of slow type I, fast type IIa and hybrid type I-IIa fibers in the soleus (SOL) and red gastrocnemius (RG) muscles from aged (30 mos old) Fisher BN male rats (n=4). Single skinned fiber techniques were used to examine the diameter (dia), absolute force (mg), peak force per cross-sectional area (Po) and maximal shortening velocity (Vo, determined by the slack test). Type I, IIa, and hybrid (fibers containing both MHC I and IIa) fibers were determined by SDS-PAGE. No differences in fiber dia were observed between SOL type I(88±2μm), RG type I (84±3μm), and RG type IIa(83±3μm) fibers. Although there was no difference in absolute force between fiber-types, Po showed fiber-type differences with Po of the RG type IIa < RG type I < RG hybrid type I-IIa fibers. The Po of slow type I fibers from the SOL and RG was not different. Significant fiber-type differences were observed in Vo. In SOL and RG, the Vo of slow type I fibers was 0.98±0.09 fl/s and 1.06±0.28 fl/s, respectively. In contrast, the RG fast type IIa fibers exhibited the highest velocities(3.13±0.40 fl/s), followed by hybrid type I-IIa fibers(2.02±0.26 fl/s). In conclusion, this study demonstrates fiber-type specific contractile properties in single skeletal muscle fibers in aged rats.