Effects of varying calreticulin Ca²⁺ buffer in SR on the simulated calcium signals and membrane voltage of embryonic cardiomyocytes. (A) Ablation of SR calreticulin content inhibits the spontaneous activity and reduces the stimulated calcium signals (middle row), whereas increased calreticulin content (1.5× control) lowers the frequency but increases the amplitude of both spontaneous and stimulated calcium signals. During stimulated APs, spontaneous calcium releases ensue due to overly large SR calcium content. (B) Effect of SR calreticulin content on the relative spontaneous frequency (f/f_control, Hz) of APs in the model cell.