Quantitative relationship between active sodium transport, expansion of endoplasmic reticulum and specialized vacuoles ("scalloped sacs") in the outermost living cell layer of the frog skin epithelium (Rana temporaria)

When an isolated frog skin (Rana temporaria) is exposed to a hydrostatic pressure difference between inside and outside bathing solutions (inside pressure higher than outside) of 20-50 cm of H2O and if under these conditions the skin is short-circuited electrically, small "vacuoles" appear light-microscopically in the outermost living cell layer in the epithelium. The number of such "vacuoles" shows a linear dependency on the rate of active sodium transport as measured by the short-circuit current. Electron-microscopically, the "vacuoles" are interpreted as previously undescribed organelles, the "scalloped sacs" which are about 0.5 mu in diameter, with a wrinkled surface and bounded by a unit membrane. This organelle is in intimate contact with sacs and tubules of smooth endoplasmic reticulum. The observed increase in the number of scalloped sacs usually is accompanied by a significant expansion of the whole system of endoplasmic reticulum. Some of the "vacuoles" seen light-microscopically must indeed be expanded cisternae of endoplasmic reticulum. The findings are discussed in light of the possibility that the scalloped sacs and the endoplasmic reticulum may be involved in active transport of sodium ions.