Effects of an outward water flow on potassium currents in a squid giant axon

The excitation of the squid giant axon was analyzed under an outward water flow through the membrane product by an osmotic gradient. The outward water flow made an undershoot of the action potential larger by about 25 mV without decreasing its peak largely. It also made EK more negative but not ENa. The effect of the outward water flow was specific for the potassium channel. The outward current increased and its decline during a long-lasting depolarization became less prominent under the outward water flow. At the same time, inward tail currents for the potassium channel decreased extraordinarily without a large change in the time course. The potassium conductance had a marked rectification in the direction of the water flow. The undershoot of the action potential under the outward water flow was very sensitive to potassium ions in the external solution. Eight mM KCl was effective to diminish the undershoot and to restore the change in EK by about 60% but gave no effect on the reduced tail current. The outward water flow effect can be explained not only by the change in a local concentration of potassium ions at the mouths of the potassium channel due to a sieving but also by the rectification in a hydrodynamic manner.