INTRASARCOMERE [CA2+] GRADIENTS AND THEIR SPATIOTEMPORAL RELATION TO CA2+ SPARKS IN RAT CARDIOMYOCYTES

1. Line-scan analyses of spontaneous Ca2+ sparks, non-propagating loca l rises in Ca2+ concentration, and the early phase of Ca2+ transients in cardiomyocytes were performed with a rapid-scanning laser confocal microscope (Nikon RCM8000) and fluo-3. 2. On electrical stimulation, p oints at which rise in Ca2+ began earliest were observed at regular sp acings of 1.82 +/- 0.26 mu m (mean +/- S.D.) along the longitudinal ax is of the cell. The points were heavily stained with di-2-ANEPEQ, whic h stains the T-tubules, indicating that they were at the Z-line. 3. Th e points where spontaneous Ca2+ sparks originated coincided with the p oints which showed faster Ca2+ elevation, i.e. the Z-line. 4. In some cases where a Ca2+ spark had occurred within about 30 ms before the ev oked Ca2+ transient, fast elevation of Ca2+ was not observed at the co rresponding Z-line, indicating the presence of a refractory period in Ca2+ release from the SR. 5. The present results provide visual eviden ce for Ca2+ release from the junctional sarcoplasmic reticulum in card iomyocytes. The presence of a refractory period in Ca2+ release after Ca2+ sparks provided new evidence that the normal Ca2+ transient may b e the summation of Ca2+ sparks.