ERNST G. HUF 215 



of Br~ and XO:r, this is without any consequence with relation to net salt up- 

 take by the epithelium of the skin, it seems, on the other hand, that I~ upsets 

 the intricate mechanism of salt uptake. It is noteworthy, also, that S04= and 

 HP04=° do not replace any appreciable amount of Cl~. These divalent anions 

 must, therefore, be regarded as rather indifferent from the standpoint of main- 

 taining optimal rate of net active salt uptake. Besides its inhibitory effect on 

 net salt uptake, I~ has other remarkable effects on skin which will be discussed 

 below. 



A chemical factor that may well turn out to be one of the important physio- 

 logical regulators in active salt uptake by frog skin is the potassium ion level 

 of the environment. In 1936, Rubin (6ia) found what might have been ex- 

 pected, namely, that frog skin, like most other tissues, contains a relatively 

 large amount of potassium, about 40 ^uEq K+/gm of wet skin. The bulk of the 

 potassium can histochemically be located in the epidermis (6ib). Shortly after- 

 wards, Steinbach (68) reported his studies on potassium equilibrium in frog skin. 

 Under certain experimental conditions, a kind of dependence of skin potassium 

 upon the external potassium concentration was obtained which, when plotted 

 graphically, looks like a power function. This, however, was not brought out 

 and not investigated further. The significance of potassium for the maintenance 

 of the skin potential was demonstrated by Fukuda (10). He found that removal 

 of potassium ions from the inside bath of the skin causes disappearance of the 

 'asymmetry potential', especially in skins of summer frogs. Interestingly 

 enough, McClure (50), in 1927, called attention (unfortunately in a somewhat 

 remote journal) to the fact that water transport across skin between isotonic 

 Ringer's solutions is smaller in the absence than in the presence of potassium. 

 Recently it has been found (26) that active salt uptake by frog skin is remark- 

 ably diminished when potassium-deficient saline solutions are used. An illustra- 

 tion of this, based on average data, is given in figure 7. The respective studies 

 were carried out using the paired bag method. This is to say that the activity 

 of one skin bag, prepared from the skin of one leg, was compared with the 

 activity of the skin bag, prepared from the other leg of the same frog. Under 

 the same conditions for a pair of bags, there is almost no difference in activity 

 of the two sides (see left part of fig. 7). 



When applied in equivalent amounts, Rb+ can almost completely, Cs+ can 

 partially substitute for K"*" in Ringer's. NH4+, however, inhibits active salt and 

 water uptake from K+ deficient salt solutions. With regard to active uptake of 

 sodium ions, the relative effectiveness of the tested cations was K+:Rb"^:Cs+: 

 XH4+ = 100:99: 76:37, with a value of 64 when none of these ions were added 

 to the salt solutions (26). It might be added that omission of Ca++ ions from 

 Ringer's solution or addition of small amounts of Mg"*"*" ions to it, do not alter 

 active salt uptake (26). Fukuda (10) reported that removal of Ca++ from 

 Ringer's does not abolish the 'asymmetry potential'. 



