164 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



POSSIBLE MECHANISMS 



Attempts to account for ionic distribution include electrical gradients in the 

 system. This follows from the fact that, with potential differences oriented as 

 in nerve fibers and in most cells — the interior negative with respect to the 

 exterior — freely dififusible cations will be accelerated inward and decelerated 

 outward, and the opposite will be true of anions. An important problem is to 

 divorce such 'passive' movement, aswell as that brought about by concentration 

 differences, from 'active' transfer achieved by coupling with the energy liber- 

 ated by metabolic reactions. Since metabolism might itself directly generate 

 potential differences, which in turn could contribute to the distribution of 

 certain ion species, a detection of this type of secondary linkage also is desir- 

 able. A theoretical relation between E and ion movement derived for this 

 purpose for passive systems has proven successful in the case of frog skin, where 

 the direct active transport only of sodium is now well established (57). On the 

 assumption of free dififusibility of ions, it is concluded that the rate of entry or 

 influx, Min, and rate of exit or outflux, Mout, of monovalent ions will, in a 

 passive system at room temperature (20°C), depend on extracellular and intra- 

 cellular concentrations (Co and d), or more strictly the thermodynamic 

 activities, and on the resting potential, E, in millivolts, as follows: 



IMin/Mout = (Co/Ci) e^/25 (2) 



This has been applied with apparent success to potassium in invertebrate 

 giant fibers, where the different influx and outflux reflect a deterioration of the 

 preparation (20). A questionable aspect of this application, aside from the 

 uncertainty introduced by the condition of the axons, is its use in systems where 

 metabolism has not been obliterated or its role clearly defined. Ussing (57) calls 

 attention to recent unpublished findings by Hodgkin and Keynes in which 

 metabolic inhibition eliminated the agreement of the potassium fluxes with 

 equalion 2. This agreement is now considered by Keynes (27) to have been a 

 'coincidence'. 



Disregarding temporarily the uncertainty introduced by the operation of 

 metabolism, we may examine the applicability of equation 2 to fibers in a steady 

 state or in equilibrium with their environment. In this instance, IMin and Mout 

 are equal, and equation 2 reduces to the form of equation i for all freely diffusible 

 ions. 



Desheathed toad nerve meets the demand for stability since it does not 

 change in ionic content for at least 40 to 48 hours in frog Ringer's (48, 52). 

 However, with this preparation the analytical and bioelectrical data are subject 

 to the usual uncertainties inherent in the use of multifibered tissue. This 

 uncertainty can be minimized by a number of techniques. For e.xample, the 

 extracellular aqueous volume, and hence the intracellular, may be determined 

 with sucrose, which, unlike chloride, probably does not enter the axons; this 



