Termination of Ca²⁺ release during Ca²⁺ sparks in rat ventricular myocytes

1. Confocal Ca²⁺ imaging was used to measure spontaneous release events (Ca²⁺ sparks) in fluo-3-loaded isolated rat ventricular myocytes. 2. The microscopic Ca²⁺ release flux underlying Ca²⁺ sparks was derived by adapting the methods used previously to describe macroscopic Ca²⁺ release from cell-averaged Ca²⁺ transients. 3. The magnitude of the local release fluxes varied from 2 to 5 μM ms^-1, depending on SR Ca²⁺ loading conditions. Following spontaneous activation, the release flux rapidly decayed (τ = 6-12 ms). The rate of termination of release flux was found to be directly related to the magnitude of the flux (r² = 0.88). 4. The rate of termination of local release flux was slowed in the presence of FK506, a compound that is known to reduce inactivation of SR Ca²⁺ channels in vitro. 5. These results suggest that termination of release flux during sparks is not due to a spontaneous stochastic decay process or local depletion of Ca²⁺ from the SR, but rather involves an active extinguishing mechanism such as Ca²⁺-dependent inactivation or adaptation.