Effect of muscle length on 45Ca efflux in resting and contracting skeletal muscle.

The increment of the slow component of 45Ca efflux from frog skeletal muscle that is produced by electrical stimulation is reduced by 70% when the muscle has been stretched to 160% of in vivo length. No change in the efflux from resting muscles or the increment in influx associated with stimulation was produced by increased length. The increased efflux produced by 8 mM caffeine was not altered by increased muscle length. Resting and action potentials were unchanged by the stretching. No morphological changes were seen in the transverse tubules or their junction with the terminal cisternae even at sarcomere lengths of 3.8 muM, but the terminal cisternae themselves were elongated at this length.