Evidence for a Functional Connection Between the Sarcoplasmic Reticulum and the Extracellular Space in Frog Sartorius Muscle

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Abstract
Electron microscopy of muscles soaked in Ringer solution containing horseradish peroxidase shows a considerable penetration of peroxidase into the terminal cisternae and the longitudinal tubules of the sarcoplasmic reticulum (SR). Taken together with observations on the effects of hypertonic solution on the dimensions of the tubular system and the terminal cisternae it seems clear that there must be a functional continuity of the sarcoplasmic reticulum and the external solution, probably through connections with the T tubes at the triad junction. These findings conflict with other reports that the probe molecules do not penetrate into the SR. The reason for these discrepancies is not yet clear. One factor which may limit the penetration of probe molecules in the tubular system is the fluid dynamics associated with active transport. The effect of ouabain on peroxidase penetration provides some support for this suggestion.