Inactivation rate constant and the number of inactivation gates

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N-type inactivation of rat Kv1.4 channels with one, two or four inactivation balls was investigated using homogenous populations of channels expressed in Xenopus oocytes. For homogenous population of channels, tandem dimeric and tetrameric constructs of Kv1.4 were made. Channels encoded y tandem cDNAs Kv1.4-Kv1.4Δ1-145 and Kv1.4-[Kv1.4Δ1-145]ィイD23ィエD2 have two or only one tethered inactivation ball, respectively, whereas Kv1.4 itself encodes channels having four inactivation balls. Kv1.4-Kv1.4 encodes channels having four inactivation balls but two in the loop connecting the subunits. The time constants for inactivation of macroscopic currents were increased significantly as the number of inactivation balls was decreased, whereas the time constants for recovery from inactivation of Kv1.4, Kv1.4-Kv1.4, Kv1.4-Kv1.4Δ1-145 and Kv1.4-[Kv1.4Δ1-145]ィイD23ィエD2 channels were not modified. The ratios of the rate constants of inactivation kィイD2inactィエD2 of Kv1.4-Kv1.4, Kv1.4-Kv1.4Δ1-145 and Kv1.4-[Kv1.4Δ1-145]ィイD23ィエD2 channels to that of Kv1.4 channel were 0.78, 0.65 and 0.4, respectively, while the ratios of the rate constant of recovery kィイD2recィエD2 of those channels to that of Kv1.4 were almost unity. The rate constants kィイD2inactィエD2 for the channel shaving two and four inactivation balls are smaller than those that would be expected if inactivation balls on each channel are independent, suggesting some interaction occurs between inactivation balls. Furthermore, noninactivating current became apparent as the number of inactivation balls on a channel was decreased.