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.
