The mitochondria-rich cell of frog skin as hormone-sensitive "shunt-path".

Further investigations about the role of the mitochondria-rich cell (MR cell) in hormone-mediated transport regulation in the epithelium of frog skin brought the following results: Unlike toad bladder, in frog skin the spontaneous potential difference cannot be reversed when Na transport is blocked. A similar situation is obtained when, in addition to transport-blockade, one applies a chemical gradient for chloride to the epithelium. Under these conditions we found that in the intact preparation as well as in the separated epithelium: (i) the reversed current (RC) is linearly related to the number of MR cells; (ii) RC is mainly carried by a passive, transcellular chloride flux inwards and (iii) RC is sensitive to nor-adrenaline (10(-7) M). The beta-blocker propranolol abolishes this effect. We propose that the MR cells are the sites of transepithelial shunt-path and that this chloride flux is transcellular. As it is hormone sensitive, it could be an important regulatory instrument for the regulation of overall salt transport (internal shorting).

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.

Some morphological aspects of active sodium transport. The epithelium of the frog skin.

A method was experimentally tested which allows simultaneous morphological and bioelectrical studies of a tissue that performs active sodium transport, i.e., the isolated, surviving frog skin. In a four cell lucite chamber with four separate electric potential and current circuits, skin specimens for morphological observation (light and electron microscopy) were fixed in situ in well-defined functional states. The rate of active sodium transport through the epithelium of Rana temporaria skin was modified by changing the strength of the electric current passed through the specimens. A marked, reversible swelling of the outermost layer of the stratum granulosum was observed during short circuiting of the skin compared to the homogeneous appearance of the epithelium under open circuit conditions. Doubling the ingoing current led to an additional small increase of the swelling or the appearance of islets of cell necrosis in the same layer. There were signs of a slight shrinkage of the underlying cell layers. The observations are discussed in the light of previous bioelectrical and morphological observations.