212 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



(2, 70). What should be well recognized is the fact that the very core of the 

 'carrier hypothesis', namely the transport of ions across a boundary in the form 

 of undissociated molecules, is to be found in the first of a series of Osterhout's 

 papers on accumulation of electrolytes which was published in 1930 (3). 



Salt accumulation at the corium side of isolated frog skin was studied by 

 applying conventional microchemical and flame photometric techniques, 

 tracer techniques, electro-chemical methods, or, mostly, a combination of these 

 methods. Conventional microchemical methods give only information about 

 net transport rates, i.e. the dijference between influx and outflux. By using 

 tracer elements, one arrives at flux values in each direction for the ion species 

 under consideration. From these, net rates of ion transport may be calculated. 

 Katzin, in 1939, introduced radioisotopes of Na+ and K+ in permeability 

 studies on frog skin (34). In 1940, he made the observation that influx of 

 sodium ion exceeds outflux of this ion (35). The reverse was found for potas- 

 sium (35). Full use of isotopes of sodium and potassium was then made by 

 Ussing (70, 71, 73) and Linderholm (45-47) who laid particular emphasis on 

 the correlation between flux values and several electric parameters of the skin. 



Results on active salt uptake by frog skin, obtained with the various methods, 

 are in good agreement and will, therefore, be discussed together. 



Factors in Active Uptake of Sodium Chloride and Skin Potential 



Under favorable conditions, the net rate of salt transport by leg skin of 

 Rana pipiens is of the order of i juEq cm~- hr."^ (28, 29). Several chemical and 

 physical factors lead to interference of active salt transport. Most of the 

 factors studied depress metabolism, electric potential and active salt and water 

 transport in frog skin. Enzyme inhibitors prove to be very useful in studies of 

 this kind. The application of cyanide and monobromoacetate demonstrates the 

 importance of oxidative and glycolytic processes for maintenance of skin 

 potential and active salt transport (6, 15-17, 48, 70). Eckstein has given evi- 

 dence that skin potentials can, to some extent, be maintained on methylene 

 blue (3a). Lactate, pyruvate, also acetate, propionate and butyrate increase 

 skin potentials and active transport of bromo- or iodoacetate poisoned skins 

 (6, 17; fig. 5). In recent studies other enzyme inhibitors, hormones, and drugs 

 have been used. Thiocyanate, dinitrophenol, and mercurial compounds inhibit 

 skin potential and active transport (9, 44, 45). Interestingly enough, it is 

 found that anticholinesterases (tetraethylpyrophosphate, eserine, procaine) 

 inhibit sodium influx, when the drugs are applied to the inside of the skin (36). 

 A number of pharmacologically diverse drugs, atropine, several curares, hista- 

 mine, pilocarpine, but not acetylcholine, when applied to the outside, induce an 

 increase in sodium influx (36). Epinephrine, when added on the inside, results 

 in an enormous increase in outflux of sodium and a considerable increase in the 

 influx. The potential drops abruptly at first, but may be followed by a second- 



