T-system optical signals associated with inward rectification in skeletal muscle

The resting potential of many excitable cells, including skeletal muscle 1–4 , cardiac muscle 5,6 , nerve cell bodies 7 and egg cells 8 , is determined by a resting potassium conductance which shows inward rectification, allowing potassium ions to move more readily inward across the cell membrane than outward. In skeletal muscle, where inward rectification has been extensively studied, a large part of this conductance is located in the T-system membranes 2,3,9–11 . However, to date, only the kinetic and voltage-dependent properties of this conductance have been studied from analyses of the membrane potential or current recorded at the fibre surface. We report here measurements, obtained using a voltage-sensing dye, of potential changes in the T-system membranes associated with the inwardly rectifying K + current. Our results show that this conductance alters the time course and significantly attenuates the amplitude of the potential change across the tubular membranes. These optical data provide new evidence for the presence of this conductance in the T-system and, when analysed using a radial cable model for the T-system, provide an estimate of the distribution of the inward rectifier conductance over the surface and T-system which is in agreement with estimates obtained by other techniques.