Eccentric exercise-induced morphological changes in the membrane systems involved in excitation-contraction coupling in rat skeletal muscle

1. Physiological evidence suggests that excitation-contraction (E-C!) coupl ing failure results from eccentric contraction-induced muscle injury becaus e of structural and morphological damage to membrane systems directly assoc iated with the E-C coupling processes within skeletal muscle fibres. In thi s study using rats, we observed the ultrastructural features of the membran e sq stems of fast-twitch (FT) and slow-twitch (ST) muscle fibres involved in E-C coupling following level and downhill running exercise. Our aim was to find out whether mechanically mediated events following eccentric exerci se caused disorder in the membrane systems: involved in E-C coupling, and h ow soon after exercise such disorder occurred. We also compared the morphol ogical changes of the membrane systems between ST and FT muscle from; withi n the same muscles. 2. Single muscle fibres were dissected from triceps brachii muscles of male Fischer 344 rats after level or downhill (16 deg decline) motor-driven tre admill running (18 m min(-1), 5 min running with 2 min rest interval, 18 bo uts). All single muscle fibres were histochemically classified into ST or F T fibres. The membranes system were visualized using Ca2+-K3Fe(CN)(6)-OsO4 techniques, and ol,served by high voltage electron microscopy (120-200 kV). 3. There were four obvious ultrastructural changes in the arrangement of th e transverse (t)-tubules and the disposition of triads after the downhill r unning exercise: (1) an increase in the number of longitudinal segments of the t-tubule network, (2) changes in the direction and disposition of triad s, (3) the appearance of caveolar clusters, and (4) the appearance of penta ds and heptads (close apposition of two or three t-tubule elements with thr ee or four elements of terminal cisternae of the sarcoplasmic reticulum). T he caveolar clusters appeared almost exclusively in the ST fibres immediate ly after downhill running exercise and again 16 h later. The pentads and he ptads appeared almost, exclusively in the FT fibres, and their numberu incr eased dramatically 2-3 days after the downhill running exercise. 4. The eccentric exercise led to tile formation of abnormal membrane system s involved in E-C coupling processes. These systems hal e unique morphologi cal features, which differ beta-een ST and FT fibres, even within the same skeletal muscle, and the damage appears to we concentrated in the FT fibres . These observations also support the idea that eccentric exercise induced E-C coupling failure is due to physical and chemical disruption of the memb rane systems involved in the E-C coupling process in skeletal muscle.