Increased optical transparency associated with excitation–contraction coupling in voltage-clamped cut skeletal muscle fibres

Abstract

CONTRACTION of a skeletal muscle fibre is triggered by depolarisation of its transverse (T-) tubules¹, which occurs physiologically with the spread of an action potential into the T-system². The T-tubule depolarisation gives rise to calcium release^3,4 from the adjacent but separate^5,6 sarcoplasmic reticulum (SR). The released calcium binds to troponin, removing the inhibition of actin–myosin interaction and enabling contractile activity⁷. Little is known about the nature of the T-tubule–SR interaction. It has been suggested that movement of charged molecules or dipoles in the T-tubule membrane is the initial voltage-sensitive step in the chain of events leading from T-tubule depolarisation to SR calcium release⁸, and membrane charge displacement currents which may reflect such movements have been observed and characterised^8–10. We report here a study of subsequent changes which may be related to calcium release.