Reverse mode of the sarcoplasmic reticulum calcium pump and load-dependentcytosolic calcium decline in voltage-clamped cardiac ventricular myocytes

We have characterized [Ca](i) decline in voltage-clamped rabbit ventricular myocytes with progressive increases in sarcoplasmic reticulum (SR) calcium load, "Backflux" through the SR calcium pump is a critical feature which a llows realistically small values for SR calcium leak fluxes to be used, Tot al cytosolic calcium was calculated from the latter part of [Ca](i) decline using rate constants for cellular calcium buffers, Intra-SR calcium buffer ing characteristics were also deduced, We found that the net SR calcium pum p flux and rate of [Ca](i) decline decreased as the SR free [Ca] rose, with pump parameters held constant. We have therefore characterized for the fir st time in intact myocytes both forward and reverse SR calcium pump kinetic s as well as intra-SR calcium buffering and SR calcium leak. We conclude th at the reverse flux through the SR calcium pump is an important factor in c omprehensive understanding of dynamic SR calcium fluxes.