Regulation of twitch tension in sheep cardiac Purkinje fibers during calcium overload

The dependence of twitch tension on the interval (Δt) between depolarizations was examined in voltage-clamped sheep cardiac Purkinje fibers under conditions of calcium overload. During the development of calcium overload (produced by sodium-pump inhibition), twitch amplitude changes from a monotonic function of Δt to an oscillatory one. We investigated the cellular processes underlying this oscillatory relationship. Measurable calcium current was blocked by D 600 (25 μM), but neither the twitch nor the oscillatory dependence of twitch tension on Δt was abolished. Caffeine (2 mM), applied to modify sarcoplasmic reticulum function, decreased the oscillatory period of the twitch/interval relationship as it increased the frequency of spontaneous fluctuations of resting tension. Our results suggest that the oscillatory relationship between twitch amplitude and Δt is not caused by changes in the calcium current per se but rather by fluctuations in the amount of releasable calcium in the sarcoplasmic reticulum. Additionally, we conclude that the calcium current may not be a necessary prerequisite for depolarization to trigger calcium release from the sarcoplasmic reticulum under conditions of calcium overload.