Mechanical activation and voltage-dependent charge movement in stretched muscle fibres

Abstract

Although properties of the intramembranous charge movement in frog skeletal muscle have now been fairly well characterised1–5, the function of this process remains uncertain6–9. Schneider and Chandler1 first suggested that the observed charge movement in sartorius fibres held near slack length in a hypertonic paralysing solution took place within the transverse (T) tubular membranes in the region of the triad feet and signalled tubular depolarisation to the sarcoplasmic reticulum (SR), thereby initiating calcium release. More recently it has been reported that much of this charge movement is abolished in semitendinosus fibres stretched well beyond slack length in hypertonic Ringer solution10. Other studies using intracellular calcium indicators, however, have shown that substantial amounts of calcium are released by brief depolarisations in muscle fibres stretched to sarcomere lengths of 4.0 µm or more, when bathed in isotonic solutions11–13. The question thus arises, is charge movement abolished in muscle fibres stretched in an isotonic Ringer solution? As calcium release persists in these conditions, an inhibition of charge movement would make involvement of this phenomenon with calcium release very unlikely. We report here the influence of stretch on charge movement and on an aspect of mechanical activation indicative of calcium release using standard, microelectrode voltage-clamp experiments on intact frog twitch fibres. We found no dramatic dependence of these properties on fibre length, and the possibility of a functional role for charge movement in the regulation of calcium release remains.