A general procedure for determining the rate of calcium release from the sarcoplasmic reticulum in skeletal muscle fibers

A general procedure for using myoplasmic calcium transients measured with a metallochromic indicator dye to calculate the time course of calcium release from the sarcoplasmic reticulum in voltage-clamped skeletal muscle fibers is described and analyzed. Explicit properties are first assigned to all relatively rapidly equilibrating calcium binding sites in the myoplasm so that the calcium content (CaF) in this pool of "fast" calcium can be calculated from the calcium transient. The overall properties of the transport systems and relatively slowly equilibrating binding sites that remove calcium from CaF are then characterized experimentally from the decay of CaF following fiber repolarization. The rate of calcium release can then be calculated as dCaF/dt plus the rate of removal of calcium from CaF. Two alternatives are assumed for the component of CaF that is due to fast binding sites intrinsic to the fiber: a linear instantaneous buffer or a set of binding sites having properties similar to thin filament troponin. Both assumptions yielded similar calcium release wave forms. Three alternative methods for characterizing the removal system are presented. The choice among these or other methods for characterizing removal can be based entirely on convenience since any method that reproduces the decay of CaF following fiber repolarization will give the same release wave form. The calculated release wave form will be accurate provided that the properties assumed for CaF are correct, that release turns off within a relatively short time after fiber repolarization, that the properties of the slow removal system are the same during and after fiber depolarization, and that possible spatial nonuniformities of free or bound calcium do not introduce major errors