Low-frequency stimulation of fast muscle affects the abundance of Ca2+-ATPase but not its oligomeric status

After chronic, low-frequency stimulation, a rapid decline in Ca2+ pump acti vity is observed during the early stages of skeletal muscle transformation. However, this variation in enzymatic activity does not coincide with a dra stic reduction in the amount of sarcoplasmic reticulum Ca2+-ATPases. To inv estigate whether changes in subunit interactions within Ca2+ pump complexes contribute to this phenomena, we performed a chemical cross-linking analys is of 4 days continuously, and 4 days discontinuously, electrostimulated fa st muscle fibers. The abundance of the slow and fast Ca2+-ATPase isoforms s arco(endo)plasmic reticulum Ca2+-ATPase types 1 and 2 was affected during t he fast-to-slow transition process, demonstrating that, even after short-te rm stimulation, distinct changes in the isoform expression pattern of muscl e proteins occur. However, the oligomeric status of both ion pump species d id not change. Hence, chemical modifications of critical enzyme domains mus t be responsible for the rapid stimulation-induced activity changes, not va riations in protein-protein interactions within Ca2+-ATPase units. Oligomer ization appears to be of central importance to the proper physiological fun ctioning of the Ca2+-ATPase and does not undergo changes during skeletal mu scle conditioning.