Calcium uptake in glycerol-extracted rabbit psoas muscle fibers. I. Biochemical properties and conditions for uptake

Rabbit psoas muscle fibers, extracted in 50% (w/v) glycerol for 20 hours, concentrate up to 300 μmol calcium per gm protein when placed in a suitable electrolyte environment at pH 6.5 containing ATP and 47Ca in the form of a Ca/EGTA metal buffer system. This property is retained at [Ca++] down to 2 × 10−8 M, the lowest studied. The kinetics of Ca pickup are consistent with a mathematical model based on diffusion of Ca as the rate‐limiting factor. The pickup is dependent upon ATP and is increased by inclusion of CP or PEP. It is promoted by oxalate, fluoride, phosphate and pyrophosphate, which share the property of forming relatively insoluble calcium salts. Ability to concentrate calcium disappears on prolonged extraction in glycerol and after preincubation with desoxycholate. These properties are similar to those previously described for a granular fraction of muscle homogenates, and it is concluded that both are probably attributable to functional remnants of sarcoplasmic reticulum.