An Analysis of Pore Size in Excitable Membranes

It is possible to develop a method of analyzing membrane pore sizes based on quite different principles from those so elegantly described by Dr. Solomon in this Symposium. This depends upon an assumption that ions in aqueous solution will not penetrate through a membrane with pores of less than I o A in radius unless the size of the ion and the size of the pore are almost equal (1). Such a situation makes it possible to equate the permeability of the membrane for an ion to the number of pores in the membrane that just fit an ion of a particular size. The conclusion from such an analysis is that the mean pore radius in excitable membranes is about 4.0 A; this value is in agreement with values deduced for the red cell membrane from quitedifferent considerations (2). The advantage of considering excitable membranes and their pore size distributions is that the marked changes in ion selectivity and conductance exhibited by such structures offer a severe test of any hypothesis offered as an explanation for the operation of such membranes. I t appears that the cycle of permeability change exhibited by the squid axon under voltage clamp conditions accounts quantitatively for the action potential that is observed (3). I t also seems a valid generalization that most electrically excitable systems rely on a similar cycle of permeability change whereby the membrane becomes selectively permeable first to Na + and later to K +. There are, however, exceptions to this mechanism: crayfish muscle is normally not electrically excitable, but is able to give an action potential under conditions in which most of the extracellular cation is Ba ++. It seems likely that the depolarization observed is a consequence of an inward Ba ++ current produced in response to a depolarization of the membrane (4). Barium is also apparently able to serve as a current carrying ion in vertebrate B and C fibers (5). It is also of interest to note that Ba ++ and K + have the same crystal radius. A second system is the plant cell Chara, where the membrane is most permeable to K + but the response to depolarization is a great increase in chloride permeability of the membrane (6), and an increase in potassium permeability. The sodium permeability is unaffected by changes in membrane potential.