Separation of active and inactive (nonphosphorylating) Ca2+-ATPase in sacroplasmic reticulum subfractions from low-frequency-stimulated rabbit muscle

Chronic low-frequency stimulation elicits in rabbit fast-twitch muscle a partial inactivation of the sarcoplasmic reticulum (SR) Ca 2+ -ATPase and Ca 2+ -uptake activities. Inactive Ca 2+ -ATPase was enriched in a light microsomal fraction by sucrose density gradient centrifugation after calcium oxalate loading in the presence of ATP. This fraction showed a reduced specific activity and phosphoprotein formation of the Ca 2+ -transport ATPase. These results suggest that the inactivation of the Ca 2+ -ATPase as induced by increased contractile activity, is confined to a specific SR vesicle population.