Expression level of cardiac ryanodine receptors dictates properties of Ca-induced Ca release.

The type 2 ryanodine receptor (RyR2) is the major Ca release channel required for Ca-induced Ca release (CICR) and cardiac excitation-contraction coupling. The cluster organization of RyR2 at the dyad is critical for efficient CICR. Despite its central role in cardiac Ca signaling, the mechanisms that control CICR are not fully understood. As a single RyR2 Ca flux dictates local CICR that underlies Ca sparks, RyR2 density in a cluster, and therefore the distance between RyR2s, should have a profound impact on local CICR. Here, we studied the effect of the RyR2 expression level ([RyR2]) on CICR activation, termination, and amplitude. The endoplasmic reticulum (ER)-targeted Ca sensor RCEPIA-1er was used to directly measure the ER [Ca] (Ca]) in the T-Rex-293 the sarco/endoplasmic reticulum Ca-ATPase (SERCA2a) stable cell line expressing human RyR2. Cells coexpressing RyR2 and SERCA2a produced periodic [Ca] depletions in the form of spontaneous Ca waves due to propagating CICR. For each studied cell, the [Ca] at which Ca waves are activated and terminated was analyzed as a function of [RyR2]. CICR parameters, such as [Ca] activation, termination, and amplitude, were inversely proportional to [RyR2] at low-intermediate levels. Increasing the sensitivity of RyR2 to cytosolic Ca lowered the [Ca] at which CICR is activated and terminated. Decreasing the sensitivity of RyR2 to cytosolic Ca had the opposite effect on CICR. These results suggest that RyR2 density in the release cluster should have a significant impact on local CICR activation and termination. Since SR Ca load is evenly distributed throughout the SR network, clusters with higher RyR2 density would have a higher probability of initiating spontaneous CICR.