The sarcoplasmic reticulum of an ultrafast lobster muscle: first evidence of a tubular configuration

The sarcoplasmic reticulum (SR) of the remarkably fast remotor muscle of lobster second antenna was investigated with regard to ultrastructure, calcium uptake and protein composition. The SR of this unique muscle dominates the volume of the cell. We were able to preserve the configuration of the SR in its native state simply by processing the muscle for electron microscopy at 0-4 degree C. For the first time the SR is seen as larger (0.1 micrometer diam.) tubules that crowd the space between myofibrils, Previous observations of this organelle have been reported that showed a rather unsatisfying discontinuous vesicular configuration of the SR. This report indicates that these structures were fixation artifacts. The uptake of calcium measured in a microsomal subfraction in the presence of ATP and oxalate does not differ significantly from similar experiments with vertebrate SR. However, gel electrophoresis of lobster SR demonstrate a single intrinsic Ca-ATPase protein of 100,000 mol, wt. with neither of the typical lower molecular weight proteins found in vertebrate SR.