Fast Inactivation of Delayed Rectifier K Conductance in Squid Giant Axon and Its Cell Bodies

Inactivation of delayed rectifier K conductance (g(K)) was studied in squid giant axons and in the somata of giant fiber lobe (GFL) neurons. Axon measurements were made with an axial wire voltage clamp by pulsing to V(K) (∼−10 mV in 50–70 mM external K) for a variable time and then assaying available g(K) with a strong, brief test pulse. GFL cells were studied with whole-cell patch clamp using the same prepulse procedure as well as with long depolarizations. Under our experimental conditions (12–18°C, 4 mM internal MgATP) a large fraction of g(K) inactivates within 250 ms at −10 mV in both cell bodies and axons, although inactivation tends to be more complete in cell bodies. Inactivation in both preparations shows two kinetic components. The faster component is more temperature-sensitive and becomes very prominent above 12°C. Contribution of the fast component to inactivation shows a similar voltage dependence to that of g(K), suggesting a strong coupling of this inactivation path to the open state. Omission of internal MgATP or application of internal protease reduces the amount of fast inactivation. High external K decreases the amount of rapidly inactivating I(K) but does not greatly alter inactivation kinetics. Neither external nor internal tetraethylammonium has a marked effect on inactivation kinetics. Squid delayed rectifier K channels in GFL cell bodies and giant axons thus share complex fast inactivation properties that do not closely resemble those associated with either C-type or N-type inactivation of cloned Kv1 channels studied in heterologous expression systems.