ERNST G. HUF 



227 



THE 'passive' and THE 'ACTIVE' ION IN SALT TRANSPORT 



The application of enzyme inhibitors, such as cyanide, bromo- or iodoacetate 

 and others, during the years from 1933 to 1938, has led to the well recognized 

 fact that in frog skin the potential difference and active salt transport are very 

 closely related phenomena (6, 15). Not well recognized or emphasized, however, 

 was an important point brought out recently by Linderholm (45), namely, that 

 electrophysiological studies on frog skin, which Galeotti did in 1904, "are in 

 fact fairly conclusive proof of an active transport of sodium" (45). The unique 

 position of sodium in the generation of the skin potential has also been demon- 

 strated by other electrophysiologists (loa). The sharp distinction between 'ac- 

 tive' and 'passive' ions in active salt transport was particularly stressed, how- 

 ever, by Ussing. Based on the fundamental principles of electrochemistry, 

 Ussing (71) and Teorell (69), independently, arrived at the expression, 



RT ln(AIin/Mout) = Mo - Mi, 



relating the ratio infiux/outflux and the electrochemical potentials of the ion 

 in the outside and inside baths. Although certain limitations regarding conclu- 

 sions drawn from such information are known (76), Ussing was able to show that 

 chloride transport in isolated frog skin can be understood from the relation 

 shown above (39). Chloride, therefore, is considered as being the 'passive' ion 

 in active salt transport. A discrepancy between theory and experiment was 

 found, however, when the results on sodium transport were subjected to this 

 test. The flux ratio was considerably larger than that calculated from the 

 equation (43). Sodium, therefore, was considered as the 'active' ion in active salt 

 uptake. The fact that older and newer electrochemical data on frog skin are in 

 agreement can now be regarded as a reliable basis for the interpretation of other 

 results. Thus, the inverse relationship which exists between net rate of salt 

 transport and the level of the sodium ion concentration at the corium side of the 

 skin was e.xplained on this basis (25). It is not difficult to understand that, since 

 sodium ion is the leading ion in active salt transport, the rate of transport will 

 diminish with increasing sodium ion concentration at the inside and that this is 

 fairly independent of the kind of anion present. 



In 1 95 1, Ussing and Zerahn (73) published an important paper which deals 

 with studies on short-circuited frog skin. Simultaneous measurements of short- 

 circuit current and ion flux have shown that net sodium flux and short-circuit 

 current, both expressed in mCoul. cm~- hr~\ were exactly equal, within the 

 limitations of measurements. What is true for the shorted skin may also apply 

 for unshorted, normal skin. An equivalent circuit for frog skin was proposed, 

 consisting of a sodium transporting mechanism of an EMF of approximately 

 90 mv, a sodium resistance of about 1500 to 2000 S2 cm~-, and an internal shunt 



