Abstract
1. High spatial resolution confocal imaging was used to investigate spontaneous calcium release events (Ca2+ sparks) in isolated rat cardiac myocytes loaded with the fluorescent calcium indicator fluo-3. 2. Frequencies of sparks at different release sites varied widely, with a few sites showing sustained activities as great as 50 times the average. Sites with frequent sparks showed more rapid recovery of activity following Ca2+ waves and locally elevated basal [Ca2+]. 3. In addition to transient sparks, some sites showed bursts of rapid flurries of spark-like events, or apparently sustained Ca2+ liberation. Bursts remained localized to individual z-lines, but adjacent sites on the same z-line could be 'driven' by a bursting site to generate similar activity. 4. Individual sites showed long-term (tens of seconds) changes in 'modes' of activity, with abrupt transitions in frequencies of sparking, and between transient sparks and sustained bursts. These transitions were not associated with changes in the amplitude of the sparks. 5. We conclude that spontaneous sparks are not stereotyped events generated with uniform probability at all sites. Instead, the Ca2+ release event in variable, and some sites have a high probability of spark generation. Both factors show long-term changes at individual sites, raising the possibility that properties of fundamental Ca2+ release units may be subject to modulation.