Measurement of sarcoplasmic reticulum Ca2+ content and sarcolemmal Ca2+ fluxes in isolated rat ventricular myocytes during spontaneous Ca2+ release

Intracellular calcium concentration (Ca2+i) and Na(+)-Ca2+ exchange currents were measured in calcium-overloaded voltage-clamped rat ventricular myocytes loaded with the Ca(2+)-sensitive fluorescent indicator indo-1. Sarcoplasmic reticulum (SR) Ca2+ content was measured from the integral of the caffeine-evoked current. In cells that had spontaneous SR Ca2+ release in 1 mM external Ca2+ concentration (Ca2+o)i raising Ca2+o increased the frequency of release with no effect on SR Ca2+ content. In quiescent cells, increased Ca2+o produced spontaneous Ca2+ release associated with increased SR Ca2+ content. Further increase of Ca2+o had no effect on SR Ca2+ content. The amount of Ca2+ leaving the cell during each release was constant over a wide range of frequencies and Ca2+o values. It appears there is a maximum level of SR Ca2+ content, perhaps because spontaneous Ca2+ release results when the content reaches a threshold. 2. From the relationship between Ca2+i and Na(+)-Ca2+ exchange current during a caffeine response, it is possible to estimate the changes in Na(+)-Ca2+ exchange current expected from a change of Ca2+i. The data show that the calcium oscillations contribute a significant fraction of the total extra Ca2+ efflux induced by increasing Ca2+o. Raising Ca2+o decreased the rate of calcium removal from the cell as measured from the rate of decay of the caffeine response, suggesting that both inhibition of Ca2+ efflux and increased Ca2+ entry account for the Ca2+ overload at elevated Ca2+o. 3. Inhibiting spontaneous SR Ca2+ release increases resting Ca2+i. The Ca2+ efflux is identical to that in the presence of release. It is concluded that spontaneous release of calcium, although potentially arrhythmogenic, is an effective way to activate Ca2+ efflux in overloaded conditions and minimizes any increase of diastolic tension.