The Two Components of the Human Atrial Action Potential

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Abstract
Action potentials studied in 36 human atrial strips at 27°C were found to be separated into two components. Simultaneous recordings with two microelectrodes demonstrated an independent conduction of the second component through selective and variable pathways. Increased separation of the two components was elicited by higher rate and lower intensity of stimulation, high K + , low Ca 2+ hypertonic solutions as well as by low temperature. Opposite procedures resulted in a more homogeneous and less variable excitation. Consequently, low rate and high intensity of stimulation allowed the study of the membrane permeabilities related to the two successive depolarizations. The first was suppressed by tetrodotoxin, the second by MnCl 2 and both by low Na + solutions. Hence, it was concluded that the separation was related to a nonhomogeneous excitation of the preparation and that the two components are triggered by two relatively independent depolarizations using different channels. Furthermore, the small amplitude of the first depolarization, without overshoot, and its modifications with stimulation intensity suggest that it might be due to an electrotonic spread or to a junctional mechanism.