Regional differences in spontaneous Ca2+ spark activity and regulation in cat atrial myocytes

Calcium sparks result from the concerted opening of a small number of Ca2+ release channels (ryanodine receptors, RyRs) organized in clusters in the membrane of the sarcoplasmic reticulum (SR). Calcium sparks represent the elementary events of SR Ca2+ release in cardiac myocytes, and their spatial and temporal summation results in whole‐cell [Ca2+]i transients observed during excitation–contraction coupling (ECC). Atrial myocytes generally lack transverse tubules; however, during ECC Ca2+ release is initiated from junctional SR (j‐SR) in the cell periphery from where activation propagates inwardly through Ca2+‐induced Ca2+ release (CICR) from non‐junctional SR (nj‐SR). Despite the structural differences in the microdomains of RyRs of j‐SR and nj‐SR, spontaneous Ca2+ sparks are observed from both types of SR, albeit at different frequencies. In cells that showed spontaneous Ca2+ sparks from j‐SR and nj‐SR, subsarcolemmal (SS) Ca2+ sparks from the j‐SR were 3–4 times more frequent than central (CTR) Ca2+ sparks occurring from nj‐SR. Subsarcolemmal Ca2+ sparks had a slightly higher amplitude, but were essentially identical in their spatial spread and duration when compared to CTR Ca2+ sparks. Sensitization of RyRs with a low concentration (0.1 mm) of caffeine led to a 107% increase in the frequency of CTR Ca2+ sparks, whereas the SS Ca2+ spark frequency increased by only 58%, suggesting that the nj‐SR is capable of much higher Ca2+ spark activity than observed normally in unstimulated cells. The L‐type Ca2+ channel blocker verapamil reduced SS Ca2+ spark frequency to 38% of control values, whereas Ca2+ spark activity from nj‐SR was reduced by only 19%, suggesting that SS Ca2+ sparks are under the control of Ca2+ influx from the extracellular space. Removal of extracellular Ca2+ eliminated SS Ca2+ sparks completely, whereas Ca2+ sparks from the nj‐SR continued, albeit at a lower frequency. In membrane‐permeabilized (saponin‐treated) atrial myocytes, where [Ca2+] can be experimentally controlled throughout the entire myocyte, j‐SR and nj‐SR Ca2+ spark frequencies were identical, and Ca2+ sparks could be observed spaced at sarcomeric distances throughout the entire cell, suggesting that all release sites of the nj‐SR can become active. Measurement of SR Ca2+ load (10 mm caffeine) revealed no difference between j‐SR and nj‐SR. The data suggest that in atrial myocytes, which lack a t‐tubular system, the nj‐SR is fully equipped with a three‐dimensional array of functional SR Ca2+ release sites; however, in intact cells under resting conditions, peripheral RyR clusters have a higher probability of activation owing to their association with surface membrane Ca2+ channels, leading to higher spontaneous Ca2+ spark activity. In conclusion, Ca2+ sparks originating from both j‐SR and nj‐SR are rather stereotypical and show little differences in their spatiotemporal properties. In intact cells, however, the higher frequency of spontaneous SS Ca2+ sparks arises from the structural arrangement of sarcolemma and j‐SR membrane and thus from the difference in the trigger mechanism.