The effects of stimulation rate on calcium‐dependent action potentials recorded from chick embryo heart cell aggregates.

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Abstract
Action potentials were recorded from aggregates of heart cells prepared from 3- or 7-day chick embryos. At 3 days the maximum rate of rise (+.ovrhdot.Vmax) was insensitive to TTX [tetrodotoxin]; at 7 days it was considerably reduced by TTX. In the presence of TTX the action potential overshoot was dependent on [Ca]o [extracellular Ca]; the results may be fitted using constant field theory and assuming that the membrane is > 100.times. more permeable to Ca than to Na or K. An increase in stimulation rate in the range 0.2-2 Hz led to an increase in overshoot and +.ovrhdot.Vmax. This effect was not seen after addition of 20 mM tetraethylammonium ions or when Sr was substituted for Ca in the external medium. These rate-dependent changes may result from partial inactivation of an outward K current.