TERMINATION OF CA2+ RELEASE DURING CA2+ SPARKS IN RAT VENTRICULAR MYOCYTES

1. Confocal Ca2+ imaging was used to measure spontaneous release event s (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+ r elease from cell-averaged Ca2+ transients. 3. The magnitude of the loc al release fluxes varied from 2 to 5 mu M ms(-1), depending on SR Ca2 loading conditions. Following spontaneous activation, the release flu x rapidly decayed (tau = 6-12 ms). The rate of termination of release flux was found to be directly related to the magnitude of the flux (r( 2) = 0.88). 4. The rate of termination of local release flux was slowe d in the presence of FK506, a compound that is known to reduce inactiv ation of SR Ca2+ channels in vitro. 5. These results suggest that term ination of release flux during sparks is not due to a spontaneous stoc hastic decay process or local depletion of Ca2+ from the SR, but rathe r involves an active extinguishing mechanism such as Ca2+-dependent in activation or adaptation.