Abstract
THE role of external Ca2+ in excitation–contraction coupling has been controversial since Sandow1 put forward the idea that contraction is initiated by an influx of Ca2+ on depolarisation. In the very thin lamellae (1 µm) of Branchiostoma myotome muscle ‘Ca2+-free’ solutions with 1 mM EGTA block twitches completely2,3. It was concluded that the Ca2+ influx during the action potential directly activates the myofilaments2. In frog muscle twitches remain unchanged in 1 mM EGTA2− (refs 4, 5). To ensure that [Ca2+] is sufficiently reduced in the narrow transverse tubular (T) system Barrett and Barrett6 raised the Ca2+ buffer concentration to 80 mM and observed that action potentials were no longer followed by twitches. In this letter we demonstrate that some further changes in the composition of the external fluids result in the restoration of contractions in both Branchiostoma and frog muscle indicating that a Ca2+ influx is not necessary for the initiation of contraction.
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References
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SPIECKER, W., MELZER, W. & LÜTTGAU, H. Extracellular Ca2+ and excitation–contraction coupling. Nature 280, 158–160 (1979). https://doi.org/10.1038/280158a0
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DOI: https://doi.org/10.1038/280158a0
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