Intrasarcomere [Ca2+] gradients and their spatio-temporal relation to Ca2+sparks in rat cardiomyocytes

Abstract

1 Line‐scan analyses of spontaneous Ca²⁺ sparks, non‐propagating local 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, points at which rise in Ca²⁺ began earliest were observed at regular spacings of 1.82 ± 0.26 μm (mean ± s.d.) along the longitudinal axis of the cell. The points were heavily stained with di‐2‐ANEPEQ, which stains the T‐tubules, indicating that they were at the Z‐line. 3 The points where spontaneous Ca²⁺ sparks originated coincided with the points 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 evoked Ca²⁺ transient, fast elevation of Ca²⁺ was not observed at the corresponding Z‐line, indicating the presence of a refractory period in Ca²⁺ release from the SR. 5 The present results provide visual evidence for Ca²⁺ release from the junctional sarcoplasmic reticulum in cardiomyocytes. The presence of a refractory period in Ca²⁺ release after Ca²⁺ sparks provided new evidence that the normal Ca²⁺ transient may be the summation of Ca²⁺ sparks.