CA2-LINES IN RAT-HEART CELLS( SPARKS INVOLVING MULTIPLE CA2+ RELEASE SITES ALONG Z)

1. High spatial resolution confocal imaging was used to investigate th e fundamental nature of 'Ca2+ sparks' in rat cardiac myocytes loaded w ith the fluorescent calcium indicator, fluo-3. 2. The sites at which c alcium sparks occurred (Ca2+ release sites) were packed closely and ir regularly in transverse planes along Z-lines (mean spacing between sit es of 0.76 mu m). In contrast, sites were spaced more regularly in the longitudinal direction, at intervals of 1.8 mu m (i.e. the sarcomere length). 3. Diffusion of released Ca2+ mas slower transversely (appare nt diffusion coefficient, D, 7.9 mu m(2) s(-1)) than longitudinally (D , 17.1 mu m(2) s(-1)). 4. Frequently, discrete sites several hundred n anometres apart transversely activated in near synchrony. The probabil ity of transverse synchronous activity fell to low levels (<20%) at si tes separated by more than 1.0 mu m. Synchronous activation was not ob served between sites on different Z-lines (i.e. separated longitudinal ly by 1.8 mu m). 5. High temporal resolution confocal microscopy (stat ionary spot) revealed Ca2+ sparks with 'stepped' rises, consistent wit h multiple sites of origin. 6. We conclude that the Ca2+ spark as orig inally described is usually not an 'elementary' event, in the sense of being indivisible, but is often comprised of yet smaller, triggered u nits of Ca2+ release.