Active solute transport across frog skin and epithelial cell systems according to the association-induction hypothesis.

The phenomenon of transport of ions, sugars, amino acids, etc. across frog skin and other epithelial systems has been commonly interpreted on the basis of the membrane-pump theory, according to which asymmetry in solute distribution as well as transport into and out of all living cells results from the permeability properties and "pump" activities of the membrane. In the present review, certain findings in the field of transepithelial transport of solutes are given new interpretation on the basis of molecular mechanisms introduced in the association-induction hypothesis, according to which "active transport" of solutes occurs only across bifacial cell systems like frog skin and intestinal epithelium but not in the maintenance of steady levels of solutes in unifacial cell systems such as muscle, nerve, and red blood cells.