Sodium-dependent Potassium Channels in Leech P Neurons
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In leech P neurons the inhibition of the Na+-K+ pump by ouabain or omission of bath K+ leaves the membrane potential unaffected for a prolonged period or even induces a marked membrane hyperpolarization, although the concentration gradients for K+ and Na+ are attenuated substantially. As shown previously, this stabilization of the membrane potential is caused by an increase in the K+ conductance of the plasma membrane, which compensates for the reduction of the K+ gradient. The data presented here strongly suggest that the increased K+ conductance is due to Na+-activated K+ (KNa) channels. Specifically, an increase in the cytosolic Na+ concentration ([Na+]i) was paralleled by a membrane hyperpolarization, a decrease in the input resistance (Rin) of the cells, and by the occurrence of an outwardly directed membrane current. The relationship between Rin and [Na+]i followed a simple model in which the Rin decrease was attributed to K+ channels that are activated by the binding of three Na+ ions, with half-maximal activation at [Na+]i between 45 and 70 mM. At maximum channel activation, Rin was reduced by more than 90%, suggesting a significant contribution of the KNa channels to the physiological functioning of the cells, although evidence for such a contribution is still lacking. Injection experiments showed that the KNa channels in leech P neurons are also activated by Li+.