Morphological aspects of some sodium transporting epithelia suggesting a transcellular pathway via elements of endoplasmic reticulum.

Electron microscopic studies of sodium transporting epithelia from frog skin, sheep choroid plexus, rabbit gallbladder and small intestine, and rat kidney revealed the presence of a complex intracellular system of tubulo-cisternal endoplasmic reticulum which appeared to connect apical (luminal) and baso-lateral cell surfaces. The system was present in the tight epithelium of frog skin but was most abundant in leaky epithelia with low transepithelial resistance and isotonic transport. The basic structural features of the system and its relationship with some associated components are described. Our result, coupled with preliminary physiological studies, indicate that developmental and seasonal (hormone-induced) changes in the configuration of the tubulo-cisternal endoplasmic reticulum may be closely correlated with specific changes in epithelial permeability. The findings are discussed in the light of the hypothesis that epithelia possess two sodium transporting systems: One based on pump sites in the plasma membrane producing a hypertonic transportate and another located in the membranes of the tubulo-cisternal endoplasmic reticulum which, due to its extensive surface, would be well suited for producing an isotonic transportate.