Estimation of the fractional sarcoplasmic reticulum Ca2+ release in intact cardiomyocytes using integrated Ca2+ fluxes

The sarcoplasmic reticulum (SR) is the main source of contraction-activating Ca2+ in the adult mammalian myocardium. The fraction of the SR Ca2+ content released at a twitch (fractional SR Ca2+ release, FR) is an important parameter for assessing the efficiency of excitation-contraction coupling under physiological and pathophysiological conditions, as well as for identification of modulators of this process. We here describe and propose an approach for FR quantitation based on the estimation of integrated Ca2+ fluxes mediated by different transporters that remove the ion from the cytosol. These fluxes may be calculated solely from the measurement of cytosolic free Ca2+ concentration ([Ca2+]i) during Ca2+ transients evoked under selective inhibition of the transporters, and from the cell Ca2+ buffering parameters available in the literature. The FR values obtained with this approach in intact rat ventricular myocytes (0.63 ± 0.04; n = 12) were comparable to those estimated in the same cell type with an already established method, based on electrophysiological measurements with the patch-clamp technique, in addition to [Ca2+]i measurement (0.69 ± 0.05; n = 6; p > 0.40). We conclude that the proposed method might be a suitable and technically simpler alternative to the electrophysiological method for FR estimation.