Frequency modulation and synchronization of spontaneous oscillations in cardiac cells

Both intact mammalian cardiac muscle and single adult Ca²⁺-tolerant myocytes, under appropriate experimental conditions, exhibit periodic, spontaneous myofilament oscillations that originate locally within a cell and propagate longitudinally as contractile waves. We have used microscopic imaging techniques to study the effect of electrical stimulation on the oscillation characteristics in single rat and rabbit myocytes. Unstimulated rat cells bathed in Ca(o) of 1-3 mM exhibited these oscillations. During stimulation at rates between 6 and 120 min^-1, oscillations did not occur in the interval between stimulated contractions, and following termination of stimulation a transient suppression of the spontaneous oscillation frequency occurred. Conversely, with higher cell Ca²⁺ loading, achieved by increasing the [Ca²⁺](o) or by addition of isoproterenol or ouabain, stimulation caused de novo oscillations in rabbit cells and increased the spontaneous oscillation frequency in rat cells to levels that resulted in their appearance between stimulated contractions. The tendency for myofilament motion to occur simultaneously at multiple foci was also increased by stimulation at high frequencies, and partial synchronization of these foci resulted in oscillations of an increased amplitude. The modulation of the spontaneous oscillation characteristics in single cells by stimulation may explain, in part, some effects of stimulation on Ca²⁺-dependent oscillatory phenomena in intact cardiac tissues.