A complementary DNA encoding the rat NMDA receptor has been cloned and characterized. The single protein encoded by the cDNA forms a receptor-channel complex that has electrophysiological and pharmacological properties characteristic of the NMDA receptor. This protein has a significant sequence similarity to the AMPA/kainate receptors and contains four putative transmembrane segments following a large extracellular domain. The NMDA receptor messenger RNA is expressed in neuronal cells throughout the brain regions, particularly in the hippocampus, cerebral cortex and cerebellum.
The slow afterhyperpolarization that follows an action potential is generated by the activation of small-conductance calcium-activated potassium channels (SK channels). The slow afterhyperpolarization limits the firing frequency of repetitive action potentials (spike-frequency adaptation) and is essential for normal neurotransmission. SK channels are voltage-independent and activated by submicromolar concentrations of intracellular calcium. They are high-affinity calcium sensors that transduce fluctuations in intracellular calcium concentrations into changes in membrane potential. Here we study the mechanism of calcium gating and find that SK channels are not gated by calcium binding directly to the channel alpha-subunits. Instead, the functional SK channels are heteromeric complexes with calmodulin, which is constitutively associated with the alpha-subunits in a calcium-independent manner. Our data support a model in which calcium gating of SK channels is mediated by binding of calcium to calmodulin and subsequent conformational alterations in the channel protein.
An intermediate conductance calciumactivated potassium channel, hIK1, was cloned from human pancreas. The predicted amino acid sequence is related to, but distinct from, the small conductance calcium-activated potassium channel subfamily, which is Ϸ50% conserved. hIK1 mRNA was detected in peripheral tissues but not in brain. Expression of hIK1 in Xenopus oocytes gave rise to inwardly rectifying potassium currents, which were activated by submicromolar concentrations of intracellular calcium (K 0.5 ؍ 0.3 M). Although the K 0.5 for calcium was similar to that of small conductance calcium-activated potassium channels, the slope factor derived from the Hill equation was significantly reduced (1.7 vs. 3.5). Single-channel current amplitudes ref lected the macroscopic inward rectification and revealed a conductance level of 39 pS in the inward direction. hIK1 currents were reversibly blocked by charybdotoxin (K i ؍ 2.5 nM) and clotrimazole (K i ؍ 24.8 nM) but were minimally affected by apamin (100 nM), iberiotoxin (50 nM), or ketoconazole (10 M). These biophysical and pharmacological properties are consistent with native intermediate conductance calcium-activated potassium channels, including the erythrocyte Gardos channel.
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