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Ca-abhängige Membranpotentialänderungen am Herzen und ihre Bedeutung für die elektro-mechanische Kopplung. Versuche mit Tetrodotoxin in Na-haltigen Lösungen

  • Hasso Scholz
Chapter

Schlüsselwörter

Ventrikuläre Trabekel Tetrodotoxin Graduierte Depolarisation Ca-Einstrom Elektro-Mechanische Kopplung 

Summary

  1. 1

    Tetrodotoxin (TTX), at a concentration of less than 10−6 g/ml, had no effect on membrane potential and contraction of isolated, thin ventricular trabeculae of sheep and calf hearts. 10−6 to 2 × 10−6 g/ml TTX decreased the rate of rise, over-shoot, and duration (phase of 90% repolarisation) of the action potential and the amplitude of contraction, without change in the resting potential and the plateau (20% repolarisation phase) of the action potential. Excitation block regularly occurred only with 10−5 to 2 × 10−5 g/ml TTX.

     
  2. 2

    In a solution containing Na and TTX (5 × 10−6 — 2 × 10−5 g/ml) graded depolarisation was possible if the preparations were stimulated by square wave pulses of 500 msec duration across a sucrose bridge. In Ca-containing solutions the time-course of the electrotonic potentials showed two steps. The second step of depolarisation (SSD) began when the membrane potential reached — 60 to — 50 mV (threshold), and tension was initiated at the same level. Contractions reached their steady-state values only after about 5 depolarisations of the same size. There was no difference in the amplitude of contractions (steady-state level) elicited in Tyrode and in Tyrode -+ TTX.

     
  3. 3

    SSD and contraction were dependent on the [Ca] e . Rate of rise (V/sec) and amplitude (mV) of SSD and tension increased with increasing [Ca] e . In Ca-free solutions the electrotonic potentials reached their steady-state levels in one step. No SSD and tension were observed in the absence of Ca, even with reversal of membrane potential.

     
  4. 4

    In Tyrode + TTX the SSD was identical with the changes in membrane potential which could be observed in Na-free solution without TTX, but in Na-free solutions, contractions already reached their maximum during the first SSD.

     
  5. 5

    SSD in Na-free solution was not blocked by TTX.

    The results indicate that the second step of depolarization in Na-free as well as in Na-containing solution + TTX is due to a Ca inward current. It is tentatively concluded that not only in Na-free but also in Na-containing solution Ca ions carry charge across the membrane of cardiac muscle during depolarisation and that this Ca inward current is an important factor in excitation-contraction coupling.

     

Key-Words

Ventricular Trabeculae Tetrodotoxin Graded Depolarisation Ca Inward Current Excitation-Contraction Coupling 

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Copyright information

© Springer-Verlag Berlin Heidelberg 1969

Authors and Affiliations

  • Hasso Scholz
    • 1
  1. 1.Pharmakologisches Institut der UniversitätMainzDeutschland

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