Aims: 

The extent and synchrony of calcium release from individual calcium release sites in cardiac myocytes varies considerably. Our aim was to compare latencies and amplitudes of calcium release events to reveal possible common determinants.

Methods: 

Local calcium release events (Ca-spikes) were evoked by calcium currents in voltage-clamped isolated rat ventricular myocytes. The cells were excited by a 70-ms depolarization from -50 to 0 mV. Ca-spikes were measured using 0.1mM fluo-3 as the calcium indicator and 1 mM EGTA to limit calcium diffusion, and compared with simulated Ca-spikes [1]. Three-dimensional convolution of fluo-3 concentration with a Gaussian kernel was used for simulation of Ca-spike images. Both the measured and simulated local calcium release events were analyzed as previously described [1].

Results: 

The amplitude as well as kinetic parameters of simulated Ca-spikes were strongly dependent on the distance of the event from the focal plane. In measured early calcium release events, the relationships between their fluorescence amplitude, time-to-peak, and duration was consistent with uniform calcium release flux amplitude, while in the case of the late calcium release events these relationships could be explained only if assuming a decreased calcium release flux amplitude.

Conclusion: 

These results indicate that late Ca-spikes have decreased calcium release flux, either because the presence of inactivated ryanodine receptors, or due to locally lower sarcoplasmic reticulum calcium contents. Support: APVT-51-031104, EU Contract No. LSHM-CT-2005-018833/EUGeneHeart, NIH-FIRCA-R03-TW-05543.

1. Zahradnikova A., jr. et al., J Physiol. 578: 677-691, 2007.