Sarcoplasmic reticulum Ca content, sarcolemmal Ca influx and the genesis of arrhythmias in isolated guinea-pig cardiomyocytes

Early afterdepolarizations are seen during the repolarization phases of the action potential and delayed afterdepolarizations appear later, usually following complete repolarization of the cell membrane potential. Both forms of afterdepolarization are linked to the occurrence of aftercontractions, seem to play a role in the generation of ventricular arrhythmias and are believed to be the result of abnormalities of intracellular Ca handling. Suggestions for the mechanisms responsible vary from both types of afterpotential being mediated by Ca release from the sarcoplasmic reticulum, to early afterdepolarization formation being due to reactivation of the L-type sarcolemmal Ca channels during the action potential. We tried to assess the functional importance of the sarcoplasmic reticulum or Ca influx in the development of afterpotentials and abnormal contractile activity in guinea-pig cardiac myocytes. Ca influx was increased using isoproterenol, Bay K8644 or increasing extracellular [Ca]. Sarcoplasmic reticulum Ca content was measured using rapid cooling contractures or caffeine-induced Na/Ca exchange current and the sarcoplasmic reticulum was inhibited using caffeine or thapsigargin. Aftercontractions associated with either early or delayed afterdepolarizations could be induced by increasing Ca influx. The increased Ca influx produced increases in sarcoplasmic reticulum Ca content and aftercontractions were associated with a larger SR Ca content. However, the sarcoplasmic reticulum was no more loaded with Ca when aftercontractions occurred than when aftercontractions did not occur. Preventing Ca sequestration by the sarcoplasmic reticulum inhibited the formation of aftercontractions. The results suggest that alterations to both Ca influx and sarcoplasmic reticulum Ca content are required to produce aftercontractions.