Abstract
The changes of sarcomere length of cultured myocardial cells during genesis of ouabain-induced fibrillatory beating were measured, and it was found that the sarcomere continued to be in the shortened state throughout the fibrillatory beating. Addition of ouabain caused gradual increases of both the Na content of myocardial cells and the rate of Ca uptake by the cells, and fibrillatory beating appeared to develop when the Na content and the rate of Ca uptake exceeded the normal levels by about 1.5 times and 2.0 times, respectively. Myocardial cells loaded with various concentrations of Na + could be prepared by incubating the cells in Ca 2+-free medium containing various concentrations of Na +, and it was found that a slight increase in the intracellular Na content from the physiological concentration caused an appreciable increase in Ca uptake by the cells. This Ca uptake was achieved by a carrier-mediated Na +Ca 2+ exchange system and the stoichiometry of Na +: Ca 2+ exchange was greater than 2 Na +:1 Ca 2+. From these observations the possible involvement of an Na +Ca 2+ exchange system in genesis of ouabain-induced arrhythmias of cultured myocardial cells is discussed.
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