Plasticity in Skeletal, Cardiac, and Smooth Muscle: Selected Contribution: Low-frequency stimulation of fast muscle affects the abundance of Ca2+-ATPase but not its oligomeric status

After chronic, low-frequency stimulation, a rapiddecline in Ca²⁺ pump activity is observed during the earlystages of skeletal muscle transformation. However, thisvariation in enzymatic activity does not coincide with a drasticreduction in the amount of sarcoplasmic reticulumCa²⁺-ATPases. To investigate whether changes in subunitinteractions within Ca²⁺ pump complexes contribute to thisphenomena, we performed a chemical cross-linking analysis of 4 dayscontinuously, and 4 days discontinuously, electrostimulated fast musclefibers. The abundance of the slow and fast Ca²⁺-ATPaseisoforms sarco(endo)plasmic reticulum Ca²⁺- ATPase types1 and 2 was affected during the fast-to-slow transition process,demonstrating that, even after short-term stimulation, distinct changesin 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 responsiblefor the rapid stimulation-induced activity changes, not variations inprotein-protein interactions within Ca²⁺-ATPase units.Oligomerization appears to be of central importance to the properphysiological functioning of the Ca²⁺-ATPase and does notundergo changes during skeletal muscle conditioning.