Effect of orthophosphate and oxalate on the cold-induced release of calcium from sarcoplasmic reticulum preparations from rabbit skeletal muscle.

The cold-induced release of calcium from sarcoplasmic reticulum preparations from both white and red muscles of the rabbit was studied. Part of the release was due to the increase in pH of the reaction mixture with cooling. Calcium release was greatly reduced or completely prevented by the inclusion of oxalate or inorganic orthophosphate in the medium. No release occurred in 5 mM oxalate. With phosphate, the proportion of the calcium previously taken up at 23 degrees C that was released at 0 degrees C became progressively smaller as the phosphate concentration was increased. When the pH was adjusted to be the same at 0 degrees C as at 23 degrees C there was little release from white muscle preparations in 10 mM phosphate and no release when the phosphate concentration was 20 mM or more. With red muscle preparations calcium was released at higher phosphate concentrations, 8% of the amount previously taken up still being released at 50 mM phosphate and a smaller amount at 100 mM phosphate. The effects of oxalate and phosphate can be explained in terms of the reduction in free calcium concentration inside the vesicles by calcium precipitants, and a difference in the temperature coefficients of calcium inflow and outflow.