The action potentials recorded from heart muscle with a suction electrode have been compared to those recorded with an intracellular microelectrode. It has been found that if the suction electrode is properly used the monophasic potentials recorded with it may be taken as a reliable index of the time of arrival of excitation at the electrode and as a reliable index of the shape of the action potential during the entire phase of repolarization. The suction electrode potentials differ from the microelectrode potentials in showing a lower rise velocity, a smaller amplitude, a quantitatively different reversal or overshoot and, in the beating heart, ‘afterpotentials’ caused by mechanical effects. When the shape of the action potential, as observed with the microelectrode, is changed by ions such as K+ or Ca++ a similar change is observed in the potential recorded with the suction electrodes.

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Isolated rabbit hearts were perfused with Krebs-Henseleit solution deficient in K, Ca or both. Ventricular monophasic action potentials registered with the suction electrode, electrocardiograms and intraventricular pressures were recorded simultaneously for periods up to 20 minutes. Perfusion with K-deficient solutions resulted in a brief initial prolongation of the entire descending limb of the action potential and of the T wave, followed by progressive increase in slope of the initial portion and decrease in slope of the terminal portion of the action potential. During this time the T wave became shorter and a U-wave-like deflection occupying the entire diastole developed. QRS showed progressive prolongation. Finally, A-V conduction disturbances, ectopic beats and ventricular fibrillation developed. The force of contraction increased slightly. Perfusion with Ca-deficient solutions decreased the slope of the initial portion while increasing the slope of the terminal portion of the action potential. This was accompanied by prolongation of the ST- segment and reversal in the direction of the T wave. The duration of the action potential reached a maximum after 40–80 seconds of perfusion, while the force of contraction decreased progessively. Perfusion with solutions without potassium and calcium prolonged the initial as well as terminal portion of the action potential.

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