Local Ca2+ transients (Ca2+ sparks) originate at transverse tubules in rat heart cells.

1. The origins of local [Ca2+]i transients (Ca2+ sparks) were studied using dual‐channel confocal laser scanning microscopy. Line scan images showing [Ca2+]i (as fluo‐3 fluorescence) and the transverse tubule membranes (as Di‐8 fluorescence) were obtained simultaneously in single rat cardiac ventricular cells. 2. Line scan images of Di‐8 fluorescence showed peaks regularly spaced at intervals of 1.83 +/‐ 0.30 microns (mean +/‐ S.D.). These peaks corresponded to the transverse tubules (T‐tubules) in cross‐section. 3. Line scan images of fluo‐3 fluorescence showed local [Ca2+]i transients (LCTs or Ca2+ sparks) evoked by electrical stimulation. 4. Eighty‐five per cent (85%) of all Ca2+ sparks evoked by electrical stimulation (n = 138, in 5 cells) occurred within 0.5 micron of a T‐tubule. Thirty per cent (30%) occurred within 1 pixel (0.20 micron) of a T‐tubule. 5. In some cells studied (3 out of 5), certain T‐tubules had a higher probability of being sites of origin of Ca2+ sparks than others. 6. These results support local control theories of excitation‐contraction coupling in which Ca2+ release from the sarcoplasmic reticulum (SR) is triggered by a high local [Ca2+]i established between the L‐type Ca2+ channels in the T‐tubules and associated ryanodine receptor(s) in the junctional SR.