Potentiation of Fractional Sarcoplasmic Reticulum Calcium Release by Total and Free Intra-Sarcoplasmic Reticulum Calcium Concentration

Abstract

Our aim was to measure the influence of sarcoplasmic reticulum (SR) calcium content ([Ca] SRT) and free SR [Ca] ([Ca] SR) on the fraction of SR calcium released during voltage clamp steps in isolated rabbit ventricular myocytes. [Ca] SRT, as measured by caffeine application, was progressively increased by conditioning pulses. Sodium was absent in both the intracellular and in the extracellular solutions to block sodium/calcium exchange. Total cytosolic calcium flux during the transient was inferred from I Ca, [Ca] SRT, [Ca] i, and cellular buffering characteristics. Fluxes via the calcium current ( I Ca), the SR calcium pump, and passive leak from the SR were evaluated to determine SR calcium release flux ( J rel). Excitation-contraction (EC) coupling was characterized with respect to both gain ( ʃ J rel /ʃ I Ca) and fractional SR calcium release. Both parameters were virtually zero for a small, but measurable [Ca] SRT. Gain and fractional SR calcium release increased steeply and nonlinearly with both [Ca] SRT and [Ca] SR. We conclude that potentiation of EC coupling can be correlated with both [Ca] SRT and [Ca] SR. While fractional SR calcium release was not linearly dependent upon [Ca] SR, intra-SR calcium may play a crucial role in regulating the SR calcium release process.