Selective inhibition of a slow‐inactivating voltage‐dependent K+ channel in rat PC 12 cells by hypoxia

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Abstract
1 Electrophysiological (single‐channel patch clamp) and molecular biological experiments (reverse transcriptase‐polymerase chain reaction) were performed to attempt to identify the O2‐sensitive K+ channel in rat phaeochromocytoma (PC12) cells. 2 Four types of K+ channels were recorded in PC12 cells: a small‐conductance K+ channel (14 pS), a calcium‐activated K+ channel (KCa; 102 pS) and two K+ channels with similar conductance (20 pS). These last two channels differed in their time‐dependent inactivation: one was a slow‐inactivating channel, while the other belonged to the family of fast transient K+ channels. 3 The slow‐inactivating 20 pS K+ channel was inhibited by hypoxia. Exposure to hypoxia produced a 50% reduction in channel activity (number of active channels in the patch × open probability). Hypoxia had no effect on the 20 pS transient K+ channels, whereas reduced O2 stimulated the KCa channels. 4 The genes encoding the α‐subunits of slow‐inactivating K+ channels for two members of the Shaker subfamily of K+ channels (Kvl.2 and Kvl.3) together with the Kv2.1, Kv3.1 and Kv3.2 channel genes were identified in PC12 cells. 5 The expression of the Shaker Kv1.2, but none of the other K+ channel genes, increased in cells exposed to prolonged hypoxia (18 h). The same cells were more resuponsive to a subsequent exposure to hypoxia (35% inhibition of K+ current measured in whole‐cell voltage clamp) compared with the cells maintained in normoxia (19% inhibition). 6 These results indicate that the O2‐sensitive K+ channel in PC12 cells is a 20 pS slow‐inactivating K+ channel that is upregulated by hypoxia. This channel appears to belong to the Shaker subfamily of voltage‐gated K+ channels.