Nachweis von zwei Komponenten in der Anstiegsphase des Aktionspotentials von Froschmyokardfasern

Summary

When recorded with high sweep speed, transmembrane action potentials of the isolated frog's myocardium exhibit regularly a step-like discontinuity of their rising phase. The upstroke of the action potential can thus be regarded as being composed of two phases: an initial phase of rapid depolarization preceding the step (phase I) which is followed by a slower phase reaching the peak of the overshoot (phase II).

Experiments were performed in order to find out whether these 2 phases can be specifically influenced a) by changes of the resting potential, b) by changes of the external cation concentrations (K⁺, Mg⁺⁺, Na⁺), c) by addition of epinephrine. The following results were obtained: Hyperpolarization and depolarization by electrotonic current lead to corresponding changes of phase I, whereas phase II remains practically unchanged. Like cathodic current depolarization due to high external potassium causes also a selective reduction of phase I. Reduction of the external Na⁺-concentration (70% replacement by sucrose) diminishes the height of phase I and suppresses phase II. High external Mg⁺⁺-concentrations (5–15 mM/l) increase the resting potential and reduce the overshoot. Concomitant with the Mg⁺⁺-dependent hyperpolarization the amplitude of phase I is augmented, whereas phase II disappears. Epinephrine increases the overshoot in K⁺- and Mg⁺⁺-rich as well as in Na⁺-depleted solutions by a selective stimulation of phase II.

It is concluded from these experiments, that the rising phase of the frog's cardiac action potential consists of two processes different in nature. Phase I seems to be especially related to the activation of the sodium-carrying-system (rising phase in a proper sense), whereas phase II represents the beginning of the plateau. The underlying mechanisms are discussed in terms of sodium and potassium currents using an analogue computer.