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Membrane potential changes in human ventricular and atrial muscle, excised from patients undergoing open-heart surgery, were recorded by micro-electrodes in vitro. Mean ventricular resting and action potentials were -87 mv. and 115 mv., respectively. The mean atrial resting potential was -70 mv., mean action potential 75 mv. Two forms of atrial action potential were found, one having conventional contour, the other with prominent spike and plateau. A disturbance in repolarization is believed to underlie the latter type of atrial potential cycle. The relation between the upstroke velocity of the action potential and the extracellular sodium concentration and membrane potential was shown to be similar to that in other mammalian cardiac tissue. The mean conduction velocity determined in ventricular fibers (1.3 M./sec.) was somewhat greater than that of the dog, and the possible relationship to hypertrophy of the cardiac fibers in the preparations studied is described. The effect of increased rate and anoxia in reducing the action potential duration is like that found in the hearts of other mammals. The conductance type of inhibition was produced by acetylcholine in spontaneously beating atrial tissue. The excitatory effect of epinephrine was preceded by a transitory inhibition. The basic mechanisms underlying the action potential, automaticity and transmitter effects, derived from investigation of other mammalian cardiac tissue are applicable to the human heart.

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