Potassium conductance causing hyperpolarization of CA1 hippocampal neurons during hypoxia.
In experiments on slices (from 100- to 150-g Sprague-Dawley rats) kept at 33 degreesC, we studied the effects of brief hypoxia (2-3 min) on CA1 neurons. In whole cell recordings from submerged slices, with electrodes containing only KMeSO4 and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, and in the presence of kynurenate and bicuculline (to minimize transmitter actions), hypoxia produced the following changes: under current clamp, 36 cells were hyperpolarized by 2.7 +/- 0.5 (SE) mV and their input resistance (Rin) fell by 23 +/- 2.7%; in 30 cells under voltage clamp, membrane current increased by 114 +/- 22.3 pA and input conductance (Gin) by 4.9 +/- 0.9 nS. These effects are much greater than those seen previously with K gluconate whole cell electrodes, but only half those seen with "sharp" electrodes. The hypoxic hyperpolarizations (or outward currents) were not reduced by intracellular ATP (1-5 mM) or bath-applied glyburide (10 microM): therefore they are unlikely to be mediated by conventional ATP-sensitive K channels. On the other hand, their depression by internally applied ethylene glycol-bis-(beta-aminoethyl ether)-N,N, N',N'-tetraacetic acid (1.1 and 11 mM) and especially 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (11-33 mM) indicated a significant involvement of Ca-dependent K (KCa) channels. The beta-adrenergic agonist isoprenaline (10 microM) reduced hypoxic hyperpolarizations and decreases in Rin (n = 4) (and in another 11 cells corresponding changes in Gin); and comparable but more variable effects were produced by internally applied 3':5'-adenosine cyclic monophosphate (cAMP, 1 mM, n = 6) and bath-applied 8-bromo-cAMP (n = 8). Thus afterhyperpolarization-type KCa channels probably take part in the hypoxic response. A major involvement of G proteins is indicated by the near total suppression of the hypoxic response by guanosine 5'-O-(3-thiotriphosphate) (0. 1-0.3 mM, n = 23) and especially guanosine 5'-O-(2-thiodiphosphate) (0.3 mM, n = 26), both applied internally. The adenosine antagonist 8-(p-sulfophenyl)theophylline (10-50 microM) significantly reduced hypoxic hyperpolarizations and outward currents in whole cell recordings (with KMeSO4 electrodes) from submerged slices but not in intracellular recordings (with KCl electrodes) from slices kept at gas/saline interface. In further intracellular recordings, antagonists of gamma-aminobutyric acid-B or serotonin receptors also had no clear effect. In conclusion, these G-protein-dependent hyperpolarizing changes produced in CA1 neurons by hypoxia are probably initiated by Ca2+ release from internal stores stimulated by enhanced glycolysis and a variable synergistic action of adenosine.
Erdemli
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Anaesthesia Research Department, McGill University, Montreal, Quebec H3G 1Y6, Canada.
Xu
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Krnjević
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D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D015687:Cell Hypoxia / Q000502:physiology; D002614:Chelating Agents / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D058788:GABA-B Receptor Antagonists; D019204:GTP-Binding Proteins / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008698:Mesylates; D008839:Microelectrodes; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D058915:Purinergic P1 Receptor Antagonists; D051381:Rats; D017207:Rats, Sprague-Dawley; D012702:Serotonin Antagonists / Q000494:pharmacology
D002614:Chelating Agents; D058788:GABA-B Receptor Antagonists; D008698:Mesylates; D015221:Potassium Channels; D058915:Purinergic P1 Receptor Antagonists; D012702:Serotonin Antagonists; C045880:methanesulfonic acid; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D007545:Isoproterenol; D002118:Calcium