Spatiotemporal characteristics of SR Ca²⁺ uptake and release in detubulated rat ventricular myocytes

In cardiac ventricular myocytes, sarcoplasmic reticulum (SR) Ca ²⁺ load is a key determinant of SR Ca²⁺ release. This release normally occurs predominantly from SR junctions at sarcolemmal invaginations (t-tubules), ensuring synchronous SR Ca²⁺ release throughout the cell. However under conditions of Ca²⁺ overload, spontaneous SR Ca²⁺ release and propagating Ca²⁺ waves can occur, which are pro-arrhythmic. We used detubulated rat ventricular myocytes to determine the dependence of Ca²⁺ wave propagation on SR Ca ²⁺ load, and the role of t-tubules in SR Ca²⁺ uptake and spontaneous release. After SR Ca²⁺ depletion, recovery of Ca ²⁺ transient amplitude (and SR Ca²⁺ load) was slower in detubulated than control myocytes (half-maximal recovery: 9.9 ± 1.4 vs. 5.5 ± 0.7:beats). In detubulated myocytes the extent and velocity of Ca²⁺ propagation from the cell periphery increased with each beat and depended steeply on SR Ca²⁺ load. Isoproterenol (ISO) accelerated recovery, increased maximal propagation velocity and reduced the threshold SR Ca²⁺ load for propagation. Ca²⁺ spark frequency was uniform across control cell width and was similar at the periphery of detubulated cells. However, internal Ca²⁺ spark frequency in detubulated cells was 75% lower (despite comparable local SR Ca²⁺ load); this transverse spark frequency profile was similar to that in atrial myocytes. We conclude that: (1) t-tubule Ca²⁺ fluxes normally control SR Ca²⁺ refilling; (2) Ca²⁺ wave propagation depends steeply on SR Ca²⁺ content (3) SR-t-tubule junctions are important in initiating SR Ca²⁺ release and (4) ISO enhances propagation of SR Ca release, but not the initiation of SR Ca release events (for given SR Ca ²⁺ loads).