Characterization of sarcoplasmic reticulum in skinned muscle cultures

The plasma membranes of chick or rat skeletal muscles, grown in cell culture, were made permeable with saponin in a solution lacking calcium. The cells were then supplied with a medium resembling the cytosol and the ATP-dependent Ca²⁺ sequestration was performed. Based on the low concentration of free Ca²⁺ in the medium (below 5 μM), the presence of mitochondrial inhibitors and the effect of drugs that interfere with sarcoplasmic reticulum (SR) function, we assume that the measured Ca²⁺ accumulation expresses SR function on the saponin-treated myotubes. The development of the SR in muscle cultures is augmented as myogenesis proceeds and depends on its occurrence. Whereas creatine kinase activity is elevated immediately following cell fusion, there is a delay of at least 1 day between myoblast fusion and the increase in Ca²⁺ accumulation in the SR. Thyroxine or triiodothyronine caused an inhibition of Ca²⁺ accumulation in rat or chick muscle cultures. This inhibition could explain some of the muscle abnormalities caused by excess of thyroid hormones. A comparison was made between a white-type (fast) and heterogeneous muscle, differentiated in cell culture. There was no significant difference in SR function, indicating the important role of innervation in specifying the properties of muscle fiber types.