Howell recently reported the effect of Ruthenium Red (RR) on the sarcoplasmic reticulum (SR) of frog skeletal muscle after long exposures to that dye. Howell observed staining of the junctional granules within the junctional SR (terminal cisternae), and fusion of the so-called intermediate cisternae that are intercalated between junctional and free SR. This is an important observation because it may provide, first, needed additional evidence for a structural definition of the so-called intermediate cisterna, and second, the fusion of the intermediate cisternae under these circumstances suggests that they might have properties that are different from, or in addition to those of the junctional and free SR, respectively. For example, membrane fusion in that region might conceivably occur in vivo under certain circumstances in which case a large proportion of the SR might become separated, electrically, from the junctional SR. As a result, at least temporary openings of the junctional SR to the extracellular space remain a possibility even during depolarization.

Wallace, N., & Sommer, J. R. (1975). Fusion of Sarcoplasmic Reticulum with Ruthenium Red. Proceedings, Annual Meeting, Electron Microscopy Society of America, 33, 500–501.