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Membrane potential and contraction were investigated in cultured chick embryonic heart cell aggregate. The cells were stimulated through the intracellular microelectrode, and the contraction of the cell aggregate was measured from the shortening of the diameter. The contraction decreased if the stimulation frequency was raised in the range between 20/min and 100/min; namely, the negative relationship between contraction strength and stimulation frequency. The peak action potential or plateau duration was not affected by the change in the stimulation frequencies unless they exceeded 60/min, but they were shortened at frequencies higher than 75/min. In the latter case, the decrease of contraction strength was related to the shortening of plateau duration. If the stimulation frequency was lowered suddenly, the following contraction was transiently increased without any noticeable change in the shape of the action potential. When TTX was added (3-10 x 10(-6) M), the rate of the spontaneous activity was reduced, which made it possible to study the contraction strength at low stimulation frequencies. In TTX medium, similar to the observations in the control medium, the relationship between contraction strength and stimulation frequency was also negative. In the medium containing TTX plus verapamil (5-9 x 10(-7) M), the peak action potential was inhibited during the repetitive stimulation dependently on the stimulation frequency, accompanied by the concomitant decrease of contraction strength. These results suggest that the negative inotropic effect of the stimulation frequency is caused in part by the change in Ca2+ inflow.

Relationship between contraction strength and stimulation frequency in cultured chick embryonic heart cells.