Summary
In frog atrial bundles it is possible under voltage clamp conditions to distinguish between a “phasic” component of mechanical response, depending on Ca++ influx, and a “slow” component, which does not directly depend on the presence of extracellular Ca++ (Vassort et Rougier, 1972). The present results suggest that the “slow” component can be abolished by substituting LiCl for NaCl. The hypothesis is advanced that a displacement of Ca++ by Na+ from some intracellular binding sites by a variation of membrane potential or [Na]i causes the “slow” phase of contraction.
Furthermore, relaxation during the phasic component is markedly slowed when LiCl or sucrose is substituted for NaCl. This may indicate that a Na+-Ca++ exchange across the surface membrane is essential for relaxation.
A Na+-Ca++ exchange (a Ca influx linked to a Na efflux in this case) may account for the contractures elicited by Na-free media.
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This work constitutes a part of “Doctorat es Sciences” No CNRS: AO 66 71. It was supported in part by contract DGRST, Paris, France.
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Vassort, G. Influence of sodium ions on the regulation of frog myocardial contractility. Pflugers Arch. 339, 225–240 (1973). https://doi.org/10.1007/BF00587374
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DOI: https://doi.org/10.1007/BF00587374