Changes of SERCA Activity have Proportionately Smaller Effects on Sarcoplasmic Reticulum Calcium Content

Abstract

The sarcoplasmic reticulum (SR) provides the majority of calcium to the systolic calcium transient. It is this rise in intracellular calcium that causes cardiac systole. The SR is, therefore, important for maintaining cardiac contractility. SR calcium content ([Ca]SR) is replenished by the sacro(endo)plasmic reticulum calcium ATP-ase (SERCA). It might be expected that reducing SERCA activity would result in an equally decreased [Ca]SR, impairing contractility. However, Andersson et al. (J. Mol. Cell. Cardiol. 47, 180-7) have shown, in a SERCA2 knock-out mouse model, that reduction of cardiac SERCA2 expression to 5 % of control levels only decreased SR calcium to 38 % of control. We now investigate how acute changes to SERCA activity affect [Ca]SR in the normal ventricular myocyte. Single isolated rat ventricular myocytes were used and intracellular calcium concentration measured with fluo-5. Experiments were performed at 37 °C, using the perforated patch technique. Systolic calcium transients were evoked with depolarizing pulses. SERCA activity was quantified from the rate constant of decay of the calcium transient and [Ca]SR from the integral of the caffeine-evoked Na-Ca exchange current. 1 μmol/l thapsigargin was applied to the cell to slow SERCA activity to varying levels. We found that [Ca]SR was proportional to (SERCA activity)1/m where the mean value for m was 3.55. This relationship can be accounted for by the fact that calcium efflux from the SR is a steep function of SR calcium. A decrease in SERCA activity will decrease [Ca]SR and even a small reduction of SR calcium will lower calcium release to balance uptake.