FAST INACTIVATION OF DELAYED RECTIFIER K-CONDUCTANCE IN SQUID GIANT-AXON AND ITS CELL-BODIES

Inactivation of delayed rectifier Ii conductance (gK) was studied in s quid giant axons and in the somata of giant fiber lobe (GFL) neurons. Axon measurements were made with an axial wire voltage clamp by pulsin g to V-K (similar to-10 mV in 50-70 mM external It) for a variable tim e and then assaying available gK 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 experimenta l conditions (12-18 degrees C, 4 mM internal MgATP) a large fraction o f g(K) inactivates within 250 ms at - 10 mV in both cell bodies and ax ons, although inactivation tends to be more complete in cell bodies. I nactivation in both preparations shows to kinetic components. The fast er component is more temperature-sensitive and becomes very prominent above 12 degrees C. Contribution of the fast component to inactivation shows a similar voltage dependence to that of g(K), suggesting a stro ng coupling of this inactivation path to the open state. Omission of i nternal MgATP or application of internal protease reduces the amount o f fast inactivation. High external Ii decreases the amount of rapidly inactivating I-K but does not greatly alter inactivation kinetics. Nei ther external nor internal tetraethylammonium has a marked effect on i nactivation kinetics. Squid delayed rectifier K channels in GFL cell b odies and giant axons thus share complex fast inactivation properties that do not closely resemble those associated with either G-type or N- type inactivation of cloned Kvl channels studied in heterologous expre ssion systems.