Zusammenfassung
An ca. 2 mm langen Purkinje-Fäden (Schaf) wurde das Membranpotential sprungweise oder stetig geändert und der dazu benötigte Membranstrom gemessen. Es wurde versucht, den etwaigen Anteil eines Calciumstromes am Membranstrom durch Weglassen oder drastische Erhöhung der extracellulären Calciumkonzentration zu ermitteln. Bei 0 Ca++ in der Außenlösung ändern sich bei einem Spannungssprung die phasischen Komponenten des Membranstroms kaum. Dagegen nimmt nach 20–60 min die Leitfähigkeit der Membran im Potentialbereich von −120 bis +20 mV zu, und die Faser depolarisiert auf einen Wert von etwa −30 mV. Diese Effekte treten in natriumhaltigen und natriumfreien Lösungen auf und sind bei Zugabe von Ca++ reversibel. Die hohe Membranleitfähigkeit in 0 Ca++ beruht nicht auf einer Erhöhung der Natrium-, Kalium- oder Chlorid-Leitfähigkeit. Bei Erhöhung des extracellulären Ca++ auf bis zu 108 mM/l tritt im untersuchten Spannungsbereich kein vergrößerter negativer Strom auf, ein vergrößerter Fluß von Calciumionen ist also nicht nachzuweisen. Die Veränderungen der Aktionspotentiale in 0 und 21,6 mM/l Ca++ werden an Hand der aufgenommenen Strom-Spannungsbeziehungen diskutiert. Die Versuche schließen eine wesentliche Beteiligung des Ca++ an den Ionenströmen aus, die während des Aktionspotentials der Purkinje-Fasern fließen.
Summary
By a voltage clamp method in short Purkinje fibers membrane current was measured during stepwise or steady changes of membrane potential. The contribution of Ca++ to the total membrane current was investigated by measuring the effects of omission and increase of the extracellular Ca++. When Ca++ is omitted the transient current flowing after a depolarizing voltage step is little affected. However, after longer exposure to 0 Ca++ the membrane conductance increases throughout the potential range of −120 to +20 mV; furthermore the fiber depolarizes to about −30 mV. These effects of omission of Ca++ are the same in Tyrode and in sodium-free solutions using either choline chloride or saccharose. The high membrane conductance in 0 Ca++ is not due to an increase of the Na+, K+ or Cl− conductance. All effects of Ca++ omission are reversible when Ca++ is readmitted. When extracellular Ca++ is increased to up to 108 mM/l no increase of a negative current is observed in the range of membrane potentials covered. There is no evidence of an increased flow of Ca++ ions. The shape of the action potentials in 0 Ca++ and 21.6 mM/l Ca++ is discussed in relation to the measured current voltage relationships. The experiments seem to exclude an appreciable contribution of Ca++ to the membrane current flowing during the action potential of the Purkinje fiber.
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Dedicatet to Professor Hans Schaefer on the occasion of his 60 th birthday.
This investigation was supported by the “Deutsche Forschungsgemeinschaft”.
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Dudel, J., Peper, K. & Trautwein, W. The contribution of Ca++ ions to the current voltage relation in cardiac muscle (Purkinje fibers). Pflügers Archiv 288, 262–281 (1966). https://doi.org/10.1007/BF00363785
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DOI: https://doi.org/10.1007/BF00363785