Removal of periaxonal potassium accumulation in a squid giant axon by outward osmotic water flow.

1. Periaxonal potassium accumulation does not occur in a squid giant axon when outward water flow is maintained by an osmotic gradient across the axolemma. Potassium concentrations in the periaxonal space were calculated from the potassium potentials (EK) for the tail K+ current. With outward water flow, the periaxonal K+ concentration was maintained at values less than or close to the K+ concentration in the bathing solution. 2. Outward osmotic water flow was produced by adding 1 M-urea to the isotonic external solution. This was sufficient to prevent K+ accumulation, but it had no effect if applied to both sides of the axolemma. 3. The thickness of the periaxonal space (theta s) and the permeability of the extracellular barrier (Ps) were estimated using a three-compartment model. Under isotonic conditions they were 25 nm and 3.7 micron/s, respectively. With outward water flow, either Ps or theta s or both must increase by a large factor, since K+ accumulation is prevented. 4. Instantaneous I-V relations with outward water flow showed outward rectification and no time-dependent changes in their shape. When external K+ concentration was increased, the curves became more linear.