Before revision: Rationale: Heart muscle contraction is  activated by a synchronized systolic Ca release from sarcoplasmic reticulum (SR) via Ca sparks. In disease, Ca sparks fail to terminate, causing a diastolic Ca leak that decreases contraction amplitude and increases the risk of arrhythmia.
After revision: Rationale: Heart muscle contraction is activated by a synchronized systolic Ca release from sarcoplasmic reticulum (SR) , a major intracellular Ca store. However, under abnormal conditions Ca leaks from the SR, decreasing heart contraction amplitude and increases the risk of arrhythmia.