[Fractionation of fragments of skeletal muscle sarcoplasmic reticulum according to their sensitivity to caffeine].

Sarcoplasmic reticulum fragments were fractionated according to the ability of caffeine to selectively block Ca2+ uptake in the population of caffeine-sensitive membranes. The membrane suspension was loaded with calcium in the presence of oxalate, Mg-ATP and caffeine, after which the Ca2+-loaded caffeine-sensitive fragments were separated by sucrose density gradient centrifugation. In Ca2+-unloaded fragments of the supernatant, the sensitivity to caffeine estimated by its ability to diminish the rate of Ca2+ uptake, Ca/ATP ratio and Ca-oxalate capacity amounted to 91-93%. The terms of protein composition, the caffeine-sensitive fragments were identified with terminal cystern membranes, while the caffeine-insensitive ones with the SR canalicular membranes. The sensitivity to caffeine may serve as a reliable criterion for estimating the relative content of terminal cystern fragments in different microsomal preparations.