Relative importance of SR load and cytoplasmic calcium concentration in the genesis of aftercontractions in cardiac myocytes.

OBJECTIVE: To determine whether calcium overload of the sarcoplasmic reticulum underlies drive train-induced aftercontractions in cardiac myocytes.
METHODS: Sarcoplasmic reticulum calcium contents were measured immediately prior to drive train-induced aftercontractions in isolated guinea pig cardiac myocytes, using caffeine application under voltage clamp conditions. Cell shortening during caffeine exposure and cell shortening during the final stimulated beat of the drive train and the delay between caffeine exposure and the onset of inward current were also used as indirect measures of sarcoplasmic reticulum load.
RESULTS: At the threshold for aftercontractions, all four measures of sarcoplasmic reticulum load showed interruption of the positive relationship between stimulation frequency and sarcoplasmic reticulum content, the sarcoplasmic reticulum being no more loaded prior to an aftercontraction than following subthreshold drive trains. Intracellular calcium concentration, estimated with the calcium-sensitive dye indo-1, was higher in cells showing aftercontractions than those not.
CONCLUSIONS: We conclude that calcium overload of the sarcoplasmic reticulum does not underlie spontaneous calcium release in this situation and the primary trigger for spontaneous release may instead be raised cytoplasmic calcium concentration.