ABSTRACT
Some salt transporting epithelia can absorb inorganic ions from extremely low external concentrations. Salt depleted frogs, for example, can absorb sodium ions from freshwater at concentrations down to 0·01 mM (Krogh, 1939) while freshwater mosquito larvae can maintain haemolymph sodium at 30 mM in external media containing as little as 1·55 × 10−3 mM sodium (Stobbart, 1974). The ability of the frogskin to absorb ions, into the blood, from such low external concentrations has been held to present difficulties (Hansen & Zerahn, 1964) for the classical model, which proposes a passive entry of external sodium ions into the cells and pumping at the inner epithelial membrane (Koefoed-Johnsen & Ussing, 1958). At least two current hypotheses exist to explain how net sodium transport from low external concentrations is achieved. The first postulates an involvement of ion pumps located on the outer epithelial membrane (Hansen & Zerahn, 1964; Leblanc, 1972) and the second that the electrical potential across the outer membrane is large enough to create an inwardly directed electro-chemical gradient to induce a passive entry of sodium ions from media containing as little as 1 mM sodium (Nagel, 1977). This communication proposes an alternative model for unidirectional salt transport from dilute environments which may be applicable to the uptake of cations and anions in a variety of salt absorptive epithelia.
