Equivalent Pore Radius of the Axolemma of Resting and Stimulated Squid Axons

The relationship between electrical potentials, ion concentrations, and ion permeabilities have been thoroughly investigated in the squid axon. The wealth of experimental results supports the ionic theory of the electrical phenomena and offers a satisfactory description of what the axolemma does at a variety of functional stages (I-3). In contrast, only suggestions exist about how the axolemma does what it does (3). A major and challenging problem is that of the nature of the mechanism of the ion permeabilities that underlie the electrical phenomena. A variety of suggestions has been made on the nature of this mechanism (4-12). The existence in the axolemma of ion pathways, channels, and /o r equivalent pores has been proposed. The ion channels are envisioned as sodium and potassium, voltage-sensitive paths, with gates that can be blocked by tetrodotoxin and tetraethyl ammonium respectively (13-19). The equivalent pores are pathways calculated from measurements of the permeability to water and some small nonelectrolyte molecules. In the resting axolemma, it has been calculated that the equivalent pores have a radius of 4-5 A and are spaced about 1000 A apart (12, 20). From analysis of ionic permeability studies the existence of similar pores has been proposed (7, 8). The present study deals with the determination of the reflection coefficient a (21) of the axolemma of resting and stimulated squid axons, for a set of penetrating nonelectrolyte molecules. The a values are then used to calculate (2226) the equivalent pore radius of the axolemma at rest and during activity. The method is essentially the same used by Villegas and Barnola (see reference 20), following Goldstein and Solomon (24).