Sarcoplasmic reticulum and L-type Ca²⁺ channel activity regulate the beat-to-beat stability of calcium handling in human atrial myocytes.

Irregularities in intracellular calcium on a beat-to-beat basis can precede cardiac arrhythmia, but the mechanisms inducing such irregularities remain elusive. This study tested the hypothesis that sarcoplasmic reticulum (SR) and L-type calcium channel activity determine the beat-to-beat response and its rate dependency. For this purpose, patch-clamp technique and confocal calcium imaging was used to record L-type calcium current (ICa) and visualize calcium in human atrial myocytes subjected to increasing stimulation frequencies (from 0.2 to 2 Hz). The beat-to-beat response was heterogeneous among a population of 133 myocytes, with 30 myocytes responding uniformly at all frequencies, while alternating and irregular responses were induced in 78 and 25 myocytes, respectively. Myocytes with uniform responses had the lowest frequency of calcium wave-induced transient inward currents (ITI; 0.4 ± 0.2 min⁻¹), ICa density (1.8 ± 0.3 pA pF⁻¹) and caffeine-releasable calcium load (6.2 ± 0.5 amol pF⁻¹), while those with alternating responses had the highest ITI frequency (1.8 ± 0.3 min⁻¹,P =0.003) and ICa density (2.4 ± 0.2 pA pF⁻¹, P =0.04). In contrast, the calcium load was highest in myocytes with irregular responses (8.5 ± 0.7 amol pF⁻¹, P =0.01). Accordingly, partial ICa inhibition reduced the incidence (from 78 to 44%, P <0.05) and increased the threshold frequency for beat-to-beat alternation (from 1.3 ± 0.2 to 1.9 ± 0.2 Hz, P <0.05). Partial inhibition of SR calcium release reduced the ITI frequency, increased calcium loading and favoured induction of irregular responses, while complete inhibition abolished beat-to-beat alternation at all frequencies. In conclusion, the beat-to-beat response was heterogeneous among human atrial myocytes subjected to increasing stimulation frequencies, and the nature and stability of the response were determined by the SR and L-type calcium channel activities, suggesting that these mechanisms are key to controlling cardiac beat-to-beat stability.