Two Components of the Cardiac Action Potential

Transmembrane potentials recorded from the rabbit heart in vitro were displayed as voltage against time (V, t display), and dV/dt against voltage (V, V or phase-plane display). Acetylcholine was applied to the recording site by means of a hydraulic system. Results showed that (a) differences in time course of action potential upstroke can be explained in terms of the relative magnitude of fast and slow phases of depolarization; (b) acetylcholine is capable of depressing the slow phase of depolarization as well as the plateau of the action potential; and (c) action potentials from nodal (SA and AV) cells seem to lack the initial fast phase. These results were construed to support a two-component hypothesis for cardiac electrogenesis. The hypothesis states that cardiac action potentials are composed of two distinct and physiologically separable "components" which result from discrete mechanisms. An initial fast component is a sodium spike similar to that of squid nerve. The slow component, which accounts for both a slow depolarization during phase 0 and the plateau, probably is dependent on the properties of a slow inward current having a positive equilibrium potential, coupled to a decrease in the resting potassium conductance. According to the hypothesis, SA and AV nodal action potentials are due entirely or almost entirely to the slow component and can therefore be expected to exhibit unique electrophysiological and pharmacological properties.