FAST-TWITCH AND SLOW-TWITCH ISOFORMS (SERCA1 AND SERCA2A) OF SARCOPLASMIC-RETICULUM CA-ATPASE ARE EXPRESSED SIMULTANEOUSLY IN CHRONICALLY STIMULATED MUSCLE-FIBERS

Using an immunohistochemical double-labeling technique, we observed th at different isoforms of sarcoplasmic reticulum Ca-ATPase are co-expre ssed in single fibers of canine fast-twitch skeletal muscles stimulate d chronically at low frequency. By 7 days of neuromuscular stimulation , the population of hybrid fibers expressing both SERCA1 and SERCA2a [ fast- and slow-twitch isoforms of sarco(endo)plasmic reticulum Ca2+-AT Pase] had increased from 1.5% to 9.2% of fibers. By 14 days of stimula tion 90% of the pure fast-twitch fibers (expressing only SERCA1) were replaced by hybrid fibers. An additional 28 days of stimulation caused all fast-twitch fibers to express SERCA2a at the same level as found in nonstimulated slow-twitch fibers (expressing only SERCA2a). At this time, one-half of the previously hybrid fibers had become pure slow-t witch fibers. The remaining one-half of the hybrid fibers expressed SE RCA1 at a very low level. Extending stimulation to 70 days did not fur ther change the percentage of fibers that were slow-twitch or hybrid. Immunoblot studies at the whole-muscle level confirmed that changes in SERCA expression at 42 days of neuromuscular stimulation were complet e. Immunohistochemical analysis of longitudinal sections of muscle sho wed that the changes in SERCA protein were uniform along the length of the muscle tiber, indicating that nuclei along its length responded e qually to chronic stimulation.