Summary
To study whether Ca ions as physiological charge carriers of the slow inward current could be completely replaced by Sr ions, voltage clamp experiments using the double sucrose gap technique were performed on isolated ventriculae and papillary muscles of cats. The separation of the slow inward current from the fast Na inward current was achieved by use of the conditioning clamp technique.
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1.
The presence of Sr in a Ca-free Tyrode solution prevented the loss of the transmembrane slow inward current. By replacement of 2.2 mM Ca by 2.2 mM Sr an identical magnitude of the slow inward current was observed. After replacement of 2.2 mM Ca by 4.2 mM Sr the slow inward current was augmented. Simultaneously, the threshold potential was lowered and the reversal potential was shifted to more positive values.
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2.
The Sr carried slow inward current is characterized by a delayed inactivation. The kinetics of activation, however, are not altered.
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3.
The slow membrane channel reacts normally towards inhibitors (D 600) or activators (isoproterenol) in the presence of Sr as charge carrier of the slow inward current. D 600 (0.5 mg/l) diminished the Sr current by 70%, isoproteronol (5 mg/l) increased the current by nearly 100%.
The results show that Ca ions as the physiological charge carriers of the slow inward current can be completely replaced by Sr ions. Besides of its function as charge carrier Ca ions are involved in the process of switching off of the conductance of the slow membrane channel.
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Kohlhardt, M., Herdey, A. & Kübler, M. Interchangeability of Ca ions and Sr ions as charge carriers of the slow inward current in mammalian myocardial fibres. Pflugers Arch. 344, 149–158 (1973). https://doi.org/10.1007/BF00586548
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DOI: https://doi.org/10.1007/BF00586548