Extracellular Ca²⁺ and excitation–contraction coupling

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 completely^2,3. 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.