Birefringence signals and calcium transients in skeletal muscle

Abstract

TWITCHES in skeletal muscle are preceded and accompanied by changes in optical properties of the muscle fibres^1–11. Measurements of birefringence give large signals which can be separated into two main components^6–8: the first begins on the falling phase of the action potential, and reaches a peak at the onset of tension development; this is followed by a late signal which is thought to be associated with development of tension by the contractile proteins^2,3,6. Since the early signal precedes tension development, it could provide an important tool for studying intervening steps in excitation–contraction coupling. This signal has recently been attributed to potential changes in the sarcoplasmic reticulum (SR) membrane, associated with Ca²⁺ release into the sarcoplasm^6,10. We have simultaneously recorded birefringence signals and changes in intracellular Ca²⁺ concentration, using arsenazo III, in frog muscle fibres. The results show that the onset of the early birefringence signal coincides with the rise in sarcoplasmic Ca²⁺ concentration, and that both can be abolished by injecting EGTA to chelate the sarcoplasmic Ca²⁺. This suggests that the early birefringence signal arises from some process dependent on the rise in sarcoplasmic Ca²⁺, and is not caused by events in the SR associated with the calcium release mechanism.