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Ca 2+ ‐inhibited non‐inactivating K + channels in cultured rat hippocampal pyramidal neurones
Author(s) -
Selyanko A. A.,
Sim J. A.
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.071bz.x
Subject(s) - tetraethylammonium , depolarization , conductance , chemistry , patch clamp , membrane potential , hippocampal formation , biophysics , reversal potential , pipette , analytical chemistry (journal) , potassium , endocrinology , biochemistry , physics , biology , chromatography , receptor , organic chemistry , condensed matter physics
1 Whole‐cell perforated‐patch recording from cultured CA1‐CA3 pyramidal neurones from neonatal rat hippocampus (20‐22 °C; [K + ] o = 2.5 mM) revealed two previously recorded non‐inactivating (sustained) K + outward currents: a voltage‐independent ‘leak’ current ( I leak ) operating at all negative potentials, and, at potentials ≥−60 mV, a time‐ and voltage‐dependent ‘M‐current’ ( I K(M) ). Both were inhibited by 1 mM Ba 2+ or 10 μM oxotremorine‐M (Oxo‐M). In ruptured‐patch recording using Ca 2+ ‐free pipette solution, I leak was strongly enhanced, and was inhibited by 1 mM Ba 2+ but unaffected by 0.5 mM 4‐aminopyridine (4‐AP), 1 mM tetraethylammonium (TEA) or 1‐10 nM margatoxin. 2 Single channels underlying these currents were sought in cell‐attached patch recordings. A single class of channels of conductance ≈7 pS showing sustained activity at resting potential and above was identified. These normally had a very low open probability ( P o < 0.1), which, however, showed a dramatic and reversible increase (to about 0.9 at ≈0 mV) following the removal of Ca 2+ from the bath. Under these (Ca 2+ ‐free) conditions, single‐channel P o showed both voltage‐dependent and voltage‐independent components on patch depolarization from resting potential. The mean activation curve was fitted by a modified Boltzmann equation. When tested, all channels were reversibly inhibited by addition of 10 μM Oxo‐M to the bath solution. 3 The channels maintained their high P o in patches excised in inside‐out mode into a Ca 2+ ‐free internal solution and were strongly inhibited by application of Ca 2+ to the inner face of the membrane (IC 50 = 122 nM); this inhibition was observed in the absence of MgATP, and therefore was direct and unrelated to channel phosphorylation/dephosphorylation. 4 Channels in patches excised in outside‐out mode were blocked by 1 mM Ba 2+ but were unaffected by 4‐AP or TEA. 5 Channels in cell‐attached patches were inhibited after single spikes, yielding inward ensemble currents lasting several hundred milliseconds. This was prevented in Ca 2+ ‐free solution, implying that it was due to Ca 2+ entry. 6 The properties of these channels (block by internal Ca 2+ and external Oxo‐M and Ba 2+ , and the presence of both voltage‐dependent and voltage‐independent components in their P o / V relationship) show points of resemblance to those expected for channels associated with both I leak and I K(M) components of the sustained macroscopic currents. For this reason we designate them K sust (‘sustained current’) channels. Inhibition of these channels by Ca 2+ entry during an action potential may account for some forms of Ca 2+ ‐dependent after‐depolarization. Their high sensitivity to internal Ca 2+ may provide a new, positive feedback mechanism for cell excitation operating at low (near‐resting) [Ca 2+ ] i .