Estimates of beat to beat handling of activator calcium using measurements of [Ca2+]i in aequorin loaded ferret cardiac muscle.

Abstract

The aims were (1) to measure simultaneously and on a beat to beat basis intracellular calcium concentration ([Ca2+]i) transients and force transients in isolated ferret cardiac trabeculae; (2) to obtain and compare independent estimates of the recirculating fraction of Ca2+ using the [Ca2+]i data and the force data (recirculating fraction is the fraction of activator Ca2+ taken up by the sarcoplasmic reticulum in each beat and, in the steady state twitch, the fraction of activator Ca2+ released by the sarcoplasmic reticulum); and (3) to estimate the amount of Ca2+ that returns to the sarcoplasmic reticulum and the amount that, during steady state contractions, enters the cytosol, presumably from the extracellular compartment, with each beat. Eight trabeculae were mounted in the myograph. The servo-controlled muscle length was 98% of the length at which developed force was maximal. A modified technique was used for chemical loading of aequorin, and a new method for computer controlled low level photon counting, storage, calibration, and analysis. [Ca2+]i transients and force transients were simultaneously recorded during potentiated beats, together with their respective decays toward control steady state [Ca2+]i transients and force transients. A modified test of postextrasystolic potentiation achieved with a brief train of rapid pacing followed by a pause was used to evoke the potentiated beats. At 2.0 mM extracellular Ca2+ ([Ca2+]o), resting [Ca2+]i was 283(SD 77) nM. The resting tension was 1.6(0.3) g.mm-2. The steady state [Ca2+]i transient and the peak potentiated [Ca2+]i transient averaged 992(165) and 1290(154) nM respectively. The corresponding tensions were 4.0(1.9) and 8.7(3.1) g.mm-2 respectively. The recirculating fraction of Ca2+ calculated from the dissipation of the potentiated [Ca2+]i transient averaged 45(4)%. This recirculating fraction was indistinguishable from the one calculated with another method from the decay of the force potentiation. This is the first study to estimate the recirculating fraction of activator Ca2+ using measurements of [Ca2+]i. The results indicate that over a wide range of [Ca2+]i and tensions the Ca(2+)-force relationship is well approximated by a straight line. At 2.0 mM [Ca2+]o it appears that some 450 nM of Ca2+ recirculates and that a similar amount per steady state beat enters the cytosol, probably from the extracellular compartment.