Calcium uptake and force development by skinned muscle fibres in EGTA buffered solutions

1. The uptake of (45)Ca and the development of force by segments of skinned muscle fibres were measured in Ca solutions buffered with EGTA. When the Ca ion concentration in these solutions was above the contraction threshold, Ca was accumulated by the sarcoplasmic reticulum during the delay phase before force developed. The uptake rate increased, and the length of the delay decreased, when the concentration of the calcium buffer was increased.2. The maximum accumulation of Ca was 2-3 mM. Force developed, ending the delay phase, when the calcium uptake approached this maximum level.3. The pattern of force development suggested that this process was often accompanied by a sudden release of accumulated Ca.4. The relation between the kinetics of (45)Ca uptake and the interaction of calcium with EGTA and the sarcoplasmic reticulum is discussed. The data indicate that at the low concentrations necessary for relaxation the sarcoplasmic reticulum takes up Ca fast enough to account for the rate at which force falls when intact muscle fibres relax.