208 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



magnitude but opposite sign" (31) appears unjustified. There is, in the words of 

 Jacobs, "no reason to beheve that solvent water, which is essentially a con- 

 tinuum with high cohesion forces, would follow exactly the same laws of diffu- 

 sion as do the entirely isolated solute molecules in a dilute solution" (31). 

 Water may be treated either as a solvent or as a solute; consequently, the 

 'permeability coefficient' for water as a solvent (Pw) must be different from the 

 'permeability coefficient' for water as a solute (P). Referring again to the in- 

 vestigations of V. Hevesy el al., one may say, then, using Jacobs' terminology, 

 that the ratio of Pw/P in frog skin is about 4. Higher coefficients for osmo- 

 permeability than for diffusion permeability were also found in many other 



50 100 150 200. 250 mm. 



Fig. 2. Action of aminophylline on isolated frog skin {Rana tern poraria) . Ringer's solution 

 on both sides. Solution on one side contained about 4% DoO. Aminophylline was added as 

 indicated by the arrow to give a concentration of 50 mg/ioo ml in the inside solution. Perme- 

 ability coefficient of skin to D2O in upper section, d.c. conductance (G) and skin potential 

 {ip) in lower section (11). 



biological structures. Since a high Pw/P ratio seems to be indicative of large 

 pores, Ussing (76) has suggested that this ratio be used as a means of estimating 

 pore sizes. The work of Prescott and Zeuthen (57) is a step in this direction. 



A number of biologically active substances, such as neuro-hypophyseal 

 extract, adrenaline, mersalyl, aminophylline, have a marked effect on net 

 water flow and several electrical parameters; however, they have only a 

 negligible effect on rate of water diffusion (11, 41; see fig. 2). Ussing has sug- 

 gested that heavy water diffusion rate may depend mainly upon the area 

 available to diffusion, whereas net flow may depend, among other things, on 

 the size of pores in the skin (74); therefore, when neuro-hypophyseal extract 

 increases the rate of net water flow, as has been found, it may do this by 

 opening pores of the skin (41, 62). 



