Significant potassium ion accumulation at the external surface of Myxicola giant axons

Potassium accumulation associated with outward membrane potassium current was investigated experimentally in Myxicola giant axon. During prolonged voltage-clamp pulses to positive transmembrane potentials, the K⁺ equilibrium potential may approach zero mV, suggesting massive K⁺ accumulation outside the axonal membrane to concentrations many-fold higher than those in the bathing medium. The potassium accumulation can be satisfactorily described by a three-compartment model, consisting of the nerve fiber, a restricted physiological periaxonal space and the bulk solution. The average thickness, θ, of the periaxonal space is calculated as 177 ± 59 A ̊, i.e., comparable to that in the squid, while the permeability coefficient of the external barrier, P_Ks, was calculated to be (1.4 ± 0.4)•10^-4 cm/s. These conclusions are well supported by morphological study.