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

Abstract

After chronic, low-frequency stimulation, a rapid decline in Ca2+ pump activity is observed during the early stages of skeletal muscle transformation. However, this variation in enzymatic activity does not coincide with a drastic reduction in the amount of sarcoplasmic reticulum Ca2+-ATPases. To investigate whether changes in subunit interactions within Ca2+ pump complexes contribute to this phenomena, we performed a chemical cross-linking analysis of 4 days continuously, and 4 days discontinuously, electrostimulated fast muscle fibers. The abundance of the slow and fast Ca2+-ATPase isoforms sarco(endo)plasmic reticulum Ca2+- ATPase types 1 and 2 was affected during the fast-to-slow transition process, demonstrating that, even after short-term stimulation, distinct changes in the isoform expression pattern of muscle proteins occur. However, the oligomeric status of both ion pump species did not change. Hence, chemical modifications of critical enzyme domains must be responsible for the rapid stimulation-induced activity changes, not variations in protein-protein interactions within Ca2+-ATPase units. Oligomerization appears to be of central importance to the proper physiological functioning of the Ca2+-ATPase and does not undergo changes during skeletal muscle conditioning.