EFFECTS OF IONS ON VASCULAR SMOOTH MUSCLE 



I 137 



table I . Ionic Radii of Alkali Metal Ions 



Li 



Na 



K 



Rb 



Cs 



NH,+ 



From Ussing et a/. (202;. 



The smaller the ionic crystal the more densely is 

 its electrical field packed and the greater its attraction 

 for water dipoles. Consequently, the size of the pack- 

 age, crystal plus water, moving in a solution, is 

 larger the smaller the crystal. Thus, the rank order 

 for the hydrated ion size is Cs < Rb < K < Na < Li 

 (table 1). This model should not be accepted un- 

 critically (160) although it does provide us with a 

 useful working framework. 



ion activity. Ions in solution are subject to interionic 

 forces which limit their availability. In consequence, 

 the measured concentration of an ion may be greater 

 or less than its reactive concentration by some 

 measurable degree defined as the activity coefficient. 

 The concentration of the substance, corrected by the 

 activity coefficient, defines the activity of the ion in 

 the given solution. Since reactions in solutions and 

 their resultant equilibria are determined by activities 

 rather than by concentrations this point has special 

 importance. As figure 2 shows, each salt has its own 

 characteristic curve relating activity to concentra- 

 tion. Standard tables are available. For similar 

 reasons, the ionization constant of salts is also of 

 import, since not all salts dissociate with equal com- 

 pleteness into ions. Activity will be symbolically 

 written (Na + ) and concentration [Na + ]. Where such 

 precision is not necessary in a given context we shall 

 simplv write Na for sodium or Na + for sodium ion. 



ion mobility. In general, the mobility of an ion in 

 free solution varies inversely with its hydrated size. 

 In fact, the concept of ion hydration was in part 

 developed to explain the relative mobilities of ions. 

 Ion mobility is ordinarily measured as velocity in a 

 standard electrical field. Table 2 shows the relation 

 of increasing size to slower velocity. 



ion penetrability or membrane permeability. Cell 

 membranes in general appear to have channels so 

 limited in size as to allow K to enter freely and just 



fig. 2. Mean activity coefficients of various electrolytes at 

 25 C. [From Prutton & Maron (161 a).] 



to exclude Na. Conway (32) has presented interesting 

 and basic data pertinent to this point (table 3). 

 Frog sartorius immersed in Ringer fluid, to which 100 

 iriM of a particular salt is added, at first loses weight 

 (osmotic withdrawal of water). Then, if the membrane 

 is freely permeable to the salt, the weight increases 

 back to its original base. Thus, at equilibrium, the 

 added salt has not upset the osmotic balance. The 

 time required to recover 50 per cent of the weight loss 

 is taken as a measure of the permeability of the mem- 

 brane for the particular ion and table 3 is so con- 

 structed. It is evident that taking KC1 as standard, 

 RbCl and CsCl enter readily, while the chlorides of 

 Na, Li, Ca, and Mg do not enter appreciably. Simi- 

 larly, in the anion series, bromide and nitrate enter 

 easily while phosphate, acetate, bicarbonate, and 

 sulfate are excluded. 



On the right side of the table Conway compares 

 the diffusion coefficients of the ions rather than rela- 

 tive ion diameter with K and points out that the 

 correspondence is far from exact (cf Rb and Cs pene- 

 tration rates with diffusion rates). 



The permeability of the cell membrane to ions is 

 not a fixed characteristic but must be expected to 

 vary physiologically and pathologically. 



